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Straight-line motion

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52278612
A tennis player hits a ball straight upward. Its height \(h\), in meters, after \(t\) seconds is modeled by \(h(t) = -4.9t^2 + 12t + 1.5\). Use the first and second derivatives to determine when the ball reaches its highest point and the height of that point.

Hints

- What condition does the first derivative satisfy at the highest point? - How does the second derivative confirm that the point is a maximum? - What does the value of \(h(t)\) represent?

Solution

1. Differentiate: \(h'(t) = -9.8t + 12\) and \(h''(t) = -9.8\). 2. Solve \(h'(t) = 0\): \(-9.8t + 12 = 0\), so \(t = \frac{60}{49} \approx 1.224\,\text{s}\). 3. Since \(h''(t) = -9.8 < 0\), the critical time gives a maximum. 4. Evaluate the height: \(h(\frac{60}{49}) = \frac{867}{98} \approx 8.85\,\text{m}\).

Answer

The ball reaches its highest point after approximately \(1.22\,\text{s}\), at a height of approximately \(8.85\,\text{m}\).
52637512
An object moves along a straight line. Its position \(s\), in meters, at time \(t\), in seconds, is \(s(t)=0.5t^3-3t^2+10t\). 1) Find formulas for the instantaneous velocity \(v(t)\) and acceleration \(a(t)\). 2) Find the velocity at \(t=2\,\text{s}\). 3) At what time is the acceleration \(0\,\text{m/s}^2\)?

Hints

- Which physical quantity is the first derivative of position? - How are velocity and acceleration related by derivatives? - Substitute the given time into the appropriate function to find an instantaneous value. - To find when acceleration has a specified value, set up and solve an equation in \(t\).

Solution

1. Velocity is the derivative of position: \(v(t)=s'(t)=1.5t^2-6t+10\). Acceleration is the derivative of velocity: \(a(t)=v'(t)=3t-6\). 2. Evaluate the velocity: \(v(2)=1.5(2)^2-6(2)+10=4\,\text{m/s}\). 3. Set acceleration equal to zero: \(3t-6=0\), so \(t=2\,\text{s}\).

Answer

1) \(v(t)=1.5t^2-6t+10\) and \(a(t)=3t-6\) 2) \(4\,\text{m/s}\) 3) \(t=2\,\text{s}\)
52643112
A small underwater robot moves along a straight path. During the first \(10\) seconds, its position, in meters, is modeled by \(s(t)=0.02t^3-0.36t^2+2.4t\), where \(t\) is measured in seconds. a) Find the robot's instantaneous velocity at \(t=5\,\text{s}\). b) Find its acceleration at the same time. Explain what the sign of the acceleration means for the robot's motion.

Hints

- The first derivative of position gives velocity. - Acceleration is the derivative of velocity. - A negative acceleration does not by itself mean slowing down; compare its sign with the velocity's sign. - Include the correct units for velocity and acceleration.

Solution

1. Differentiate position to find velocity: \(v(t)=s'(t)=0.06t^2-0.72t+2.4\). 2. Evaluate: \(v(5)=0.06(5)^2-0.72(5)+2.4=0.3\,\text{m/s}\). 3. Differentiate velocity to find acceleration: \(a(t)=v'(t)=0.12t-0.72\). 4. Evaluate: \(a(5)=0.12(5)-0.72=-0.12\,\text{m/s}^2\). 5. Because the velocity is positive and the acceleration is negative, the robot is slowing down at that instant.

Answer

a) \(v(5)=0.3\,\text{m/s}\) b) \(a(5)=-0.12\,\text{m/s}^2\). Since velocity and acceleration have opposite signs, the robot is slowing down.
52643712
A ball is thrown straight upward. Its height, in meters, after \(t\) seconds is modeled by \(h(t) = -5t^2 + 20t + 1.2\). a) Find the ball's instantaneous velocity at \(t = 1\,\text{s}\). b) Find when the ball reaches its highest point and determine that height.

Hints

- How are height and velocity related? - What is the vertical velocity at the highest point? - Which operation gives the rate of change of a function?

Solution

1. Differentiate height: \(v(t) = h'(t) = -10t + 20\). 2. Evaluate \(v(1) = -10(1) + 20 = 10\,\text{m/s}\). 3. At the highest point, \(v(t) = 0\). Solve \(-10t + 20 = 0\) to get \(t = 2\,\text{s}\). 4. Evaluate the height: \(h(2) = -5(2^2) + 20(2) + 1.2 = 21.2\,\text{m}\).

Answer

a) \(10\,\text{m/s}\) b) The ball reaches its highest point at \(t = 2\,\text{s}\), at a height of \(21.2\,\text{m}\).
52816112
A \(1200\,\text{kg}\) test vehicle moves along a straight track. Its position, in meters, is \(s(t)=\frac{1}{12}t^3+t^2+5t\), where \(t\) is measured in seconds. Find the net force on the vehicle at \(t=4\,\text{s}\). Use \(F=ma\).

