IB Physics HL 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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Question 1

Multiple choiceMedium1 mark

A 3.0 kg box is stationary on a horizontal platform. A person exerts a 10 N downward force on the box. Take $g=10\ \text{N kg}^{-1}$ What is the magnitude of the vertical force that the box exerts on the platform?

Show answer and explanation

Answer: C. Translational equilibrium of the box gives $N - mg - 10 = 0$ so the platform’s force on the box is $N = (3.0)(10)+10 = 40\ \text{N}$ By Newton’s third law, the box exerts an equal-magnitude force on the platform.

A: Treats the third-law forces as cancelling, although they act on different objects, giving $0\ \text{N}$
B: Balances only the box’s weight, omitting the person’s downward force, giving $30\ \text{N}$
D: Counts the person’s downward force twice, giving $30+2(10)=50\ \text{N}$
Remember: sum forces on one object for equilibrium; third-law pairs act on different objects.

Question 2

Paper 2Medium4 marks

A block is attached to a fixed support by a stretched horizontal spring and rests on a rough horizontal table. It remains at rest, and no other horizontal forces act on it. Figure 1 gives the block’s mass and the spring’s constant and extension. Use $g=9.81\ \mathrm{m\,s^{-2}}$

(a)

Calculate the magnitude of the force exerted by the spring on the block.

[1]

(b)

Determine the minimum coefficient of static friction between the block and the table for the block to remain at rest.

[3]

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