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AP Physics C Mechanics Reference Frames and Relative Motion MCQ

Reference Frames and Relative Motion AP  Physics C Mechanics MCQ – Exam Style Questions etc.

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Reference Frames and Relative Motion AP  Physics C Mechanics MCQ

Unit: 1. Kinematics 

Weightage : 10-15%

AP Physics C Mechanics Exam Style Questions – All Topics

Question

 

Student 1 is standing on a cart holding a small stone, while student 2 is standing at rest on the ground, as shown in the figure above. The cart is moving at a constant speed v in the +x-direction, as indicated by the coordinate system shown. Student 1 drops the stone precisely when passing student 2. Which of the following best represents the path of the falling stone relative to student 1 and the path of the falling stone relative to student 2 ?

Answer/Explanation

Ans:C

Question

 Two stones, represented in the figure above, are thrown from the same height with the same initial speed. Stone A is thrown vertically downward and stone B is thrown horizontally. If the stones are thrown at the same time and air resistance is negligible, which of the following is true?

(A) The two stones will reach the ground at the same time with the same speed.
(B) The two stones will reach the ground at the same time but with different speeds.
(C) Stone A will reach the ground first, but stone B will have the greater speed just before hitting the ground.
(D) Stone A will reach the ground first, but the two stones will have the same speed just before they hit the ground.
(E) Stone A will reach the ground first, and will have the greater speed just before hitting the ground.

Answer/Explanation

Ans:D 

Both stone A and stone B initially have the same potential energy \(U_i \)and kinetic energy . \(K_i \)Both stones therefore have the same\( K _f \)at ground level, when 0.\( U_f \)= Substituting into conservation of energy to solve for the speeds of the stones when

                                  \( U_i+K_i=U_f+K_f\)
they reach the ground yields  \(mgh+\frac{1}{2}mv^2_0=0+\frac{1}{2}mv^2_{f}\);thus , \(v_f=\sqrt{v^2_{0}+2gh}\)

because both stones have the same initial speed, they will both have the same speed when they reach the ground.

Question

A cart is sliding down a low friction incline. A device on the cart launches a ball, forcing the ball perpendicular to the incline, as shown above. Air resistance is negligible. Where will the ball land relative to the cart, and why?
(A) The ball will land in front of the cart, because the ball’s acceleration component parallel to the plane is greater than the cart’s acceleration component parallel to the plane.
(B) The ball will land in front of the cart, because the ball has a greater magnitude of acceleration than the cart.
(C) The ball will land in the cart, because both the ball and the cart have the same component of acceleration parallel to the plane.
(D) The ball will land in the cart, because both the ball and the cart have the same magnitude of acceleration.
(E) The ball will land behind the cart, because the ball slows down in the horizontal direction after it leaves the cart.

Answer/Explanation

Ans:C

The cart’s acceleration is g sin \(\theta\), down the plane, the ball’s acceleration is g, straight down. (So the magnitudes of acceleration are different and choice D is wrong.) The component of the ball’s acceleration along an axis parallel to the plane is also g sin \(\theta\), equal to the ball’s acceleration component.

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