Hints

- Acceleration is the second derivative of position. - Find the acceleration at the specified time before applying \(F=ma\). - Check that the units combine to give newtons.

Solution

1. Differentiate position to find velocity: \(v(t)=s'(t)=\frac{1}{4}t^2+2t+5\). 2. Differentiate again to find acceleration: \(a(t)=s''(t)=\frac{1}{2}t+2\). 3. Evaluate: \(a(4)=4\,\text{m/s}^2\). 4. Apply Newton's second law: \(F=ma=(1200\,\text{kg})(4\,\text{m/s}^2)=4800\,\text{N}\).

Answer

The net force is \(4800\,\text{N}\).
52222112
A toy rocket is launched straight upward. Its height \(h\), in meters, after \(t\) seconds is modeled by \(h(t) = -5t^2 + 30t + 0.5\). a) Find the rocket's velocity function \(v(t)\). b) Find when the rocket reaches its highest point and determine its maximum height. c) Find the rocket's velocity when it hits the ground.

Hints

- How are position and velocity related? - What is the velocity when the rocket changes direction? - What equation describes the moment when the rocket reaches the ground? - What does a negative velocity mean in this context?

Solution

1. Velocity is the derivative of position: \(v(t) = h'(t) = -10t + 30\). 2. At the highest point, \(v(t) = 0\). Solve \(-10t + 30 = 0\) to get \(t = 3\,\text{s}\). Then \(h(3) = -5(3^2) + 30(3) + 0.5 = 45.5\,\text{m}\). 3. The rocket hits the ground when \(h(t) = 0\). The positive solution of \(-5t^2 + 30t + 0.5 = 0\) is \(t = 3 + \frac{\sqrt{910}}{10} \approx 6.017\,\text{s}\). 4. Evaluate the velocity at that time: \(v(t) = -10t + 30 = -\sqrt{910} \approx -30.17\,\text{m/s}\). The negative sign indicates downward motion.

Answer

a) \(v(t) = -10t + 30\) b) The rocket reaches its highest point at \(t = 3\,\text{s}\), and the maximum height is \(45.5\,\text{m}\). c) The impact velocity is approximately \(-30.17\,\text{m/s}\), or \(30.17\,\text{m/s}\) downward.
52278512
An arrow is shot straight upward with an initial velocity of \(45\,\text{m/s}\). Its height \(h\), in meters, after \(t\) seconds is modeled by \(h(t) = -4.905t^2 + 45t\). a) Find the time when the arrow reaches its maximum height. b) Find the maximum height. c) Give the time interval during which the arrow is rising. Justify your answer using the first derivative.

Hints

- What physical quantity does the derivative of height represent? - What is the vertical velocity at the instant the arrow changes direction? - How does the sign of the first derivative indicate whether the height is increasing? - The motion begins at \(t = 0\).

Solution

1. Differentiate: \(h'(t) = -9.81t + 45\). 2. Set the velocity equal to zero: \(-9.81t + 45 = 0\), so \(t = \frac{500}{109} \approx 4.587\,\text{s}\). 3. Evaluate the height: \(h(\frac{500}{109}) = \frac{11250}{109} \approx 103.21\,\text{m}\). 4. The arrow is rising when \(h'(t) > 0\). Solving \(-9.81t + 45 > 0\) gives \(0 \le t < \frac{500}{109}\). Therefore, the arrow rises on \([0, \frac{500}{109})\) seconds, approximately \([0, 4.587)\) seconds.

Answer

a) \(t = \frac{500}{109} \approx 4.59\,\text{s}\) b) \(\frac{11250}{109}\,\text{m} \approx 103.21\,\text{m}\) c) \([0, \frac{500}{109})\) seconds, approximately \([0, 4.59)\) seconds
52637612
Two model cars move along a test track. Their positions, in centimeters, at time \(t\), in seconds, are \(s_1(t)=6t^2+2t\) \(s_2(t)=t^3+3t^2+5t\). 1) Which car has the greater instantaneous velocity at \(t=0\,\text{s}\)? Justify your answer algebraically. 2) Find the time \(t>0\) when the cars have equal acceleration.

Hints

- The slope of a position function is instantaneous velocity. - Velocity is the rate of change of position. - Acceleration is the rate of change of velocity. - Equal accelerations can be found by setting the acceleration functions equal.

Solution

1. Differentiate to find velocity: \(v_1(t)=12t+2\) and \(v_2(t)=3t^2+6t+5\). At \(t=0\), \(v_1(0)=2\,\text{cm/s}\) and \(v_2(0)=5\,\text{cm/s}\), so car 2 has the greater initial velocity. 2. Differentiate again: \(a_1(t)=12\) and \(a_2(t)=6t+6\). Set them equal: \(12=6t+6\), giving \(t=1\,\text{s}\).

