Grade 5
The Human Knowledge Project
Unit 12 — Forces, Motion, and Energy
Welcome back!
Objects move in many different ways.
A bicycle accelerates, a baseball changes direction, and a roller coaster speeds up and slows down.
Scientists study motion and the forces that cause it.
Today you will learn how forces and energy help explain the movement of objects.
What Is a Force?
A force is a push or a pull.
Forces can:
- start motion
- stop motion
- speed up an object
- slow down an object
- change the direction of motion
Every movement involves one or more forces.
Balanced and Unbalanced Forces
Sometimes forces are equal.
These are called balanced forces.
Balanced forces do not change an object's motion.
When forces are unequal, they are called unbalanced forces.
Unbalanced forces cause an object to accelerate, slow down, stop, or change direction.
Gravity
Gravity is a force of attraction between objects that have mass.
Earth's gravity pulls objects toward the ground.
Gravity:
- keeps us on Earth's surface
- causes objects to fall
- helps keep planets in orbit around the Sun
Gravity acts everywhere in the universe.
Friction
Friction is a force that opposes motion.
Examples include:
- shoes gripping the ground
- bicycle brakes slowing a wheel
- rubbing your hands together
Friction can be helpful or harmful.
Sometimes engineers try to reduce friction.
At other times they increase it for safety.
Motion
Motion describes how an object's position changes over time.
Scientists describe motion using:
- distance
- direction
- speed
- time
Careful measurement allows scientists to compare different kinds of motion.
Energy
Energy is the ability to do work or cause change.
Common forms include:
- mechanical energy
- heat energy
- light energy
- sound energy
- electrical energy
- chemical energy
Energy can often change from one form to another.
Potential and Kinetic Energy
Potential energy is stored energy.
Examples include:
- a stretched rubber band
- water behind a dam
- a book resting on a shelf
Kinetic energy is the energy of motion.
Examples include:
- a rolling ball
- a moving bicycle
- flowing water
Objects often change between potential and kinetic energy.
Simple Machines
Simple machines make work easier.
The six classic simple machines are:
- lever
- wheel and axle
- pulley
- inclined plane
- wedge
- screw
Many complex machines combine several simple machines.
Activity
Observe five examples of motion.
For each example identify:
- the force involved
- whether the forces are balanced or unbalanced
- the type of energy involved
- whether friction is helping or opposing the motion
Then identify three simple machines in your home or school.
Explain how each makes work easier.
Practice
Answer these questions.
What is a force?
What is the difference between balanced and unbalanced forces?
How does gravity affect objects?
Why is friction important?
What is the difference between potential and kinetic energy?
Name the six simple machines.
Review
Today you learned:
- forces are pushes and pulls
- balanced and unbalanced forces affect motion differently
- gravity attracts objects with mass
- friction opposes motion
- energy exists in many forms
- potential and kinetic energy describe different kinds of energy
- simple machines help people perform work more efficiently
Understanding forces and energy helps explain many everyday events and technologies.
Looking Ahead
Next we will explore Earth, the Solar System, and space science, learning how our planet fits into the larger universe and how scientists investigate the cosmos.