AP Physics C: Mechanics : Momentum and Impulse

Study concepts, example questions & explanations for AP Physics C: Mechanics

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Example Questions

Example Question #143 : Mechanics Exam

A baseball player hits a \(\displaystyle \small 150g\) baseball initially moving at \(\displaystyle \small 35\frac{m}{s}\), returning it at a speed of \(\displaystyle \small 45\frac{m}{s}\) along the same path. If the ball was in contact with the bat for \(\displaystyle \small 1ms\), what magnitude of force did the ball experience during the moment of contact?

Possible Answers:

\(\displaystyle 42kN\)

\(\displaystyle 12kN\)

\(\displaystyle 530N\)

\(\displaystyle 1.5kN\)

\(\displaystyle 80kN\)

Correct answer:

\(\displaystyle 12kN\)

Explanation:

Relevant equations:

\(\displaystyle J = m\Delta v\)

\(\displaystyle J = F_{avg}\Delta t\)

Evaluate the impulse based on the mass and change of velocity.

\(\displaystyle J = m\Delta v = (0.150kg)(80\frac{m}{s})=12 \frac{kg*m}{s}\)

Use the total impulse and time in the second equation.

\(\displaystyle m\Delta v = F_{avg}\Delta t\)

\(\displaystyle 12\frac{kg*m}{s}=F_{avg}(0.001s)\)

Solve for the average force.

\(\displaystyle F_{avg}=\frac{12\frac{kg*m}{s}}{0.001s}=12,000N = 12kN\)

Example Question #11 : Momentum

Which of the following could be used as units of impulse?

Possible Answers:

\(\displaystyle \frac{kg*m^2}{s}\)

\(\displaystyle \frac{kg*m}{s^3}\)

\(\displaystyle \frac{kg*m^2}{s^2}\)

\(\displaystyle \frac{kg*m}{s}\)

\(\displaystyle \frac{kg*m}{s^2}\)

Correct answer:

\(\displaystyle \frac{kg*m}{s}\)

Explanation:

Relevant equations:

\(\displaystyle J = \Delta p = m\Delta v\)

\(\displaystyle J = F \Delta t\)

Impulse is defined as change in momentum, so has the same units as momentum. These units can easily be found using the given equations.

\(\displaystyle J=m\Delta v=(kg)(\frac{m}{s})=\frac{kg*m}{s}\)

\(\displaystyle J=F\Delta t=(N)(s)=N*s\)

Example Question #142 : Ap Physics C

A pitcher throws a 0.15kg baseball at \(\displaystyle 40\frac{m}{s}\) towards the batter and the batter hits the ball with his bat. The ball leaves the bat in the opposite direction at a speed of \(\displaystyle 45\frac{m}{s}\). Calculate the impulse experienced by the baseball.

Possible Answers:

\(\displaystyle -17.1N\cdot s\)

\(\displaystyle -6.25N\cdot s\)

\(\displaystyle -3.5N\cdot s\)

\(\displaystyle 0N\cdot s\)

\(\displaystyle -12.5N\cdot s\)

Correct answer:

\(\displaystyle -12.5N\cdot s\)

Explanation:

To calculate the impulse, know that it is equal to the change in momentum.

\(\displaystyle \textbf{J}=\Delta\textbf{p}=\textbf{p}_f-\textbf{p}_i\)

Write the impulse equation in terms of mass and velocity.

\(\displaystyle \textbf{J}=m\textbf{v}_f-m\textbf{v}_i\)

In our case, the initial velocity of the baseball is \(\displaystyle \textbf{v}_i=40\ \text{m/s}\) and its final velocity is \(\displaystyle \textbf{v}_f=-45\ \text{m/s}\), where the negative sign indicates that the ball is traveling in the opposite direction. Also, m = 0.15kg. The impulse on the ball can be calculated below.

\(\displaystyle \textbf{J}=(0.15\ \text{kg})(-45\ \text{m/s})-(0.15\ \text{kg})(40\ \text{m/s})=-12.75{N\cdot s}\)

The negative sign tells that the force on the baseball is opposed to the original momentum.

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