Answer

1) Car 2; its velocity is \(5\,\text{cm/s}\), compared with \(2\,\text{cm/s}\) for car 1. 2) \(t=1\,\text{s}\)
52642112
A research vehicle moves along a straight test track. Its position, in meters, is \(s(t) = 0.2t^3 - 1.5t^2 + 4t + 10\), where \(t\) is measured in seconds. a) Find the instantaneous velocity at \(t = 5\,\text{s}\). b) Find when the vehicle's velocity is minimized. What is its acceleration at that time?

Hints

- How are position, velocity, and acceleration related by derivatives? - What does instantaneous velocity mean in terms of \(s(t)\)? - At an interior minimum of velocity, what must the acceleration equal? - Track the units through each derivative.

Solution

1. Differentiate position to find velocity: \(v(t) = s'(t) = 0.6t^2 - 3t + 4\). 2. Evaluate \(v(5) = 0.6(5^2) - 3(5) + 4 = 4\,\text{m/s}\). 3. Differentiate velocity to find acceleration: \(a(t) = v'(t) = 1.2t - 3\). 4. A minimum of velocity occurs where \(a(t) = 0\). Solve \(1.2t - 3 = 0\) to get \(t = 2.5\,\text{s}\). Because \(v''(t) = 1.2 > 0\), this critical time gives a minimum. 5. At that time, \(a(2.5) = 0\,\text{m/s}^2\).

Answer

a) \(4\,\text{m/s}\) b) The velocity is minimized at \(t = 2.5\,\text{s}\), when the acceleration is \(0\,\text{m/s}^2\).
52642212
An object moves along a straight line with position \(s(t)=\frac{1}{6}t^3-2t^2+6t\), where \(s\) is measured in meters and \(t\geq0\) is measured in seconds. a) Find the velocity and acceleration at \(t=2\,\text{s}\). Describe the object's motion at that instant. b) Find another time \(t>2\) when the object is momentarily at rest. Find the acceleration at that time.

Hints

- What must velocity equal when an object is momentarily at rest? - When velocity is zero, its sign on either side determines whether the object reverses direction. - Find rest times by solving a quadratic equation for \(v(t)=0\). - Differentiate position twice to obtain acceleration.

Solution

1. Differentiate: \(v(t)=s'(t)=\frac{1}{2}t^2-4t+6\) and \(a(t)=v'(t)=t-4\). 2. At \(t=2\), \(v(2)=0\,\text{m/s}\) and \(a(2)=-2\,\text{m/s}^2\). The object is momentarily at rest and reverses direction because the velocity changes from positive to negative. 3. For another rest time, solve \(v(t)=0\): \(\frac{1}{2}t^2-4t+6=0\). 4. Multiplying by \(2\) and factoring gives \(t^2-8t+12=(t-2)(t-6)=0\). 5. The other time is \(t=6\,\text{s}\), and \(a(6)=2\,\text{m/s}^2\).

Answer

a) \(v(2)=0\,\text{m/s}\) and \(a(2)=-2\,\text{m/s}^2\). The object is momentarily at rest and reverses direction. b) The other rest time is \(t=6\,\text{s}\), when \(a(6)=2\,\text{m/s}^2\).
52642612
An object moves along a line. Its position, in meters, at time \(t\) seconds is \(s(t) = t^3 - 9t^2 + 24t\). a) Find the times when the object is momentarily at rest. b) Find the acceleration at those times. c) Describe how the direction of motion changes at each time.

Hints

- The object is momentarily at rest when its velocity is zero. - Differentiate velocity to obtain acceleration. - Factor the velocity function and examine its sign on either side of each zero. - A sign change in velocity indicates a reversal of direction.

Solution

1. Differentiate position: \(v(t) = s'(t) = 3t^2 - 18t + 24 = 3(t - 2)(t - 4)\). 2. Set \(v(t) = 0\). The object is at rest at \(t = 2\,\text{s}\) and \(t = 4\,\text{s}\). 3. Differentiate velocity: \(a(t) = v'(t) = 6t - 18\). 4. Evaluate: \(a(2) = -6\,\text{m/s}^2\) and \(a(4) = 6\,\text{m/s}^2\). 5. The velocity changes from positive to negative at \(t = 2\), so the object reverses from the positive direction to the negative direction. At \(t = 4\), velocity changes from negative to positive, so the object reverses back to the positive direction.

Answer

a) \(t = 2\,\text{s}\) and \(t = 4\,\text{s}\) b) \(a(2) = -6\,\text{m/s}^2\) and \(a(4) = 6\,\text{m/s}^2\) c) At \(t = 2\), motion changes from the positive direction to the negative direction. At \(t = 4\), it changes from the negative direction to the positive direction.
52643212
A freight train begins moving in a rail yard. During the first \(30\) seconds, its position is modeled by \(s(t)=0.005t^3+0.06t^2\), where \(s\) is measured in meters and \(t\) in seconds. a) Find the train's velocity after \(20\) seconds in both \(\text{m/s}\) and \(\text{km/h}\). b) Find its acceleration at \(t=20\,\text{s}\).

Hints

- Use derivatives to relate position, velocity, and acceleration. - To convert \(\text{m/s}\) to \(\text{km/h}\), multiply by \(3.6\). - The slope of a position-time graph is instantaneous velocity.

Solution

1. Differentiate position: \(v(t)=s'(t)=0.015t^2+0.12t\). 2. Evaluate: \(v(20)=0.015(20)^2+0.12(20)=8.4\,\text{m/s}\). 3. Convert units: \(8.4\cdot3.6=30.24\), so the velocity is \(30.24\,\text{km/h}\). 4. Differentiate again: \(a(t)=s''(t)=0.03t+0.12\). 5. Evaluate: \(a(20)=0.03(20)+0.12=0.72\,\text{m/s}^2\).

Answer

a) \(8.4\,\text{m/s}\), or \(30.24\,\text{km/h}\) b) \(0.72\,\text{m/s}^2\)
52644912
Two vehicles start at the same time. Their positions, in meters, are \(s_1(t)=\frac{1}{6}t^3+2t^2+4t\) and \(s_2(t)=t^2+10t\), where \(t\geq0\) is measured in seconds. Find the instantaneous acceleration of each vehicle at the time when their instantaneous velocities are equal.

Hints

- Differentiate position once for velocity and twice for acceleration. - At the requested time, the two velocity functions have equal values. - Discard a negative time because the model uses \(t\geq0\).

Solution

1. Differentiate to find velocity: \(v_1(t)=\frac{1}{2}t^2+4t+4\) and \(v_2(t)=2t+10\). 2. Set the velocities equal: \(\frac{1}{2}t^2+4t+4=2t+10\). 3. Simplifying gives \(t^2+4t-12=0=(t-2)(t+6)\). Since \(t\geq0\), the relevant time is \(t=2\,\text{s}\). 4. Differentiate velocity: \(a_1(t)=t+4\) and \(a_2(t)=2\). 5. At \(t=2\), \(a_1(2)=6\,\text{m/s}^2\) and \(a_2(2)=2\,\text{m/s}^2\).

Answer

At \(t=2\,\text{s}\), vehicle 1 has acceleration \(6\,\text{m/s}^2\), and vehicle 2 has acceleration \(2\,\text{m/s}^2\).
52645012
For \(t\geq0\), the positions of test objects \(A\) and \(B\), in meters, are modeled by \(s_A(t)=\frac{1}{12}t^3+\frac{1}{2}t^2+2t\) \(s_B(t)=\frac{3}{4}t^2+3t\), where \(t\) is measured in seconds. a) Find the time \(t>0\) when the objects have equal instantaneous acceleration. b) Determine which object has the greater velocity at that time. Justify your answer algebraically.

Hints

- The first derivative of position is velocity, and the second derivative is acceleration. - Express equal acceleration by setting the two acceleration functions equal. - Substitute the time from part a) into both velocity functions.

Solution

1. Differentiate twice. For object \(A\), \(v_A(t)=\frac{1}{4}t^2+t+2\) and \(a_A(t)=\frac{1}{2}t+1\). For object \(B\), \(v_B(t)=\frac{3}{2}t+3\) and \(a_B(t)=\frac{3}{2}\). 2. Set the accelerations equal: \(\frac{1}{2}t+1=\frac{3}{2}\), so \(t=1\,\text{s}\). 3. Evaluate the velocities: \(v_A(1)=3.25\,\text{m/s}\) and \(v_B(1)=4.5\,\text{m/s}\). 4. Since \(4.5>3.25\), object \(B\) has the greater velocity.

Answer

a) \(t=1\,\text{s}\) b) Object \(B\); \(v_B(1)=4.5\,\text{m/s}\), while \(v_A(1)=3.25\,\text{m/s}\).
52755012
A test vehicle moves along a straight track. Its position \(s(t)\), in meters from a fixed origin in the positive direction, is modeled for \(t\ge0\) by \(s(t)=\sqrt{4t^2+900}\), where \(t\) is measured in seconds. a) Find the vehicle's initial position. b) Find its instantaneous velocity \(v(t)=s'(t)\) at \(t=20\,\text{s}\). c) Show that \(s''(t)=\frac{3600}{(4t^2+900)^{\frac{3}{2}}}\). Interpret the sign of \(s''(t)\) in terms of the vehicle's velocity.

Hints

- Use the chain rule for the square-root function. - Position, velocity, and acceleration are connected by successive derivatives. - Differentiate the velocity using the product rule or quotient rule. - A positive second derivative means the first derivative is increasing.

