Every time you open a door, kick a ball, ride a bicycle or place a book on a table, forces are involved.
Forces can start motion, stop motion, change speed, change direction or change the shape of an object.
Understanding forces helps us explain many ordinary events that happen around us every day.
What is a force?
A force is a push or a pull acting on an object.
Examples include:
- pushing a shopping cart
- pulling open a drawer
- kicking a football
- gravity pulling an object toward Earth
- friction acting between surfaces
A force has both strength and direction.
A strong push can affect an object differently from a gentle push.
A push to the left can also affect motion differently from a push to the right.
Forces can change motion
Motion describes how the position of an object changes with time.
A force can cause an object to:
- start moving
- speed up
- slow down
- stop
- change direction
A force does not always cause visible movement, however.
Sometimes several forces act at the same time and balance one another.
Example: Kicking a stationary ball
A football resting on the ground stays at rest until something causes its motion to change.
When you kick the ball, your foot exerts a force on it.
The ball begins moving.
The force changed the state of motion of the ball.
Example: Catching a moving ball
A moving ball can also have its motion changed.
When you catch it, your hands exert forces that reduce its speed until the ball stops relative to you.
This is another example of force changing motion.
Example: Turning a bicycle
A force can change direction even if an object continues moving.
When a bicycle turns a corner, forces between the tyres and road help change the direction of motion.
A change in direction is a change in motion.
More than one force can act at once
Objects usually experience several forces at the same time.
Imagine a book resting on a table.
Earth pulls the book downward because of gravity.
The table pushes upward on the book.
The book remains at rest because these vertical forces balance under ordinary conditions.
What is net force?
The net force is the overall effect of all the forces acting on one object.
To understand the net force, you must consider both the strength and direction of the individual forces.
Imagine two people pushing a box in the same direction.
Their pushes can combine to produce a larger overall force.
If they push in opposite directions, one force may partly or completely oppose the other.
Balanced forces
Forces are balanced when their combined effect gives a net force of zero.
Balanced forces do not cause an object's velocity to change.
This means an object with balanced forces can remain at rest.
It can also continue moving in a straight line at constant speed.
Balanced does not always mean stationary
This is an important idea.
Imagine an object already moving at constant speed in a straight line.
If the forces on it remain balanced, its motion does not need to stop.
A net force is required to change its velocity.
Unbalanced forces
When the forces acting on an object do not balance, the net force is not zero.
The object's velocity changes.
It may:
- speed up
- slow down
- change direction
- do more than one of these at the same time
Example: A box pushed across the floor
Suppose you push a box to the right.
Friction acts in the opposite direction and resists the sliding motion.
If your push is stronger than the opposing friction, the forces are unbalanced and the box can accelerate to the right.
If the forces balance while the box is already moving, it can move at constant velocity.
What is acceleration?
Acceleration means a change in velocity.
This can mean:
- getting faster
- getting slower
- changing direction
So acceleration does not only mean speeding up.
A car slowing for a traffic light is also accelerating in the scientific sense because its velocity is changing.
Force and acceleration
A net force causes acceleration.
If the mass stays the same, a larger net force produces a larger acceleration.
For example, a stronger push on the same toy cart can produce a greater change in motion than a weaker push.
Mass also matters
The mass of an object affects how easily its motion changes.
Compare pushing an empty shopping cart with pushing the same cart when it is full of heavy items.
If you apply the same force, the heavier cart generally has less acceleration.
More mass means more resistance to a change in motion.
What is inertia?
Inertia is the tendency of an object to resist a change in its motion.
An object at rest tends to remain at rest unless a net external force acts on it.
An object already moving tends to continue moving with constant velocity unless a net external force changes that motion.
Objects with more mass have more inertia.
Why do moving objects usually slow down?
You may wonder why a ball rolling across the ground eventually stops.
If objects naturally continue moving unless a net force acts, why does the ball not roll forever?
The answer is that forces such as friction and air resistance act on the ball.
These forces change its motion.
Friction
Friction is a force associated with surfaces interacting.
It can oppose sliding motion or prevent surfaces from beginning to slide.
Friction often acts in a direction that resists relative motion between surfaces.
Friction can slow an object
Slide a book gently across a table.
It eventually stops.
Friction between the book and table acts against the sliding motion.
The net force changes the speed of the book.
Friction can also be useful
Friction is not simply a problem.
