# Forces

In this topic, we meet some existing ideas
and add to our understanding.
Work done is the energy
required to move
something. It is also the energy required to overcome
the friction that was stopping the body from
moving. For example, the friction between a box
and the floor, the work done is the energy
required to overcome friction and move it for a
specific distance.

The formula is:

Work Done (J) = Force (N) x Distance moved in that direction(M)

Somethings are not moved along, in fact, they
can be lifted or lowered. If an item is lifted,
then it gains Gravitational Potential Energy.
This can be calculated using another simple
formula:

Change in GPE (J) = Mass (Kg) x g (10N/Kg on Earth) x change in height (M)

This can be both positive (item gets higher) or negative (item is lowered).
For both GPE and Work Done, the rate at which
the item is moved can be calculated in the form
of Power. If it moves quickly, it has more power
and more slowly, less power. The power can be
calculated:

Power (W) = Energy (J) (whether work done or change in GPE) / Time (s)

When an object is moving, it has
kinetic
energy, another form of energy that we have met
before but this time, we can give it values. It
can be calculated using the slightly more
complicated formula of:

Kinetic Energy (J) = ½ x Mass (Kg) x Velocity^{2} (m/s)

An object that is moving has an amount of
kinetic energy and remember that it will
require that exact amount of energy to bring it
to a complete stop. Doesn't matter how fast or
slow it happens (the power), it will always need
that amount of energy.

When
moving objects collide, there is a special way
of describing their energies and how they react
in the collision. We call this measure, the
object's Momentum.

It is a vector quantity like velocity so has
a specific direction and it is calculated using
both the object's mass and its velocity.

Momentum (kgm/s) = Mass (Kg) x Velocity (m/s)

As you can see form the formula, the heavier
an object is, the more momentum it will have and
the faster an object is travelling, the more
momentum it will have.
Providing no other external forces are
applied, there is a law called
The Conservation
of Momentum. This means that the total momentum
before a collision is equal to the total
momentum after a collision. If the cars bounce
apart or move off together in one direction,
they will have the same momentum as they did
when they started.

In an Explosion, objects at the centre will move
away in opposite directions. As they are moving
apart, one has a positive momentum and the other
is negative. Because they are equal and one
positive and the other negative, if you add them
together, the total is 0 kgm/s.

An example, is a bullet leaving a gun. The
gun and the bullet both have the same momentum
but as they are in opposite directions, they add
up to 0 kgm/s.

"To every action, there is an equal and
opposite reaction".

Just like the power measure to tell how
quickly an object was being moved, we have
Impact Forces to tell us how quickly the
momentum of an object is changed. If the
object's momentum is reduced quickly, it has a
large Impact Force and if it is changed slowly,
it has a small Impact Force.

If
you are unlucky enough to be in a car crash, you
want the forces on your body to be as small as
possible. Small impact forces can be achieved by
reducing the car's momentum more slowly. To
absorb some of the energy, cars have
crumple
zones. These areas at the front and back of the
car help to reduce the impact force and use some
of the kinetic energy of the car to bend and
twist the metal that makes up these zones. You
are also protected by seatbelts, air bags,
collapsible steering wheels and a central cage
that should not crumple to protect the
passengers.

This topic relies on your basic understanding
of maths. For example, you should be able to
close your eyes and picture a car travelling at
30 miles per hour. If it travels at that speed
for 1 hour it will have moved 30 miles. So if it
travelled for 30 minutes it will have moved 15
miles and so on.

Speed is the distance an object travels
divided by the time it took. Remember to use SI
units. In this topic refer only to meters,
seconds and meters per second.

Speed = Distance/Time

Velocity is different from speed as it has a
direction. You can have a speed of 3m/s but
velocity must have a direction e.g. 3m/s north.

Acceleration is the rate of change of
velocity or how quickly something speeds up or
slows down. It is given by the formula:

Acceleration = (Final velocity - initial velocity) / Time taken

Or more simply: a=(v-u)/t

The units are meters per second per second (m/s^{2})or ms^{-2}

.
If an object is slowing, acceleration will be
negative. Distance vs Time and Velocity vs Time graphs
are used to display information about journeys.
It is important to be able to translate the
information on a graph into what a person or car
is doing to generate that shape on a graph.

Forces
are one of the most fundamental parts of physics
and they are always measured in Newtons (N).
When objects interact with each other and exert a
force, or push off each other, they exert an
equal and opposite force. For example, if you
lean onto a desk, you are applying a force to
the surface and the desk is applying exactly the
same amount of force back at you to stop you
from falling through it. This is an action and
reaction force.

Note that the force due to Gravity, weight, always
acts towards the center of Earth-straight down.

If you
add up all of the forces acting on a body and
the overall force, called the resultant force,
is zero, then the body does not move or
continues to move at the same speed in the same
direction (this means same velocity from the
previous page).

If you are still or moving in the direction of a
resultant force (which is not 0), you will accelerate in that
direction. If you are moving in the opposite
direction of the resultant force, you will
decelerate until you stop and then begin to
accelerate in the direction of the resultant
force.

Resultant forces always cause a change in
velocity which means that they cause
acceleration. Remember, if the velocity doesn't
change, there is no resultant force.

This can
be calculated by the simple formula,
F =m x a

Force in
Newtons is equal to the mass in kilograms
multiplied by the acceleration in meters per
second squared.

When a
car travels at a constant velocity, the forces
are balanced, no resultant force. The forces of
friction and drag trying to slow the car are
equal and opposite to the forces from the engine
pushing it forward. Linking to the formula
above, F=m x a, the faster the car, the more
deceleration is needed to stop in a specified
distance, the greater the force needed.

Heavier vehicles need more force to decelerate
because they have more mass;

- Faster
vehicles need more force to decelerate because
they need to decelerate more

When you
learn to drive, you will see in the highway code
thinking distance, braking distance and stopping
distance.

-Thinking distance is how far you travel between
seeing a hazard and you applying the brakes.
This time increases if you are tired or have
been on drugs or alcohol. Your reaction time
slows with every bit of alcohol so even if you
are under the legal limit, you are still more at
risk of a collision.

-Breaking distance is how far the car travels
from when you apply the brakes to the car coming to
a complete stop. This distance goes up if: tyres
are worn, brakes are worn, roads are poorly
surfaced or on gravel or mud, if there is snow,
ice or water on the roads

Finally,
we must look back to a fundamental idea which is
gravity. Any object that falls to Earth
accelerates at about 10 m/s^{2}. On
Earth, weight can be calculated using the
formula F=m x a. This time, the force due to
gravity is equal to the mass of the object
multiplied by the acceleration due to gravity on
Earth which is 10 m/s^{2}. We can
therefore work out that a car with a mass of
1000Kg will have a weight of 10000N. This car
will always have a mass of 1000Kg no matter
which planet it sits on or even if floating
through space but the weight will change.

Finally finally - objects all accelerate to
Earth at the same rate BUT air resistance or
drag will slow some down. A feather will take
longer to hit the floor than a pin because the
feather has a lot more air to push out of the
way before it gets to the floor.