Solution

1. a) \(s(0)=\sqrt{900}=30\), so the initial position is \(30\,\text{m}\) from the origin. 2. b) By the chain rule, \(s'(t)=\frac{4t}{\sqrt{4t^2+900}}\). Therefore, \(s'(20)=\frac{80}{50}=1.6\,\text{m}/\text{s}\). 3. c) Differentiate \(s'(t)=4t(4t^2+900)^{-\frac{1}{2}}\): \(s''(t)=4(4t^2+900)^{-\frac{1}{2}}-16t^2(4t^2+900)^{-\frac{3}{2}}\). 4. Combining terms gives \(s''(t)=\frac{4(4t^2+900)-16t^2}{(4t^2+900)^{\frac{3}{2}}}=\frac{3600}{(4t^2+900)^{\frac{3}{2}}}\). 5. Since \(s''(t)>0\) for \(t\ge0\), the velocity \(s'(t)\) is increasing.

Answer

a) \(30\,\text{m}\) b) \(1.6\,\text{m}/\text{s}\) c) \(s''(t)=\frac{3600}{(4t^2+900)^{\frac{3}{2}}}>0\), so the vehicle's velocity is increasing.
52816212
An object with mass \(0.5\,\text{kg}\) moves along an axis with position \(s(t)=\frac{2}{3}t^3-6t^2+10t\), where \(s\) is measured in meters and \(t\) in seconds. a) Find the net force function \(F(t)\), in newtons. b) At what time \(t>0\) is the net force zero? c) Find the object's velocity at the time found in part b).

Hints

- Use the second derivative of position to find acceleration before applying \(F=ma\). - Zero net force means the force function equals \(0\). - Velocity is the first derivative of position. - A negative velocity indicates motion in the negative direction.

Solution

1. Differentiate position: \(v(t)=s'(t)=2t^2-12t+10\). 2. Differentiate again: \(a(t)=v'(t)=4t-12\). 3. Apply \(F=ma\): \(F(t)=0.5(4t-12)=2t-6\). 4. Set the force equal to zero: \(2t-6=0\), so \(t=3\,\text{s}\). 5. Evaluate the velocity: \(v(3)=2(3)^2-12(3)+10=-8\,\text{m/s}\).

Answer

a) \(F(t)=2t-6\), in newtons b) \(t=3\,\text{s}\) c) \(-8\,\text{m/s}\)
52893912
A test vehicle moves along a straight road. Its velocity, in \(\text{m/s}\), is \(v(t)=0.5t^2+2t\), where \(t\) is measured in seconds. 1) Find the average acceleration on \([2,4]\). 2) Find the instantaneous acceleration at \(t=2\,\text{s}\). 3) Explain the mathematical relationship between acceleration \(a(t)\) and a position function \(s(t)\).

Hints

- Average acceleration is the average rate of change of velocity over an interval. - Distinguish an average rate from an instantaneous rate. - Acceleration measures the rate of change of velocity. - Relate velocity to position, then differentiate once more.

Solution

1. Compute the endpoint velocities: \(v(2)=6\,\text{m/s}\) and \(v(4)=16\,\text{m/s}\). The average acceleration is \(\frac{v(4)-v(2)}{4-2}=\frac{16-6}{2}=5\,\text{m/s}^2\). 2. Differentiate velocity: \(a(t)=v'(t)=t+2\). Thus \(a(2)=4\,\text{m/s}^2\). 3. Velocity is the first derivative of position, \(v(t)=s'(t)\), and acceleration is the derivative of velocity. Therefore, \(a(t)=s''(t)\).

Answer

1) \(5\,\text{m/s}^2\) 2) \(4\,\text{m/s}^2\) 3) \(a(t)=s''(t)\)
52894012
The position of an object is \(s(t)=t^3-6t^2+12t\), where \(s\) is measured in meters and \(t\) in seconds. 1) Find the instantaneous velocity function \(v(t)\). 2) Find the instantaneous acceleration function \(a(t)\). 3) Find the time when the acceleration is \(0\,\text{m/s}^2\). 4) Find the average acceleration on \([1,3]\) and compare it with the instantaneous acceleration at \(t=2\,\text{s}\).

Hints

- Position, velocity, and acceleration are connected by successive derivatives. - Zero acceleration means the acceleration function equals \(0\). - Use a difference quotient of velocity to find average acceleration. - Consider why a linear acceleration function has its midpoint value equal to its average over a symmetric interval.

Solution

1. Differentiate position: \(v(t)=s'(t)=3t^2-12t+12\). 2. Differentiate velocity: \(a(t)=v'(t)=6t-12\). 3. Solve \(6t-12=0\), giving \(t=2\,\text{s}\). 4. The endpoint velocities are \(v(1)=3\,\text{m/s}\) and \(v(3)=3\,\text{m/s}\). Thus the average acceleration is \(\frac{v(3)-v(1)}{3-1}=0\,\text{m/s}^2\). Also, \(a(2)=0\,\text{m/s}^2\), so the two values are equal.