Without enough friction:
- walking would be difficult
- tyres could not grip a road effectively
- many brakes would not work properly
- objects could easily slip from your hands
Friction can help us control motion.
Too much friction can be unwanted
In other situations, friction can make movement harder.
Parts in machines may rub against one another and lose energy through heating.
Engineers often design systems to control friction depending on whether it is useful or unwanted.
Gravity
Gravity is a force of attraction between objects with mass.
Near Earth's surface, Earth exerts a gravitational force that pulls objects toward the planet.
This force is responsible for the weight of an object.
Dropping an object
If you release a ball, gravity pulls it downward.
The ball accelerates toward Earth.
Air resistance may also act, but gravity is the main downward force in a simple falling example.
Mass and weight are not the same
Mass describes a property of the object related to the amount of matter and its inertia.
Weight is the gravitational force acting on the object.
In everyday speech, people often use the words as though they mean the same thing.
In physics they are different quantities.
The support force from a surface
Consider a book resting on a table again.
Gravity pulls the book downward.
The table exerts an upward contact force on the book.
This supporting force prevents the book from simply moving through the table.
Physicists call this type of surface force a normal force.
Forces can act through contact
Many forces occur when objects touch.
Examples include:
- a hand pushing a door
- a rope pulling a bucket
- a table supporting a book
- friction between a tyre and road
These are contact interactions.
Some forces can act without direct contact
Objects do not always need to touch for a force to act.
Gravity is one example.
Magnetic forces can also act across a distance.
If two magnets push apart or pull together without touching, a force is still acting.
Force is measured in newtons
The standard scientific unit of force is the newton.
Its symbol is N.
A force measuring 5 N is stronger than a force measuring 2 N when we compare only their magnitudes.
Direction must also be considered when determining their combined effect.
Representing forces with arrows
Scientists often draw arrows to show forces.
An arrow can communicate:
- which direction the force acts
- which object experiences the force
- the relative strength of the force
A longer arrow may be used to represent a stronger force when a diagram uses a consistent scale.
Example: A book on a table
You could draw:
- one arrow downward for the gravitational force on the book
- one arrow upward for the support force from the table
If the book remains at rest and these are the only important vertical forces, the arrows can be drawn with equal lengths to show that they balance.
Example: Pulling a wagon
A wagon can experience several forces:
- a forward pull
- resistance from friction and rolling effects
- gravity downward
- support from the ground upward
The motion depends on the net effect of all the forces acting on the wagon.
Newton's first law in everyday life
Newton's first law describes the tendency of an object to keep its current state of motion unless a net external force acts.
A stationary object stays stationary unless a net force changes that.
A moving object continues at constant velocity unless a net force changes its speed or direction.
A passenger in a stopping car
Imagine sitting in a moving car that stops suddenly.
Your body tends to continue moving forward because of inertia.
A seat belt exerts a force that helps change your motion with the car.
This is one important reason seat belts are essential for safety.
Newton's second law in everyday life
Newton's second law connects net force, mass and acceleration.
For the same object, increasing the net force increases the acceleration.
For the same net force, increasing the mass reduces the acceleration.
Shopping cart comparison
Imagine two identical carts.
One is empty.
The other is loaded with heavy items.
If you push both with the same force, the empty cart changes its motion more easily because it has less mass.
Newton's third law in everyday life
Forces between interacting objects occur in pairs.
If one object exerts a force on another object, the second object exerts an equal strength force in the opposite direction on the first object.
The two forces act on different objects.
Walking is an example
When you walk, your foot pushes backward on the ground.
The ground pushes forward on your foot.
That interaction helps move you forward.
Jumping is another example
When you jump, your feet push downward on the ground.
The ground pushes upward on you.
This upward force helps accelerate your body upward.
Why equal opposite interaction forces do not cancel
This can be confusing.
The two forces in an interaction pair act on different objects.
Your foot pushes on the ground.
The ground pushes on your foot.
Because they do not both act on the same object, we do not add them together when finding the net force on your body alone.
Balanced forces and interaction pairs are different
Balanced forces are forces on the same object whose combined effect is zero.
An interaction pair consists of forces on two different objects.
These are two different ideas.
Example: A person standing still
Consider a person standing on the floor.
Gravity pulls the person downward.
The floor pushes the person upward.
These forces on the person can balance.
At the same time, the person pushes downward on the floor and the floor pushes upward on the person as an interaction pair.
