Newton’s 3 Laws of Motion Explained: Real-Life Examples, F = ma and Practice Quiz

Quick answer: Newton’s three laws of motion explain how forces affect motion. The first law says an object maintains its state of rest or constant straight-line velocity unless acted on by a net external force. The second law relates net force, mass and acceleration: F_net = m × a for constant mass. The third law says interacting objects exert forces on each other that are equal in magnitude and opposite in direction.

This physics lesson for U.S. middle-school and beginning high-school students provides everyday examples, two worked calculations, a safe demonstration, a misconception guide, and an eight-question practice quiz. It follows the explanations in NASA Glenn Research Center’s Newton’s Laws of Motion educational resource.

What students should know before starting

Motion means change in position over time. Speed describes how fast something moves; velocity also includes direction. Acceleration is a change in velocity per unit time, whether the object speeds up, slows down, or turns. A force is a push or pull, measured in newtons (N). The net force is the vector sum of all forces acting on the same object.

Newton’s first law: inertia

Definition: An object at rest stays at rest, and a moving object continues with constant velocity in a straight line, unless a nonzero net external force acts on it. Inertia is an object’s resistance to changes in velocity. More mass generally means greater inertia.

Real-life example 1: a book on a desk

A closed textbook remains at rest on a level desk because the downward pull of gravity and the upward normal force from the desk balance. That does not mean no forces act on the book; rather, the net force is zero.

Real-life example 2: a passenger wearing a seat belt

When a car slows suddenly, a passenger tends to keep moving forward at the vehicle’s earlier velocity. A correctly worn seat belt provides a force that slows the passenger with the vehicle. This is why “inertia is a force that pushes you forward” is an incorrect explanation: the passenger’s motion continues because of inertia, while the belt supplies a real force to change it.

Does a moving object always need a force to keep going?

No. If an object already moves at constant velocity, it does not need a net force to maintain that velocity. On Earth, friction and air resistance commonly slow moving objects, which can make the opposite seem true. In nearly frictionless conditions, motion continues much longer without an additional push.

Newton’s second law: net force, mass and acceleration

For a constant-mass object, the law can be written F_net = m × a. This gives three useful forms:

• Net force: F_net = m × a.
• Acceleration: a = F_net / m, for m ≠ 0.
• Mass: m = F_net / a, for a ≠ 0.

Use kilograms (kg) for mass, meters per second squared (m/s²) for acceleration, and newtons (N) for force. One newton equals one kg·m/s². Net means all relevant forces must be combined with their directions—not merely the largest force.

Worked example 1: find acceleration

Problem: A 3 kg classroom cart experiences a 12 N net horizontal force. What is its horizontal acceleration? Assume constant mass.

1. Write the formula: a = F_net / m.
2. Substitute: a = 12 N / 3 kg.
3. Answer: 4 m/s², in the direction of the net force.

Worked example 2: opposing forces

Problem: A 5 kg object is pushed rightward with 20 N while friction acts leftward with 5 N. Other horizontal forces are negligible. What is the horizontal acceleration?

1. Take right as positive. The net horizontal force is 20 N − 5 N = 15 N to the right.
2. Calculate a = 15 N / 5 kg = 3 m/s² to the right.
3. Notice that using 20 N instead of the net 15 N would produce the wrong answer.

For a general object whose mass changes, Newton’s law is expressed more broadly in terms of the rate of change of momentum. The familiar F_net = m × a form is appropriate for the constant-mass beginner exercises here. Review algebra skills if rearranging these equations is new to you.

Newton’s third law: equal and opposite forces on different objects

If object A pushes object B with a certain force, object B simultaneously pushes object A with a force of the same magnitude in the opposite direction. The two forces act on different objects. Therefore they do not cancel when calculating the net force on either one object alone.

Example 1: walking

Your shoe pushes the ground backward. The ground exerts an equal forward static-friction force on your shoe. This interaction helps you walk forward; other forces also matter.

Example 2: swimming

A swimmer pushes water backward, and the water pushes the swimmer forward. The forces are paired but belong to different bodies.

Example 3: a NASA rocket at launch

In a rocket engine, the rocket exerts force on expelled exhaust gases; the gases exert a reaction force on the rocket, helping generate thrust. The rocket rises when the total net upward force is enough to accelerate it upward against gravity and other opposing forces. Equal action-reaction forces do not imply zero thrust because they act on different objects. See NASA’s rocket principles lesson.

Image context: The accompanying real NASA photograph shows the Space Launch System rocket lifting off on the uncrewed Artemis I mission at Kennedy Space Center, Florida, on November 16, 2022. It illustrates rocket propulsion, not a laboratory measurement of the forces in this exercise.

How do the three laws work together?

Consider a soccer ball. Before the kick, balanced forces leave the ball at rest (first law). The kick creates a net force and changes the ball’s velocity (second law). During contact, the foot pushes the ball while the ball pushes back on the foot (third law). Afterward, gravity, drag, and ground contact may change the ball’s direction and speed. One event can be explained using all three laws.

Safe classroom activity: test motion with a rolling ball

1. Work on a level indoor surface clear of people and breakable objects. Use a soft ball, not a heavy projectile.
2. Release the ball gently with similar pushes on a smooth surface and then on a rough carpet.
3. Mark approximate stopping distances using a ruler or tape measure.
4. Record differences and discuss friction, surface irregularities, and the challenge of repeating exactly the same push.

This activity illustrates how opposing forces influence motion. It does not by itself provide an accurate numerical measurement of Newton’s laws. Never run vehicle braking experiments in traffic.

Common mistakes students should avoid

• “Zero net force means zero velocity.” It means zero acceleration, so an object may still move at constant velocity.
• “A heavier object always accelerates more.” Under the same net force, a larger constant mass produces less acceleration.
• “Action and reaction cancel.” They occur on different objects.
• “Acceleration means only speeding up.” Turning or slowing also changes velocity.
• “Force and mass are measured in the same units.” Force is measured in N; mass is measured in kg.

Eight practice questions

1. Which law explains an object staying at rest when net force is zero?
2. Does a moving object need a net force to maintain constant straight-line velocity?
3. What is the acceleration when a 16 N net force acts on a 4 kg object?
4. A 2 kg cart accelerates at 3 m/s². What net force acts on it?
5. A cart receives 9 N rightward and 4 N leftward. What is the net horizontal force?
6. What object receives the reaction force when a swimmer pushes water backward?
7. Are the two forces in a third-law pair applied to the same object?
8. Can an object accelerate even if its speed stays constant?

Answer key with reasoning

1. First law: inertia.
2. No; a zero net force allows constant velocity.
3. 16 / 4 = 4 m/s².
4. 2 × 3 = 6 N in the direction of acceleration.
5. 5 N rightward.
6. The swimmer; water exerts the forward reaction force on the swimmer.
7. No, different interacting objects.
8. Yes. Changing direction, such as turning in a circle, is acceleration.

Related science study guides

Continue with Physics & Motion, use Mathematics & Algebra for equation practice, and explore Space & Astronomy for applications of forces in spaceflight.

Official sources and image rights

• NASA Glenn Research Center — Newton’s Laws of Motion
• NASA Glenn Research Center — Rocket Principles
• NASA — Artemis I Mission Overview
• NASA — Media and Image Use Guidelines

Featured-image credit: NASA/Bill Ingalls, Artemis I launch, November 16, 2022. NASA U.S. government photograph; public-domain use subject to NASA media guidelines, including no implication of endorsement.