Forces, Friction, and Newton’s Laws
Why does a pushed box eventually stop, and why does a seatbelt matter in a sudden stop? The answers come from forces and Newton’s three laws of motion. These rules describe how everything moves, and once you know them, a whole category of physics questions becomes predictable.
This lesson covers forces, friction, and Newton’s three laws.
A force is a push or a pull. Newton’s three laws describe how forces change motion: objects keep doing what they are doing unless a force acts (first law), force equals mass times acceleration (second law), and every action has an equal and opposite reaction (third law). Friction is a force that opposes motion between surfaces.
What are Newton’s three laws?
Newton’s first law (inertia) says an object at rest stays at rest, and an object in motion stays in motion at constant speed and direction, unless an unbalanced force acts on it. Newton’s second law gives the exact relationship between force, mass, and acceleration:
[ F = ma ]Here (F) is the net force, (m) is the mass, and (a) is the acceleration. The same force gives a small mass a large acceleration and a large mass a small one. Newton’s third law says that for every action there is an equal and opposite reaction: when you push on a wall, the wall pushes back on you with equal force.
| Law | Idea |
|---|---|
| First | Objects resist changes in motion (inertia) |
| Second | (F=ma) |
| Third | Equal and opposite reactions |
What is friction?
Friction is a force that resists motion when two surfaces touch. It always acts opposite to the direction of motion, which is why a sliding box slows and stops. Rougher surfaces and heavier objects produce more friction. Friction can be a nuisance, wearing down machine parts, but it is also essential: without it, you could not walk, and cars could not grip the road.
A worked example
Suppose a net force of (20) newtons pushes a (4) kilogram cart. What is its acceleration? Rearrange the second law to (a=dfrac{F}{m}), then:
[ a = dfrac{20}{4} = 5text{ m/s}^2 ]The cart speeds up by (5) meters per second every second. Double the mass and the acceleration would be halved, exactly as (F=ma) predicts.
Watch: A Short Video Lesson
Step by Step Science walks through this skill clearly in a few minutes. It is a helpful companion to the reading above:
A routine for force questions
- Identify the forces acting and whether they are balanced.
- Balanced forces mean no change in motion (first law).
- For acceleration, use (F=ma), solving for whichever quantity is unknown.
- Remember friction opposes motion.
- Look for action-reaction pairs (third law).
Practice questions
- State Newton’s first law in your own words.
- Write the formula for Newton’s second law.
- A (10) N force acts on a (2) kg object. What is its acceleration?
- In which direction does friction act?
- Give an everyday example of Newton’s third law.
- True or false: heavier objects and rougher surfaces produce more friction.
Answers:
- An object keeps its motion (or stays at rest) unless an unbalanced force acts on it.
- (F=ma).
- (a=dfrac{10}{2}=5text{ m/s}^2).
- Opposite to the direction of motion.
- Pushing on a wall and feeling it push back (or a rocket pushing gas down and being pushed up).
- True.
Where this fits
Newton’s laws build on the description of motion in motion: speed, velocity, and acceleration, and they lead into forces and Newton’s laws and momentum, work, and power. Find all topics on the ASVAB General Science Learning Hub.
Recommended Prep Books
These study guides and practice books help you keep building momentum as you prepare:
Related to This Article
More math articles
- 5th Grade MEA Math Worksheets: FREE & Printable
- 10 Most Common 8th Grade MCAS Math Questions
- How to Solve Word Problems Involving the Percentage of a Number?
- Best Calculators for Linear Algebra and Calculus
- Best Laptop for Math Students In 2024
- Free Grade 4 English Worksheets for Wisconsin Students
- Reading and Writing Numbers for 5th Grade: Standard, Word, and Expanded Form
- The Best Grade 3 Math Book for Arizona Students
- The 8 Best Webcams for Online Teachers
- Free Grade 5 English Worksheets for South Dakota Students
What people say about "Forces, Friction, and Newton's Laws: Definition & Examples"?
No one replied yet.