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-PS3-4
Plan and conduct an investigation to provide evidence that the transfer of thermal energy when two components of different temperature are combined within a closed system results in a more uniform energy distribution.
HS-PS4-5
Communicate technical information about how some technological devices use the principles of wave behavior and wave interactions with matter to transmit and capture information and energy.
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.
HS-ETS1-3
Evaluate a solution to a complex real-world problem based on prioritized criteria and trade-offs that account for a range of constraints.
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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