IB Physics SL A.2 Forces and momentum questions

Exam-style multiple choice and written questions with full markschemes. Check multiple choice answers straight away, or type, photograph or draw a written answer and get it marked.

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Multiple choice Written

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Question 1

Paper 2Medium6 marks

A trolley of mass $24\ \mathrm{kg}$ is pulled horizontally along a level track by a rope. The rope exerts a forward force of $48\ \mathrm{N}$ while the track exerts a frictional force of $12\ \mathrm{N}$ backwards. These forces remain constant for $4.0\ \mathrm{s}$ At the start of this interval, the trolley is moving forwards at $1.5\ \mathrm{m\,s^{-1}}$ Ignore any other horizontal forces.

The rope force is then reduced, and the trolley continues at constant velocity. The frictional force remains $12\ \mathrm{N}$

(a)

Calculate the trolley’s acceleration during the $4.0\ \mathrm{s}$ interval.

[2]

(b)

Determine the trolley’s speed at the end of the $4.0\ \mathrm{s}$ interval.

[1]

(c)

Using Newton’s first law, explain why the horizontal resultant force on the trolley is zero when it moves at constant velocity.

[1]

(d)

State the Newton’s third-law force pair corresponding to the frictional force exerted by the track on the trolley.

[2]

Question 2

Multiple choiceHard1 mark

Blocks A, B and C, of masses 2.0 kg, 3.0 kg and 5.0 kg respectively, are in contact in that order on a horizontal track. A 50 N horizontal force pushes B to the right. Friction forces on A, B and C are 2.0 N, 5.0 N and 3.0 N respectively, all to the left. The blocks move together. What is the force exerted by C on B?

Show answer and explanation

Answer: B. For all three blocks, the resultant force is $50-2.0-5.0-3.0=40\ \text{N}$ so their acceleration is $40/(2.0+3.0+5.0)=4.0\ \text{m s}^{-2}$ to the right. For C, the contact force from B satisfies $F-3.0=5.0(4.0)$ giving $F=23\ \text{N}$ to the right. The force exerted by C on B is equal in magnitude and opposite in direction: $23\ \text{N}$ to the left.
A: uses $m_Ca=20\ \text{N}$ as the contact force, omitting friction on C.
C: gives the force on C by B, not the equal-and-opposite force on B.
D: treats the resultant force on all three blocks as the force at the B–C contact.
Remember: interaction forces act on different bodies and are equal in magnitude and opposite in direction.

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