Unit 10 — Forces and Motion

Welcome back!

Have you ever pushed a swing, kicked a ball, or ridden a bicycle?

Every movement happens because of forces.

Forces help explain why objects start moving, stop moving, speed up, slow down, or change direction.

Today you will learn about some of the forces we experience every day.

What Is a Force?

A force is a push or a pull.

Forces can:

• start motion
• stop motion
• speed up motion
• slow down motion
• change direction

Almost every movement you see is caused by one or more forces.

Motion

Motion means a change in position over time.

If an object moves from one place to another, it is in motion.

Examples include:

• a rolling ball
• a flying bird
• a moving car
• a falling leaf

Objects at rest are not moving.

Gravity

Gravity is the force that pulls objects toward Earth.

Because of gravity:

• apples fall from trees
• rain falls from clouds
• people stay on the ground
• planets orbit the Sun

Gravity acts on everything that has mass.

Friction

Friction is a force that slows motion.

It happens when two surfaces rub against each other.

Examples include:

• bicycle brakes
• shoes gripping the ground
• rubbing your hands together
• sliding a book across a table

Without friction, walking would be very difficult.

Balanced and Unbalanced Forces

Sometimes forces are equal.

These are called balanced forces.

Balanced forces do not change an object's motion.

When one force is stronger than another, the forces are unbalanced.

Unbalanced forces can make an object start moving, stop, speed up, slow down, or change direction.

Simple Machines

Simple machines make work easier.

Examples include:

• lever
• wheel and axle
• pulley
• inclined plane
• wedge
• screw

Simple machines help people lift, move, and build things more efficiently.

Energy and Motion

Objects in motion have energy.

Energy can change from one form to another.

For example:

• A moving bicycle has motion energy.
• A stretched rubber band stores energy.
• Rolling a ball transfers energy to the ball.

Energy and motion are closely connected.

Activity

Look around your home or classroom.

Find:

• one example of gravity
• one example of friction
• one simple machine

Describe how each one works.

Practice

Answer these questions.

What is a force?

What is motion?

How does gravity affect us?

What does friction do?

Name four simple machines.

Review

Today you learned:

• forces are pushes and pulls
• motion means changing position
• gravity pulls objects toward Earth
• friction slows motion
• balanced and unbalanced forces affect movement differently
• simple machines make work easier

Understanding forces helps explain how objects move throughout the world around us.

Looking Ahead

Next we will explore Earth and space.

You will discover the planets, the Moon, the Sun, stars, and the amazing universe beyond our world.