We can't actually see forces, but we see their effect
on objects. We see the Moon orbit the Earth, objects fall to the ground,
and birds fly - all of these are because of forces.
force is a push or a pull. We measure forces in "Newtons"
Sir Isaac Newton (1642
1 Newton isn't
a very big force: it's about the weight of an apple.
the direction is important.
- the speed of
- the direction that
an object is moving in
- the shape
of an object.
is a force that acts towards the centre of the
This means that,
wherever you are in the world, "down" is always towards
the ground - even though your "down" isn't the same direction
as anybody else's.
pull of the Earth is what gives objects weight.
Thus weight is a force - it's how hard the Earth is pulling
on an object.
The Earth pulls
on every kilogramme with a force of ten Newtons.
We say that the Earth's gravitational field strength (at ground
level) is 10 Newtons per kilogramme (10 N/kg) (Actually,
it's more like 9.81, but for GCSE we usually call it 10.)
In other words, an object with a
mass of 1kg
has a weight of 10N.
is a very weak force, you need a very large mass in order
to get a noticeable gravitational pull.
An odd thing about gravity - it always attracts objects and never
on the question to reveal the answers)
How hard does the Earth's gravity pull on a 2kg object?
What is the weight of a 4kg rock?
What is the weight of a 100 gramme plastic toy?
Isaac Newton came up with three "Laws
of Motion" back in the 17th century, which we still find useful today.
For the moment, we need:-
says that if the forces on an object are in balance,
the object's speed and direction of motion won't
(In other words, if you leave it alone, it'll carry on doing whatever
it was doing already.)
the forces on an object are in balance,
then the object's velocity is constant.
This means that
if it's not moving, it'll stay still; or if it is moving,
it'll continue in a straight line at a constant speed .
What if the
forces aren't in balance? See Acceleration page
|Here is a picture
of a parchustist, descending at a constant speed.
- What forces
- What can
you say about the size of the forces?
on the picture to see the answer
forces act on an object, we can add them together to find out what's
going to happen. The combined effect of all the forces on an object
is called the resultant force.
GCSE, you'll usually only be asked to deal with forces which are
all in a straight line.
This makes it really easy!
if I push a car with a force of 500N, and somebody else pushes the
other way with a force of 400N, what is the resultant force on the
Click here for answer.
If these pictures aren't moving, click
the forces are not in a straight line, we must use other techniques,
such as a "triangle of forces", to find the resultant.
this picture isn't moving, click on
|| Newton's Third
every action has an equal and opposite reaction.
other words, when you push or pull something, it pushes or pulls
back at you. Forces are two-way affairs, they're about the way that
objects affect each other.
you're pushing down on your chair with a force of, maybe,
500N, and the chair is pushing back with a force of 500N.
The forces are balanced, and as you're already sat still,
you stay that way.