C. Forces · Forces

Types of Forces and What They Do

C2.1 C3.1About 65 minutes

Learning goals

I can define a force and state its SI unit, the newton.
I can sort forces into contact forces and action-at-a-distance forces.
I can identify gravity, normal force, friction, tension, applied force and air resistance acting on an object, and give the direction of each.
I can describe how an unbalanced force changes an object's velocity.

Words to know

force
A push or a pull on an object. Force is a vector: it has a size (magnitude) and a direction.

newton (N)
The SI unit of force. 1 N = 1 kg·m/s², the force that gives a 1 kg mass an acceleration of 1 m/s².

contact force
A force that acts only when two objects touch, such as friction or the normal force.

action-at-a-distance force
A force that acts across space without touching, such as gravity, electric force or magnetic force. Also called a non-contact force or field force.

gravitational force (Fg)
The force of attraction between masses. Near Earth's surface it pulls every object straight down, toward Earth's centre. Also called weight.

normal force (FN)
The push of a surface on an object, perpendicular ("normal") to the surface.

friction (Ff)
A contact force that acts along a surface and opposes sliding (kinetic friction) or attempted sliding (static friction).

tension (FT)
The pulling force exerted by a rope, string, cable or chain. It always pulls along the rope, away from the object.

air resistance
Friction from air on a moving object. It acts opposite to the object's motion through the air and grows as speed grows.

applied force (Fa)
A push or pull from a person or machine in direct contact with the object.

Why does a puck stop?

A hockey puck slides across an outdoor rink on a cold January afternoon. It glides a long way, but eventually it slows down and stops. Or it hits the boards and bounces back. Or a stick flicks it sideways.

In every case the puck's velocity changes: its speed, its direction, or both. In the Kinematics unit you learned to describe changes in velocity using acceleration. This unit is about why velocity changes. The answer, every time, is a force.

The study of forces and how they cause motion is called dynamics. Its rules were worked out by Galileo Galilei and Isaac Newton, and they still let engineers design everything from snow tires to roller coasters.

Remember from earlier grades

In Grade 6 you learned that weight is the pull of gravity on an object, measured in newtons, and that on Earth gravity pulls on each kilogram with about 9.8 N. In the Grade 6 flight unit you saw that balanced forces keep a plane's motion steady while unbalanced forces change it. In Grade 7 you described forces by their magnitude, direction and point of application. Now we make all of this precise enough to calculate with.

What is a force?

A force is a push or a pull. Because a force has a size and a direction, it is a vector, just like displacement and velocity in the Kinematics unit. We draw a force as an arrow. The arrow points in the direction of the push or pull, and its length shows the size.

The SI unit of force is the newton (N). One newton is about the weight of a medium apple (a mass of about 100 g). In base units, 1 N = 1 kg·m/s². You will see why in the lesson on Newton's second law.

A force can change an object's speed, its direction, or its shape (a squashed tennis ball). In this unit we focus on how forces change motion.

Two families of forces

Contact forces act only when objects touch:

Applied force (Fa): a push or pull from a person or machine.
Normal force (FN): the push of a surface. It is always perpendicular to the surface. A table pushes up on a book; a wall pushes out on your hand.
Friction (Ff): acts along a surface and opposes sliding. Static friction acts when the surfaces are not sliding (it stops a parked car from rolling). Kinetic friction acts when they are sliding.
Tension (FT): the pull of a rope, string or cable. It always pulls, never pushes, and acts along the rope.
Air resistance (drag): friction from air (or water). It points opposite to the motion through the fluid and grows with speed.
Spring (elastic) force: the push or pull of something stretched or squashed.

Action-at-a-distance forces act across space:

Gravitational force (Fg): every mass attracts every other mass. Near Earth it points straight down.
Electric force and magnetic force: between charges and magnets.

Inside matter, contact forces are really electromagnetic forces between the atoms of the two surfaces. Physicists say all forces come from four fundamental forces: gravitational, electromagnetic, strong nuclear and weak nuclear.

Every force in this unit fits into one of two families. Contact forces need touching; action-at-a-distance forces, like gravity, act across empty space.

Force	Symbol	Direction	Example
Gravitational	Fg	straight down (toward Earth's centre)	a falling maple leaf
Normal	FN	perpendicular to the surface, pushing away from it	the floor holding you up
Friction	Ff	along the surface, opposing sliding	skates scraping to a stop
Tension	FT	along the rope, pulling away from the object	a rope towing a toboggan
Applied	Fa	whatever way it is pushed or pulled	pushing a shopping cart
Air resistance	Fair	opposite to the motion through the air	a parachute opening

The six forces you will meet most often. In textbooks the letters after F are written small, as subscripts.

How forces change velocity

What matters is the unbalanced (net) force, which you will learn to add up in the next lesson. If the forces on an object balance, its velocity does not change. If they do not balance:

a net force in the same direction as the velocity makes the object speed up;
a net force opposite to the velocity makes it slow down;
a net force sideways to the velocity makes it change direction.

So friction on a sliding puck slows it down. Gravity on a dropped ball speeds it up. A stick hitting the puck from the side turns it.

The dashed arrow is the puck's velocity; the solid arrow is an unbalanced force. A force along the motion changes the speed; a force across the motion changes the direction.

