Worked checkpoints

These quick prompts are designed for class discussion or self-check before moving to momentum and energy.

A 4 kg trolley is pulled right with 18 N while friction is 10 N left. What is its acceleration?

Net force \(= 18 - 10 = 8\text{ N}\) to the right, so \(a = F_{\text{net}}/m = 8/4 = 2\text{ m s}^{-2}\) to the right.

Why can a book stay at rest on a table even though gravity is acting?

Gravity pulls down, but the table exerts an equal normal force upward. Net force is zero, so acceleration is zero.

A rocket pushes exhaust gas backward. Which Newton's law explains forward thrust?

Newton's third law. The rocket pushes gas backward; the gas pushes the rocket forward with an equal and opposite force.

In the motion lab, what happens to the velocity-time line when acceleration is set to zero?

The line becomes horizontal, showing constant velocity. Position then changes linearly with time.