Systems and Center of Mass AP Physics 1 MCQ

Systems and Center of Mass AP Physics 1 MCQ – Exam Style Questions etc.

Systems and Center of Mass AP Physics 1 MCQ

Unit: 2. Force and Translational Dynamics

Weightage : 10-15%

AP Physics 1 Exam Style Questions – All Topics

Question

For which of the following motions of an object must the acceleration always be zero?
(A) Any motion in a straight line
(B) Simple harmonic motion
(C) Any motion at constant speed
(D) Any single object in motion with constant momentum

▶️Answer/Explanation

Ans:D

Solution: Motion at constant speed includes, for example, motion in a circle, in which the direction of the velocity changes and thus acceleration exists. Constant momentum for a single object means , that the velocity doesn’t change.

Question

After the Throw After the Catch

(A) Boat moves forward forward Boat moves

(B) Boat moves forward Boat moves backward

(C) Boat moves forward Boat does not move

(D) Boat moves backward Boat does not move

(E) Boat moves backward Boat moves forward

Answer/Explanation

Ans:C

Question

The 1.0-m-long nonuniform plank, shown above, has weight 1000 N. It is to be supported by two rods, A and B, as shown above. The center of mass of the plank is 30 cm from the right edge. Each support bears half the weight of the plank. If support B is 10 cm from the right-hand edge, how far from the left-hand edge should support A be?
(A) 0 cm
(B) 10 cm
(C) 30 cm
(D) 50 cm
(E) 70 cm

Answer/Explanation

Ans:

D—Choose any point at all as the fulcrum; say, the center of mass. Rod B supports 500 N, and is located 20 cm from the fulcrum, producing a total counterclockwise torque of 10,000 N·cm. Rod A also supports 500 N; call its distance from the fulcrum “x”. So 10,000 = 500x, and x = 20 cm. This means Rod A is located 20 cm left of the center of mass, or 50 cm from the left edge.

Question

Thin rods A, B, and C all have uniform mass distibution and are all of equal length L. The center of mass of the three-rod system is shown above. If both A and C have a mass M, the ratio of the mass of bar B to the mass of bar A, \(\frac{m_B}{m_A}\) , is
A \(1/1\)

B \(2/1\)

C \(4/1\)

D \(6/1\)

E \(8/1\)

Answer/Explanation

Ans:D

Using the center of mass equation in the horizontal direction and setting the position of bar B as zero:

\(x_{cm}=\frac{(m_B)(0)+(M)(L/2)+(M)(L/2)}{(m_B+M+M)}=\frac{L}{8}\therefore \frac{ML}{(m_B+2M)}=\frac{L}{8}\therefore 8M=m_B+2M\therefore m_B=6M\therefore m_B/m_A=6/1\).

Question

A 5-kilogram sphere is connected to a 10-kilogram sphere by a rigid rod of negligible mass, as shown above.

Which of the five lettered points represents the center of mass of the sphere-rod combination?

A A

B B

C C

D D

E E

Answer/Explanation

Ans:B

Question

As shown above, two students sit at opposite ends of a boat that is initially at rest. The student in the front throws a heavy ball to the student in the back. What is the motion of the boat at the time immediately after the ball is thrown and, later, after the ball is caught? (Assume that air and water friction are negligible.) Immediately
After the Throw After the Catch
(A) Boat moves forward Boat moves forward
(B) Boat moves forward Boat moves backward
(C) Boat moves forward Boat does not move
(D) Boat moves backward Boat does not move
(E) Boat moves backward Boat moves forward

Answer/Explanation

Ans:C

Solution: As the system started at rest and the only forces on the system are between parts of the system, the center of mass must remain stationary. While the ball is in motion toward the back of the boat, the boat will move slightly forward. Once the ball is stationary, so is the boat.

Systems and Center of Mass AP Physics 1 MCQ