Newton's First Law Examples in Physics

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Newton's First Law.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Physics.

Concept Recap

An object at rest stays at rest, and an object in motion continues moving at constant velocity in a straight line, unless acted upon by.

Things keep doing what they're doing unless something pushes or pulls them.

Read the full concept explanation →

How to Use These Examples

  • Read the first worked example with the solution open so the structure is clear.
  • Try the practice problems before revealing each solution.
  • Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Motion doesn't require a force—changing motion requires a force.

Common stuck point: Objects on Earth slow down due to friction, not because motion naturally stops.

Sense of Study hint: When applying Newton's first law, first determine whether the net force on the object is zero. If the net force is zero, the object either remains at rest or continues at constant velocity. If the object is accelerating or decelerating, a net force must be acting — identify it.

Worked Examples

Example 1

easy
A book rests on a table. The weight of the book is 15 \text{ N} downward. What is the normal force exerted by the table, and what is the net force on the book?

Solution

  1. 1
    The book is at rest, so by Newton's first law, the net force must be zero.
  2. 2
    The normal force must balance the weight: N = W = 15 \text{ N upward}
  3. 3
    Net force: F_{\text{net}} = N - W = 15 - 15 = 0 \text{ N}

Answer

N = 15 \text{ N upward}, \quad F_{\text{net}} = 0 \text{ N}
Newton's first law states that an object at rest stays at rest unless acted on by a net external force. A book on a table is in equilibrium because the normal force balances gravity.

Example 2

medium
A hockey puck slides across a frictionless ice surface at 5 \text{ m/s}. What is the puck's velocity after 10 \text{ seconds}?

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

easy
A car travels at a constant velocity of 20 \text{ m/s} on a straight road. What is the net force acting on the car?

Example 2

medium
A bus travels at 12 \text{ m/s} and brakes suddenly. A backpack on the floor slides forward. According to Newton's first law, what was the backpack's velocity immediately before friction acted on it?
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Background Knowledge

These ideas may be useful before you work through the harder examples.

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