Worked example

Example 1: Naming the forces

Name every force acting on each object and give its direction.

(a) A textbook resting on a desk. (b) A curling stone sliding across the ice after it is released. (c) A lamp hanging from the ceiling by a chain.

Start with gravity. Every object near Earth has Fg, down.
Then look for everything touching the object. Each thing touching it can give a contact force.
(a) The desk touches the book: normal force, up. Nothing is trying to slide it, so there is no friction. Forces: Fg down, FN up.
(b) The ice touches the stone: normal force up, and kinetic friction backward (opposite the sliding). The curler's hand is no longer touching it, so there is no forward force. (Air resistance is tiny and can be ignored.)
(c) The chain touches the lamp: tension, up along the chain. Forces: Fg down, FT up.

(a) Fg down, FN up. (b) Fg down, FN up, kinetic friction backward. (c) Fg down, FT up.

Worked example

Example 2: Force and velocity

A skydiver jumps from a plane. Describe the forces on her and how each affects her velocity: (a) just after she jumps, (b) after her parachute opens.

The forces are gravity (down) and air resistance (up, opposite her motion). Gravity is action-at-a-distance; air resistance is a contact force.
(a) Just after jumping she is moving slowly, so air resistance is small. Gravity is much larger, so the net force is down, the same direction as her velocity. She speeds up.
(b) The open parachute catches much more air. Air resistance becomes larger than gravity, so the net force points up, opposite her downward velocity. She slows down (while still moving downward) until the two forces balance.

(a) Gravity is larger, net force down, she speeds up. (b) Air resistance is larger, net force up, she slows down until the forces balance.

Just for fun

Why is the normal force such a good friend?

Because it always supports you, and it never takes sides: it stays perpendicular.

Hands-on activity

Force hunt

You need: a book, a piece of string (about 50 cm), a smooth table, a sheet of paper and a pencil.

Rest the book on the table. List every force on it and draw an arrow for each.
Tie the string around the book and pull it slowly across the table. Which new forces appear? (Tension forward, kinetic friction backward.)
Pull very gently, so the book does not move. What is stopping it? (Static friction.) Pull harder until it starts to slide. Did it feel harder to start it or to keep it moving?
Lift the book by the string and hold it still. Now which forces act? (Fg down, FT up.)
Hold the sheet of paper flat and drop it, then crumple it into a ball and drop it again from the same height. Which force made the difference? (Air resistance.)

Write one sentence for each step saying whether the book's or paper's velocity changed, and which force caused it.

Your turn

What is the SI unit of force, and what is it equal to in base units?

Classify each force as contact or action-at-a-distance: (a) friction, (b) gravity, (c) tension, (d) magnetic force, (e) normal force.

A fridge magnet holds a note on the fridge door. Name the forces on the magnet.

A canoe glides across a lake after the paddler stops paddling. Which force slows it down?

In which direction does the normal force act on a skier standing on a flat, level hill top? On a sloped hill?

A ball is thrown straight up. While it rises, which way does gravity point, and what does it do to the ball's speed?

More practice: warm-up

Is force a scalar or a vector? Why?

Which force always acts perpendicular to a surface?

Which force can only pull, never push?

About how many newtons does a 1 kg bag of flour weigh on Earth?

Name the force that keeps a parked car from rolling down a gentle driveway.

More practice: on your own

A box sits on the floor of a moving truck that is speeding up. The box does not slide. Which force speeds the box up? Explain.

A puck slides to the right across ice and slows down. Draw (in words) the velocity arrow and the friction arrow.

Explain why air resistance on a cyclist is much larger at 40 km/h than at 10 km/h.

A crane lifts a steel beam at a constant speed. Name the forces on the beam. What does "constant speed" tell you about them?

Your friend says, "Once a ball leaves my hand, my throwing force keeps pushing it forward." What is wrong with this?

Challenge

A book is pressed against a wall by your hand, pushing horizontally, and it does not fall. Name all four forces on the book with their directions, and say which force balances which.

A satellite orbits Earth far above the atmosphere. Which force or forces act on it? Is its velocity constant? Explain.

Explain, using the idea of electromagnetic forces between atoms, why the normal force from a table gets bigger when you put a heavier book on it.

Check your understanding

Quiz

Which of these is an action-at-a-distance force?

The normal force on an object always acts:

1 N is equal to:

A ball rolls to the right on a carpet and slows down. Friction on the ball points:

A hockey puck sliding north is struck from the side by a stick pushing east. The puck will mainly:

Which force is responsible for a parked car not sliding down a slight slope?

For parents and teachers

Big idea: This lesson builds the vocabulary of the unit (C2.1) and names the common forces and their effects on velocity (C3.1).

How to help: Pick everyday objects (a mug on the table, a backpack on a hook, a car braking) and ask your teen, "What is touching it? What force does each thing give? Which way?" Always start with gravity, then list contact forces.

Common difficulties:

Inventing a "force of motion" that carries a thrown ball along. Once contact ends, the push ends.
Drawing the normal force straight up on a slope. It is perpendicular to the surface.
Mixing up mass (kg) and force (N). A 1 kg object weighs about 9.8 N on Earth.
Thinking friction always opposes motion. Static friction can actually make things move, such as a box riding in an accelerating truck, or your shoes pushing you forward when you walk.