The Three Laws Practice- Newton's Laws Exercises
What Are Newton's Three Laws of Motion?
Sir Isaac Newton published his three laws of motion in 1687. They form the foundation of classical mechanics and explain how objects move—or fail to move—when forces act on them.
These laws aren't abstract theory. Engineers use them to build bridges. Car designers use them to crash-test vehicles. Basketball players use them without even knowing it every time they shoot.
You need to understand these laws to solve most physics problems. Here's everything you need to know, with exercises to test yourself.
The First Law of Motion: Law of Inertia
An object at rest stays at rest. An object in motion stays in motion with the same speed and direction. This is inertia—objects resist changes to their state of motion.
No net force means no change in motion. That's it.
Real Examples
- When a car suddenly stops, your body lurches forward. Your body wanted to keep moving at the same speed.
- A hockey puck sliding on ice keeps sliding far longer than a puck on carpet because ice has less friction.
- Satellites continue orbiting Earth for decades with minimal thrust because there's no air resistance in space to slow them down.
First Law Practice Questions
Q1: A book sits on a table. No forces are pushing or pulling it. Why doesn't the book start moving on its own?
Answer: The book has no reason to change its state. It wants to stay at rest. No net force means no acceleration.
Q2: A spacecraft in deep space (virtually no gravity, no friction) turns off its engines. What happens?
Answer: It keeps moving in a straight line forever at the same speed. The engines aren't needed to maintain motion—only to change it.
The Second Law of Motion: Force Equals Mass Times Acceleration
This is the math most physics problems ask for:
F = ma
Force equals mass times acceleration. Units: Newtons (N) for force, kilograms (kg) for mass, meters per second squared (m/s²) for acceleration.
Breaking Down the Formula
Force and acceleration are directly proportional. Double the force, double the acceleration—assuming mass stays the same.
Mass and acceleration are inversely proportional. Double the mass, halve the acceleration—if force stays the same.
A 2 kg object with 10 N of force applied gets 5 m/s² acceleration. A 4 kg object with the same 10 N force gets only 2.5 m/s².
Second Law Practice Questions
Q1: A 5 kg object accelerates at 3 m/s². What force is applied?
Answer: F = 5 Ă— 3 = 15 N
Q2: You push a 20 kg shopping cart with 50 N of force. What is the acceleration?
Answer: a = F/m = 50/20 = 2.5 m/s²
Q3: A 1000 kg car needs to accelerate at 4 m/s². How much force is required?
Answer: F = 1000 Ă— 4 = 4000 N
The Third Law of Motion: Action and Reaction
For every action, there is an equal and opposite reaction.
Forces always come in pairs. When you push on a wall, the wall pushes back on you with equal force in the opposite direction.
Common Misconceptions
The action and reaction forces do not cancel out. They act on different objects.
When you push the wall, you experience the wall's push back on you. The wall experiences your push on it. Different objects, different force pairs.
Third Law Practice Questions
Q1: A swimmer pushes backward on the water. What happens?
Answer: The water pushes forward on the swimmer with equal force. The swimmer moves forward.
Q2: A rocket fires exhaust gases backward. Why does the rocket move forward?
Answer: The exhaust gases are pushed backward (action). The gases push the rocket forward (reaction). The rocket accelerates forward.
Q3: You stand on the floor. Name the action-reaction force pair between you and the floor.
Answer: Action: You push down on the floor with your weight. Reaction: The floor pushes up on you with equal force. That's why you don't fall through.
Quick Reference: Comparing Newton's Three Laws
| Law | Statement | Key Concept | Common Example |
|---|---|---|---|
| First Law | Objects at rest stay at rest; objects in motion stay in motion | Inertia / Resistance to change | Seatbelt snapping you forward in a car crash |
| Second Law | F = ma | Force causes acceleration | Pushing an empty cart vs. a full cart |
| Third Law | Every action has an equal, opposite reaction | Force pairs on different objects | Walking (push backward, move forward) |
How to Solve Newton's Laws Problems
Follow these steps for any mechanics problem:
- Draw a free body diagram. Show all forces acting on the object with arrows. Label them clearly (gravity, normal force, friction, applied force, etc.).
- Identify the net force. Add up all forces in each direction. Forces in opposite directions subtract.
- Choose your axis. Usually, put the direction of motion on one axis.
- Apply F = ma. Set up equations for each direction.
- Solve for the unknown. Isolate the variable you need. Check your units.
Example Problem with Full Solution
Problem: A 10 kg box sits on a flat surface. You pull it with 30 N of horizontal force. Friction is 10 N. Find the acceleration.
Step 1: Forces acting: Applied force (30 N forward), Friction (10 N backward).
Step 2: Net force = 30 - 10 = 20 N forward.
Step 3: Apply F = ma
a = F/m = 20/10 = 2 m/s²
Common Mistakes Students Make
- Confusing mass and weight. Weight is a force (W = mg). Mass is intrinsic. A 10 kg object has 98 N of weight on Earth, but still only 10 kg of mass.
- Forgetting friction. Real problems almost always include friction unless stated otherwise.
- Mixing up action-reaction pairs. Remember: they act on different objects.
- Using the wrong units. Mass in kg, force in N, acceleration in m/s². Convert everything before calculating.
- Ignoring the normal force. Objects on surfaces experience an upward normal force that cancels gravity (when flat).
Practice Problems: Mixed Difficulty
Try these without checking the answers first.
Easy: A 3 kg ball rolls with 15 N of force. What is its acceleration?
Answer: a = 15/3 = 5 m/s²
Medium: A 50 kg person stands on a scale in an elevator accelerating upward at 2 m/s². What does the scale read? (Hint: The scale measures the normal force.)
Answer: N = m(g + a) = 50(10 + 2) = 600 N
Hard: Two blocks (5 kg and 10 kg) are pushed together with a force of 90 N on a frictionless surface. What is the acceleration of the system?
Answer: Total mass = 15 kg. a = F/m = 90/15 = 6 m/s²
Key Takeaways
- Objects resist changes to motion due to inertia.
- F = ma connects force, mass, and acceleration.
- Forces come in pairs—action equals reaction, different objects.
- Draw free body diagrams before solving anything.
- Check your units before you calculate.
That's Newton's Three Laws. Practice the problems until the formulas feel automatic. That's how you actually learn this stuff—not by reading about it.