Force is push or pull.


Unbalanced forces make an object accelerate


Forces on an object are usually balanced: forces in one direction are equal to forces in the opposite direction:

forces balanced
Examples of Balanced Forces
No acceleration.

Example: The forces are in balance at the top of this bridge tower.

suspension bridge

The cables pull downwards equally to the left and right, and that is balanced by the tower's upwards push. (Does the tower push? Yes! Imagine you stand there instead of the tower.)

We can model the forces like this:

suspension bridge forces

By putting them head-to-tail we see they close back on themselves, meaning the net effect is zero:

suspension bridge forces
The forces are in balance.


But when forces are unbalanced the object accelerates.

forces unbalanced
Examples of Unbalanced Forces.
There is acceleration.

ball kick


Kicking a ball is an unbalanced force that makes it accelerate very quickly.

Once it loses contact with your foot that force (and its acceleration) stops.


The ball would continue to travel in a straight line at a fixed velocity, except that other forces act on the ball:

ball forces: kick, gravity, air

Acceleration: an increase or decrease in velocity

A decrease in velocity can also be called deceleration.

Force and Acceleration are linked.

The more force, the more acceleration.

But objects with more mass are harder to accelerate

The more mass, the less acceleration.

Imagine trying to kick a stone ball:

ball kick massive stone

The famous formula is:

F = ma

"Newton's Second Law of Motion"
(F is force, m is mass and a is acceleration)

Dividing both sides by m makes it clearer:

f/m = a


The idea that mass resists acceleration is called inertia.

Inertia means the object wants to continue to do what it does.

If it is at rest (in relation to us), it continues to be at rest. If it is moving East at 20 m/s, it continues moving East at 20 m/s. etc.

container ship

This oil tanker has a large inertia. It is:


It seems that mass doesn't like being pushed around!


In honor of Sir Isaac Newton the unit of force is the Newton (abbreviation is N).

A Newton is the force it takes to make 1 kg change its velocity by 1 m/s every second.

In other words:   1 kg changes its velocity by 1 m/s every second
Or:   1 kg changes 1 m/s per second
Or:   1 kg  m/s  /s

The two lots of "/s" (per second) become "/s2" (per second squared), so:

1 Newton is 1 kg m/s2 (kilogram meter per second squared)

Example: A 2000kg car goes from 0 to 15 m/s in 5 seconds. What is the force?

The acceleration is 15 m/s over 5 seconds, or 15 m/s 5 s = 3 m/s s = 3 m/s2

Now let's calculate the force:

F = ma

F = 2000 kg × 3 m/s2

F = 6000 kg m/s2

F = 6000 N

F = 6 kN

The force is 6 kN (6 kiloNewtons).

Forces are Interactions

stone finger force


In the words of Sir Isaac Newton (his Third Law of Motion):

"To every action there is always opposed an equal reaction"

Today we say "Every action has an equal and opposite reaction"

He also gave this example:

"If you press a stone with your finger, the finger is also pressed by the stone."


Force has magnitude and direction. So it is a vector:

vector magnitude and direction

Acceleration also has magnitude and direction. So it too is a vector.


force beam strut


Read the page devoted to Force Calculations


Note: when we say something is "at rest" or "moving at 4 m/s" we forget to say "in relation to me" or "in relation to the ground", etc.

earth rotate

Think about this: are you really standing still? You are on planet Earth which is spinning at 40,075 km per day (about 1675 km/h or 465 m/s), and moving around the Sun at about 100,000 km/h, which is itself moving through the Galaxy.

It is all relative!



Newton's Three Laws

Back in 1687 Newton wrote three laws about motion, which basically are: