# Find the magnitude of the momentum change of the ball?

• paulimerci
In summary, the conversation discusses a 2D momentum problem with an elastic collision. The vector diagrams show that the momentum does not change in the y direction, but does change in the x direction. The magnitude of the momentum change can be calculated using the equation Delta p = -2mvcos theta. The conversation ends with a question about whether the calculation was done correctly.
paulimerci
Homework Statement
A tennis ball of mass m rebounds from a racquet with the same speed v as it had
initially as shown. The magnitude of the momentum change of the ball is
(A) 0 (B) 2mv (C) 2mv sin theta (D) 2mv cos theta
Relevant Equations
Conservation of momentum
I understand that it is a 2D momentum problem with an elastic collision;
Looking at the vector diagrams below, I notice that the velocity vectors initial and final in the y direction are in the same direction, indicating that momentum does not change, whereas the velocity vectors initial and final in the x direction are opposite each other, indicating that momentum does change.
Therfore,
$$\Delta p = p_f - p_i$$
$$= -mvcos\theta -mvcos\theta$$
$$\Delta p = -2mvcos\theta$$

Have I done it right?

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paulimerci said:
Homework Statement:: A tennis ball of mass m rebounds from a racquet with the same speed v as it had
initially as shown. The magnitude of the momentum change of the ball is
(A) 0 (B) 2mv (C) 2mv sin theta (D) 2mv cos theta
Relevant Equations:: Conservation of momentum

I understand that it is a 2D momentum problem with an elastic collision;
Looking at the vector diagrams below, I notice that the velocity vectors initial and final in the y direction are in the same direction, indicating that momentum does not change, whereas the velocity vectors initial and final in the x direction are opposite each other, indicating that momentum does change.
Therfore,
$$\Delta p = p_f - p_i$$
$$= -mvcos\theta -mvcos\theta$$
$$\Delta p = -2mvcos\theta$$

Have I done it right?
Looks good.

TSny said:
Looks good.
Thank you!

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