Forces can change shape too
Not every force causes an object to move noticeably.
A force can also deform an object.
Examples include:
- squeezing a sponge
- stretching an elastic band
- compressing a spring
- bending a flexible ruler
The object may return to its original shape after the force is removed, or it may remain changed depending on the material and strength of the force.
Everyday example: Opening a door
You push the door with your hand.
Your force can make the door rotate around its hinges.
The effect depends partly on where and how you push.
This everyday action shows that forces can change rotational motion as well as straight line motion.
Everyday example: Braking a bicycle
When bicycle brakes are applied, forces act to reduce the motion of the wheels.
Friction between tyres and the road is also important for controlling the bicycle.
The bicycle slows because its velocity is changing under the effect of forces.
Everyday example: Throwing a ball upward
Your hand first exerts a force that launches the ball upward.
After the ball leaves your hand, gravity continues pulling downward.
The ball slows as it rises, briefly reaches its highest point and then accelerates downward.
Air resistance also affects real motion.
Common mistake 1: Thinking a moving object always needs a forward force
An object does not need a forward net force merely to continue moving at constant velocity.
A net force is needed to change its velocity.
In everyday situations, friction often means we must keep applying a force to maintain motion.
Common mistake 2: Thinking no movement means no forces
A stationary object can have several forces acting on it.
A book resting on a table experiences gravity and an upward support force.
If the forces balance, the book does not accelerate.
Common mistake 3: Thinking balanced forces always mean the object is at rest
Balanced forces mean the net force is zero.
An object can be at rest or moving at constant velocity when its net force is zero.
Common mistake 4: Thinking friction always acts backward relative to the ground
Friction acts to oppose relative slipping or the tendency to slip between contacting surfaces.
Its direction depends on the situation.
For example, friction from the ground can act forward on your foot while you walk.
Common mistake 5: Confusing force with motion
A force is not motion itself.
Force is an interaction that can change motion.
An object can already be moving even when the net force on it is zero.
Common mistake 6: Thinking heavier objects always move faster when pushed
Mass affects acceleration.
If the same net force acts on two objects with different masses, the object with greater mass generally has the smaller acceleration.
Practice 1
A football is resting on grass.
A player kicks it.
What caused the ball's motion to change?
A force from the player's foot acted on the ball.
Practice 2
A book remains still on a table.
Does that mean no forces act on it?
No.
Gravity acts downward and the table provides an upward support force.
These forces can balance.
Practice 3
A toy car rolls across a rough floor and gradually stops.
Which force helps explain the slowing?
Friction.
Practice 4
Two people push the same box to the right.
What happens to their pushes?
The forces combine in the same direction.
Practice 5
Two people push an object with equal strength in opposite directions.
If these are the only horizontal forces, what is the horizontal net force?
Zero.
Practice 6
An object is moving in a straight line at constant speed.
Must the net force be forward?
No.
Constant velocity is consistent with a net force of zero.
Practice 7
You push backward on the ground while walking.
What force helps push you forward?
The ground exerts a forward force on your foot.
Practice 8
A ball changes direction while keeping approximately the same speed.
Has its motion changed?
Yes.
A change in direction is a change in velocity.
Practice 9
You push an empty cart and a heavily loaded cart with similar forces.
Which is likely to accelerate more?
The empty cart because it has less mass.
Practice 10
A magnet attracts a metal object without touching it.
Does this count as a force?
Yes.
Some forces can act without direct physical contact.
A simple way to analyse forces
When looking at an everyday situation, ask four questions.
- Which object am I studying?
- What forces act on that object?
- Which direction does each force act?
- Do the forces balance, or is there a nonzero net force?
These questions prepare you for drawing force diagrams in the related lesson.
The main idea
A force is a push or pull with strength and direction.
Several forces can act on the same object at once.
Their combined effect is called the net force.
When the net force is zero, the object's velocity does not change.
When there is a nonzero net force, the object accelerates, meaning its speed, direction or both can change.
Gravity, friction and support forces are common examples in everyday situations.
Mass affects how strongly an object's motion changes for a given net force.
Forces also occur through interactions between objects, and the forces in an interaction pair act on different objects.
Once you learn to identify the forces acting on one object, many everyday movements become much easier to explain.
Continue learning
The related lesson will help you identify and describe the forces acting on everyday objects using simple force diagrams.