Answer

1) \(v(t)=3t^2-12t+12\) 2) \(a(t)=6t-12\) 3) \(t=2\,\text{s}\) 4) The average acceleration is \(0\,\text{m/s}^2\), equal to \(a(2)\).
52923812
An autonomous test vehicle moves along a straight track. For the first \(10\) seconds, its position, in feet, is modeled by \(s(t) = -0.5t^3 + 9t^2\), where \(t\) is measured in seconds. a) Find the velocity function \(v(t)\) and the vehicle's velocity at \(t = 4\,\text{s}\). b) When does the vehicle reach its maximum velocity? c) Find the maximum velocity in feet per second and miles per hour.

Hints

- Velocity is the derivative of position. - To maximize velocity, examine the derivative of the velocity function. - Confirm that the critical time lies within the first \(10\) seconds. - Use \(5280\,\text{ft} = 1\,\text{mi}\) and \(3600\,\text{s} = 1\,\text{h}\).

Solution

1. Differentiate position: \(v(t) = s'(t) = -1.5t^2 + 18t\). 2. Evaluate \(v(4) = -1.5(4^2) + 18(4) = 48\,\text{ft/s}\). 3. Acceleration is \(a(t) = v'(t) = -3t + 18\). Set \(a(t) = 0\): \(-3t + 18 = 0\), so \(t = 6\,\text{s}\). Since \(v''(t) = -3 < 0\), the velocity is maximized then. 4. Evaluate \(v(6) = -1.5(6^2) + 18(6) = 54\,\text{ft/s}\). 5. Convert units: \(54 \cdot \frac{3600}{5280} \approx 36.82\,\text{mph}\).

Answer

a) \(v(t) = -1.5t^2 + 18t\); \(v(4) = 48\,\text{ft/s}\) b) \(t = 6\,\text{s}\) c) \(54\,\text{ft/s} \approx 36.82\,\text{mph}\)
53273812
During a prototype vehicle test on a straight track, the velocity for the first \(30\) seconds is modeled by \(v(t)=0.01t^3-0.6t^2+9t\), for \(0\le t\le30\), where \(v\) is measured in meters per second and \(t\) in seconds. a) Find the maximum velocity during the test and the time when it occurs. b) Find when the velocity is decreasing most rapidly, and give the corresponding acceleration. c) Find the total distance traveled during the first \(30\) seconds.

Hints

- Find critical times of the velocity and compare them with the interval endpoints. - Acceleration is the derivative of velocity. - The most rapid decrease occurs where acceleration is smallest. - Total distance is the integral of speed; first check whether velocity changes sign. - Apply the power rule carefully when integrating.

Solution

1. Differentiate: \(v'(t)=0.03t^2-1.2t+9\). The critical times are \(t=10\) and \(t=30\). Compare the velocity at the endpoints and the interior critical time: \(v(0)=0\), \(v(10)=40\), and \(v(30)=0\). Therefore, the maximum velocity is \(40\,\text{m/s}\) at \(t=10\,\text{s}\). 2. Acceleration is \(a(t)=v'(t)\). Its derivative is \(a'(t)=v''(t)=0.06t-1.2\), which is zero at \(t=20\). Since acceleration is an upward-opening quadratic, this is its minimum. The acceleration is \(a(20)=-3\,\text{m/s}^2\), so the velocity is decreasing most rapidly then; the deceleration magnitude is \(3\,\text{m/s}^2\). 3. The velocity factors as \(v(t)=0.01t(t-30)^2\), so it is nonnegative throughout the interval. Thus total distance equals \(\int_0^{30}v(t)\,dt\). An antiderivative is \(0.0025t^4-0.2t^3+4.5t^2\). Evaluation gives \(675\,\text{m}\).

Answer

a) \(40\,\text{m/s}\) at \(t=10\,\text{s}\). b) At \(t=20\,\text{s}\), the acceleration is \(-3\,\text{m/s}^2\), corresponding to a deceleration magnitude of \(3\,\text{m/s}^2\). c) \(675\,\text{m}\).
53418012
A bicyclist starts from rest, speeds up, reaches a maximum speed, and then slows down. The function \(v\) gives the bicyclist’s speed as a function of time \(t\), measured in seconds. a) When does the bicyclist reach maximum speed, and what is that speed? b) Find the acceleration, the instantaneous rate of change of speed, at \(t=10\,\text{s}\). c) When is the bicyclist’s acceleration greatest? d) Compare the instantaneous rates of change of speed at \(t=5\,\text{s}\) and \(t=25\,\text{s}\). What do they indicate about the motion?
Figure for problem 534180

Hints

- Acceleration is the derivative of speed with respect to time. - Maximum speed occurs at the highest point of the speed graph. - Maximum acceleration occurs where the graph rises most steeply. - The sign of the slope indicates whether speed is increasing or decreasing.

