Grade 7
The Human Knowledge Project
Unit 7 — Energy, Forces, Motion, and Simple Machines
Welcome back!
Every movement we observe—from a rolling ball to a rocket launching into space—is governed by the laws of physics.
Physics helps us understand how objects move, why they move, and how energy is transferred throughout the universe.
Today you will investigate energy, forces, motion, and the simple machines that make work easier.
What Is Energy?
Energy is the ability to do work or produce change.
Energy exists in many forms.
Examples include:
- kinetic energy
- potential energy
- thermal energy
- chemical energy
- electrical energy
- light energy
- sound energy
- nuclear energy
Although energy may change from one form to another, it is conserved.
Kinetic and Potential Energy
Kinetic energy is the energy of motion.
Examples include:
- a moving bicycle
- a flowing river
- a flying bird
Potential energy is stored energy.
Examples include:
- a stretched rubber band
- water behind a dam
- a book resting on a shelf
Many systems continuously convert potential energy into kinetic energy and back again.
Forces
A force is a push or a pull.
Forces can:
- start motion
- stop motion
- speed up motion
- slow down motion
- change direction
- change shape
Forces are measured in newtons (N).
Balanced and Unbalanced Forces
When forces acting on an object are equal, they are balanced.
Balanced forces do not change an object's motion.
When forces are unequal, they are unbalanced.
Unbalanced forces cause changes in motion.
Understanding force balance helps explain why objects accelerate or remain at rest.
Newton's Laws of Motion
Sir Isaac Newton described three important laws of motion.
First Law
An object remains at rest or continues moving in a straight line unless acted upon by an unbalanced force.
Second Law
Greater force produces greater acceleration, while greater mass requires more force to produce the same acceleration.
Third Law
For every action, there is an equal and opposite reaction.
These laws explain motion throughout everyday life and throughout the universe.
Friction
Friction is a force that opposes motion.
Examples include:
- shoes gripping the ground
- bicycle brakes
- rubbing hands together
Friction can be helpful or harmful depending on the situation.
Engineers often design systems to either increase or reduce friction.
Work and Power
In physics, work occurs when a force moves an object over a distance.
Power describes how quickly work is done.
Machines often increase efficiency by reducing the effort needed to perform work.
Simple Machines
Simple machines make work easier by changing the size or direction of a force.
The six classical simple machines are:
- lever
- wheel and axle
- pulley
- inclined plane
- wedge
- screw
Many complex machines combine several simple machines.
Energy in Everyday Life
Energy and forces are involved in countless activities.
Examples include:
- transportation
- sports
- construction
- electricity generation
- robotics
- manufacturing
- space exploration
Understanding physics helps engineers design safer and more efficient technologies.
Activity
Complete the following:
- identify examples of kinetic and potential energy
- observe balanced and unbalanced forces
- investigate friction using different surfaces
- identify ten examples of simple machines
- explain how each machine changes force
- design a device using at least two simple machines
Record your observations and explain your conclusions.
Practice
Answer these questions.
What is energy?
How do kinetic and potential energy differ?
What is a force?
What is the difference between balanced and unbalanced forces?
What is friction?
Name the six simple machines.
Review
Today you learned:
- energy exists in many forms
- kinetic energy is associated with motion
- potential energy is stored energy
- forces change motion
- balanced and unbalanced forces have different effects
- Newton's laws explain motion
- simple machines make work easier
Physics helps us understand movement, energy, and the design of countless technologies that improve our daily lives.
Looking Ahead
Next we will investigate Earth's dynamic systems, including the atmosphere, weather, climate, plate tectonics, and the processes that continually shape our planet.