Grade 10 · Newton's Laws and Momentum · Newton's Laws
Newton's Three Laws
Inertia, F = m x a, and why forces always come in pairs.
Learning objectives
- State Newton's three laws.
- Use F = m x a in calculations.
- Identify action-reaction force pairs.
NGSS alignment
HS-PS2-1
Analyze data to support the claim that Newton's second law of motion describes the mathematical relationship among the net force on a macroscopic object, its mass, and its acceleration.
MS-PS2-1
Apply Newton's third law to design a solution to a problem involving the motion of two colliding objects.
MS-PS2-2
Plan an investigation to provide evidence that the change in an object's motion depends on the sum of the forces on the object and the mass of the object.
MYP criteria
- Criterion A — Knowing and Understanding
- Criterion C — Processing and Evaluating
ENGAGE
Why seat belts exist
When a bus brakes suddenly, standing passengers lurch forwards even though nothing pushed them.
Think about it
- What were the passengers doing before the brakes were applied?
- Which law explains their motion?
EXPLAIN
The three laws
| Law | Statement | Everyday example |
|---|---|---|
| First | An object stays at rest or at constant velocity unless a resultant force acts | Passengers lurch forward when a bus brakes |
| Second | Resultant force = mass x acceleration | The same push accelerates a shopping trolley more when it is empty |
| Third | Every action has an equal and opposite reaction on another object | A swimmer pushes water back and moves forward |
Key equation
F = m x a. A resultant force of 60 N on a 20 kg mass gives an acceleration of 3 m/s squared.
- Inertia is the tendency of an object to resist a change in motion; more mass means more inertia
- Action-reaction pairs act on two different objects, never on the same object
- Weight is the force of gravity: W = m x g
Common misconception
Action and reaction do not cancel out, because they act on different objects. If they cancelled, nothing could ever accelerate.
Watch
Newton's Laws: Crash Course Physics
CrashCourse · 10 min
All three laws with worked examples.
Before you watch: Which law explains a swimmer moving forwards?
- Write the second law as an equation.
- Why do action-reaction pairs not cancel?
Interactive simulation · PhET
Forces and Motion: Basics
Use the Acceleration tab. Keep the force fixed and change the mass on the cart.
While you explore
- What happens to acceleration when mass doubles?
- How does this match F = m x a?
Key vocabulary
- Acceleration
- How quickly speed or direction changes.
- Inertia
- The tendency of an object to keep doing what it is doing.
Practice questions
0/2 correct
Level 1 · Criterion A
A resultant force of 40 N acts on a 10 kg mass. Its acceleration is:
Level 2 · Criterion A
A book rests on a table. The reaction pair to the book's weight acts:
MYP criterion tasks
Level 3 · Criterion C
Two students push identical trolleys with the same force, but one trolley carries a heavy load. Use Newton's second law to compare their accelerations, and state what data you would collect to test your prediction.
Reflect & track
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