Solution

1. The highest point on the speed graph occurs at \(t=20\,\text{s}\), where \(v=10\,\text{m/s}\). 2. Acceleration is the tangent slope of the speed graph. At \(t=10\,\text{s}\), the slope is approximately \(0.75\,\text{m/s}^2\). 3. Acceleration is greatest where the speed graph rises most steeply. This occurs at the inflection point \(t=10\,\text{s}\). 4. At \(t=5\,\text{s}\), the slope is approximately \(0.56\,\text{m/s}^2\), so the bicyclist is speeding up. At \(t=25\,\text{s}\), the slope is approximately \(-0.94\,\text{m/s}^2\), so the bicyclist is slowing down.

Answer

a) At \(t=20\,\text{s}\), with a maximum speed of \(10\,\text{m/s}\) b) Approximately \(0.75\,\text{m/s}^2\) c) At \(t=10\,\text{s}\) d) At \(5\,\text{s}\), the rate is positive and the bicyclist is speeding up. At \(25\,\text{s}\), the rate is negative and the bicyclist is slowing down.
53422212
An autonomous vehicle travels on a test track. The function \(v\) gives its speed in meters per second as a function of time \(t\), measured in seconds. The acceleration \(a\) is the instantaneous rate of change of speed. Which graph, a, b, or c, represents the acceleration over time? Justify your choice by comparing the graphs.
Figure for problem 534222

Hints

- Acceleration is the derivative of speed. - Find times when the speed graph has horizontal tangent lines. - Determine whether speed is increasing or decreasing at the beginning. - Match both the zeros and the sign pattern.

Solution

1. Acceleration is the derivative of the speed function: \(a(t)=v^{\prime}(t)\). 2. The speed graph has local maxima at \(t=2\) and \(t=10\) and a local minimum at \(t=6\). Therefore, the acceleration graph must have zeros at \(t=2\), \(t=6\), and \(t=10\). 3. Graph c has zeros at \(t=0\), \(t=4\), \(t=8\), and \(t=12\), so it does not fit. Graphs a and b have the required zeros. 4. At \(t=0\), the speed is increasing, so \(a(0)>0\). Graph a starts with a positive value, while graph b starts with a negative value. Therefore, graph a represents the acceleration.

Answer

Graph a represents the acceleration.
53443912
The graphs of functions \(f\), \(g\), and \(h\) describe an object moving for \(8\,\text{s}\). One graph represents position \(s(t)\) in meters, one represents velocity \(v(t)\) in meters per second, and one represents acceleration \(a(t)\) in meters per second squared. Match each graph to the correct quantity. Justify your answer using relationships such as extrema and zeros.
Figure for problem 534439

Hints

- Velocity is the derivative of position, and acceleration is the derivative of velocity. - At a local maximum or minimum of a function, its derivative is zero. - Compare where one graph increases or decreases with the sign of a possible derivative graph. - Differentiating a cubic produces a quadratic, and differentiating a quadratic produces a linear function.

Solution

1. For straight-line motion, \(s^{\prime}(t)=v(t)\) and \(v^{\prime}(t)=a(t)\). 2. Graph \(h\) has a local maximum at \(t=2\) and a local minimum at \(t=6\). Graph \(g\) is zero at those same times, and its sign agrees with when \(h\) increases or decreases. Therefore, \(g=h^{\prime}\), so \(h\) is position and \(g\) is velocity. 3. Graph \(g\) has a local minimum at \(t=4\). Graph \(f\) is zero at \(t=4\), and its sign agrees with when \(g\) decreases or increases. Therefore, \(f=g^{\prime}\), so \(f\) is acceleration.

Answer

Graph \(h\): position \(s(t)\) Graph \(g\): velocity \(v(t)\) Graph \(f\): acceleration \(a(t)\)
53477412
During the first \(12\) seconds of a test drive, an electric-car prototype's velocity is modeled by \(v(t)=-0.05t^3+0.6t^2\), where \(t\) is measured in seconds and \(v(t)\) is measured in meters per second. a) Find when the vehicle reaches its maximum velocity and state that velocity. b) Acceleration is the instantaneous rate of change of velocity. Find when acceleration is greatest and state its value. c) Find the total distance traveled during the \(12\)-second interval. d) Find the average velocity over the interval.
Figure for problem 534774

Hints

- Recall the derivative and integral relationships among position, velocity, and acceleration. - Use the first derivative of velocity to locate its maximum. - Analyze the acceleration function to find its maximum. - Average velocity can be found from total displacement divided by total time.

Solution

1. \(v'(t)=-0.15t^2+1.2t=-0.15t(t-8)\). Comparing critical points and endpoints on \([0,12]\) shows that the maximum occurs at \(t=8\), where \(v(8)=12.8\,\text{m/s}\). 2. The acceleration is \(a(t)=-0.15t^2+1.2t\). Since \(a\) is a downward-opening parabola, its maximum occurs at \(t=4\), where \(a(4)=2.4\,\text{m/s}^2\). 3. Velocity is nonnegative on \([0,12]\), so total distance is \(\int_0^{12}v(t)\,\text{d}t=[-0.0125t^4+0.2t^3]_0^{12}=86.4\,\text{m}\). 4. The average velocity is \(\frac{1}{12}\int_0^{12}v(t)\,\text{d}t=\frac{86.4}{12}=7.2\,\text{m/s}\).

Answer

a) At \(t=8\,\text{s}\); \(12.8\,\text{m/s}\) b) At \(t=4\,\text{s}\); \(2.4\,\text{m/s}^2\) c) \(86.4\,\text{m}\) d) \(7.2\,\text{m/s}\)
53490212
The graph shows the altitude \(h\), in meters, of a research drone as a function of time \(t\), in seconds, for \(0\leq t\leq 30\). a) Use the graph to estimate the drone's altitude after \(10\,\text{s}\). Also estimate the time interval during which the drone is at least \(40\,\text{m}\) above the ground. b) Use the graph to estimate the drone's instantaneous vertical velocity at \(t=10\,\text{s}\). c) Explain the meaning of \(h(t+10)=h(t)+15\) in this context. Use the graph to decide whether the equation is true when \(t=10\), and justify your answer.
Figure for problem 534902

Hints

- At \(t=10\), move vertically from the time axis to the graph, then read the corresponding altitude. - Find where the graph crosses the horizontal level \(h=40\). - Approximate the slope of a tangent line at \(t=10\). - Interpret each side of the equation as an altitude at a particular time.

Solution

1. At \(t=10\), the graph passes through approximately \((10,45)\), so the drone's altitude is about \(45\,\text{m}\). 2. The graph intersects \(h=40\) at approximately \(t=8.1\) and \(t=29.0\). Therefore, the drone is at least \(40\,\text{m}\) high for approximately \(8.1\leq t\leq 29.0\). 3. Estimate the slope of the tangent line at \((10,45)\). A reasonable slope estimate is \(h^{\prime}(10)\approx 2.5\,\text{m/s}\), so the drone is rising at about \(2.5\,\text{m/s}\). 4. The equation means that, starting at time \(t\), the drone is exactly \(15\,\text{m}\) higher \(10\) seconds later. 5. For \(t=10\), the graph gives \(h(10)\approx 45\) and \(h(20)\approx 60\). Since \(60=45+15\), the equation is true when \(t=10\).

Answer

a) The altitude after \(10\,\text{s}\) is about \(45\,\text{m}\). The drone is at least \(40\,\text{m}\) high for approximately \(8.1\leq t\leq 29.0\). b) The instantaneous vertical velocity is approximately \(2.5\,\text{m/s}\) upward. c) The equation means that the drone is \(15\,\text{m}\) higher \(10\) seconds after time \(t\). It is true for \(t=10\) because \(h(20)\approx 60\) and \(h(10)\approx 45\).
52662912
During a \(60\)-second test, a prototype vehicle's velocity is modeled by \(v(t)=0.8t^2e^{-0.1t}\), where \(t\) is measured in seconds and \(v(t)\) is measured in meters per second. 1. Find the maximum acceleration the vehicle reaches during the test. 2. Find the distance the vehicle travels during the first \(20\) seconds. You may use the antiderivative \(V(t)=(-8t^2-160t-1600)e^{-0.1t}\).

Hints

- Recall the relationships among position, velocity, and acceleration. - What condition must hold at an interior point where acceleration has a maximum? - How do you evaluate a definite integral when an antiderivative is given? - Include appropriate units in each answer.

Solution

1. The acceleration is \(a(t)=v'(t)=(1.6t-0.08t^2)e^{-0.1t}\). 2. Critical points of \(a\) satisfy \(a'(t)=v''(t)=(0.008t^2-0.32t+1.6)e^{-0.1t}=0\). This gives \(t=20\pm10\sqrt{2}\). Comparing \(a(t)\) at these times and at the endpoints \(t=0\) and \(t=60\) shows that the maximum occurs at \(t=20-10\sqrt{2}\approx5.86\), where \(a(t)\approx3.69\,\text{m/s}^2\). 3. The distance traveled during the first \(20\) seconds is \(\int_0^{20}v(t)\,\text{d}t=V(20)-V(0)=-8000e^{-2}+1600\approx517.32\,\text{m}\).

Answer

1. Approximately \(3.69\,\text{m/s}^2\) 2. Approximately \(517.32\,\text{m}\)

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