01 / 20

Apply Motion
and Force

Understand how objects move and why, using Newton's Laws and the relationship \( F = ma \).

GED Science

What Is Motion?

An object is in motion when its position changes over time. We describe motion using three connected quantities.

Speed
How fast position changes: \( \text{speed} = \dfrac{\text{distance}}{\text{time}} \)
Velocity
Speed with a direction (a vector).
Acceleration
How quickly velocity changes over time.

What Is a Force?

A force is a push or a pull that can change an object's motion. Forces are measured in newtons (\( \text{N} \)).

box applied force friction Net force determines the resulting motion

Why It Matters

Motion and force describe nearly everything around you: cars braking, balls falling, rockets launching, and your own steps.

1
Everyday safety. Seatbelts and airbags work because of forces and inertia.
2
Engineering. Bridges, vehicles, and machines all balance forces.
3
GED test. You'll interpret diagrams and apply \( F = ma \) to solve problems.

Newton's First Law

at rest stays still in motion keeps moving

Inertia is an object's resistance to changes in its motion. More mass means more inertia.

Newton's Second Law

The acceleration of an object depends on the net force and its mass:

More Force
Same mass + bigger \( F \) means bigger acceleration.
More Mass
Same \( F \) + bigger \( m \) means smaller acceleration.

Newton's Third Law

A B A pushes B B pushes A

Example: when you push off a wall, the wall pushes back on you with equal force.

Mass vs. Weight

These are often confused, but they measure different things.

Mass Weight
What it isAmount of matterForce of gravity on mass
Unitkilograms (\( \text{kg} \))newtons (\( \text{N} \))
Changes with location?NoYes
Formula—\( W = m \times g \)

How to Apply \( F = ma \)

1
Identify what you know. List the given values for \( F \), \( m \), or \( a \).
2
Find the unknown. Decide which variable you need to solve for.
3
Rearrange the formula. \( a = \dfrac{F}{m} \) or \( m = \dfrac{F}{a} \).
4
Plug in and solve. Keep units consistent and include the unit in your answer.

Example 1: Find the Force

A \( 4 \ \text{kg} \) cart accelerates at \( 3 \ \text{m/s}^2 \). What net force is needed?

Example 2: Find Acceleration

A net force of \( 20 \ \text{N} \) acts on a \( 5 \ \text{kg} \) box. What is its acceleration?

5 kg 20 N

Example 3: Find Weight

An object has a mass of \( 10 \ \text{kg} \). What is its weight on Earth where \( g = 9.8 \ \text{m/s}^2 \)?

Remember: the mass stays \( 10 \ \text{kg} \) anywhere, but weight changes if gravity changes.

Common Mistakes

✕ Mixing mass and weight
Mass is in \( \text{kg} \); weight is a force in newtons (\( \text{N} \)). They are not the same.
✕ Forgetting net force
Use the combined force, accounting for friction and opposing pushes.
✕ Dropping units
Always carry units through. An answer needs \( \text{N} \), \( \text{kg} \), or \( \text{m/s}^2 \).

Quick Summary

Law Idea
First (Inertia)Objects keep their motion unless a net force acts
Second \( (F=ma) \)Force causes acceleration based on mass
Third (Action-Reaction)Forces come in equal, opposite pairs

Question 1 of 5

A \( 6 \ \text{kg} \) object accelerates at \( 2 \ \text{m/s}^2 \). What net force acts on it?

Question 2 of 5

Which law explains why a passenger lurches forward when a car suddenly stops?

Question 3 of 5

A \( 30 \ \text{N} \) force acts on a \( 5 \ \text{kg} \) box. What is its acceleration?

Question 4 of 5

An astronaut travels to the Moon. Which statement is true?

Question 5 of 5

When you jump off a small boat onto a dock, the boat moves backward. Which law is this?

You've Got Motion Down!

You can now describe motion, identify forces, and apply all three of Newton's Laws.