Newton's Laws

Overview

Newton’s laws connect forces to motion. The main step in most problems is to identify the resultant force first, then link that resultant force to acceleration or change in momentum.

What You Need to Know

• Mass is the property of an object that resists a change in motion.
• Use W = mg to connect weight, mass, and gravitational field strength.
• F = ma uses the resultant force, not just one force from the diagram.
• Acceleration is in the same direction as the resultant force.
• Newton’s laws describe inertia, force and acceleration, and action-reaction pairs.
• Use force as a rate-of-change idea when motion is better described through momentum.

How to Work Through It

1. Start by drawing free-body diagrams and identifying all forces on one object.
2. Resolve forces where needed and calculate the resultant force.
3. Use F = ma to find acceleration, force, or mass.
4. Practise explaining action-reaction pairs without placing both forces on the same object.

Check Your Understanding

• Why is mass not the same as weight?
• Which force belongs in F = ma?
• What does zero resultant force tell you about acceleration?
• Why do action-reaction forces not cancel each other on the same object?

Common Mistakes

• Using a single applied force in F = ma without subtracting opposing forces.
• Saying an object at constant velocity has no forces acting on it.
• Drawing action-reaction forces on the same free-body diagram.
• Forgetting that weight depends on gravitational field strength.

Next Steps

• Practise force diagrams before starting calculations.
• Use these laws again in impulse and momentum questions.