What is a force?
■ Forces are what makes things move,
like a push or a pull.
2) Make a
of a moving
of a moving
Force is measured in
Newton (N). The more
Newton, the bigger the
How do we
Force can be measured
using a Newton meter
Different types of
Occurs when the material or object
exerting the force touches the object
or material on which the force acts.
Occurs when the materials or objects
do not touch each other.
- Impact force
- Strain force
- Air resistance
- Water resistance
- Magnetic force
- Gravitational force
force- When a moving object
collides with a stationary
- Impact force can be large
ex. Hammer striking nail.
- OR small ex. Air particles
striking the skin.
force- When some materials are
twisted or bent, they exert a
force opposite the direction
of the force acting on them.
- These materials are called
- The force they undergo
when deformed is called the
Look at the racket and the tennis
Draw arrows to represent the
- Tension is a strain
- Exerted by a stretched
rope, spring or string.
- Occurs when an object
(being pushed or pulled)
moves over the surface of
- As the push or pull on the
object increases, friction
- The friction that exits
between two objects when
there is no movement is
- Push or pull = static friction (no movement)
- Push or pull , than friction the object starts to
- When the object moves there is still friction, it is
called sliding friction.
- The force of the sliding friction is than the
A closer look at friction
- The surfaces of objects are not
- Under the microscope it can be
seen that they have tiny
projections with hollows between
Metal objects appear smooth to our
Have a look what they look like under
- When projections from one
surface meet the projections from
another surface they stick.
- These connections between the
surface produce the force of
friction between the objects.
- If a liquid is placed between the
two surfaces, the projections are
forced apart a little and the
number of connections is
- Reducing connections means
- Reducing friction can be helpful
or cause problems.
- Example: Water running
between car tyres and the road
reduces friction and increases
the chance of skidding.
- But, oil between moving metal
parts of an engine reduces
friction and reduces wear on the
- To increase friction between 2
surfaces we should press the
surfaces together more strongly.
- This makes the projections press
against each other more and
increases the connections
between the surfaces.
Friction AND ROAD SAFETY
Driver in a
sees a hazard
Speed Thinking distance /m Braking distance /m Total stopping
48 km /h
9 14 23
15 38 53
112 km /h
21 75 96
Thinking and braking distance
■ Thinking distance
■ The distance travelled by the car before the driver reacts
and applies the brakes.
■ Braking distance
■ The distance covered by the car after the brakes are
applied and before the car stops.
Other forces affecting speed
Friction Air resistance Water resistance
When an object moves through air it pushes air out of the
Air moves over the object sides and pushes back on the
This push on the object is called air resistance or DRAG.
The value of air resistance depends on the size and shape of an
Many cars are designed to move with low resistance.
The cars body is designed like a wedge to cut it’s way through the
air . And the surfaces are curved to allow the air to flow over the
sides with minimum drag .
Shapes that are designed to reduce air resistance are called
■ A dragster is a vehicle that accelerates very quickly.
■ At the end of the race they are slowed down by
brakes and a parachute.
■ The parachute has a large surface area that offers
high air resistance ( drag ) that slows down the
dragster and stops it in a short distance.’
The air resistance
produced by a parachute
is also used to bring sky
divers safely to the
The resistance of the gases
in the atmosphere of other
planets in the solar system is
used to land space probes
■ When an object moves through water it
pushes water out of the way.
■ Water moves over the object sides and
pushes back on the object .
■ This push on the object is called water
resistance or DRAG.
■ Object that move through water quickly have a streamlined
■ A barracuda which moves quickly through water has a much
more streamlined shape than the slow-moving sun fish.
■ Water resistance affects the movement of boats and
ships on the water surface .
■ Boats designed for high speed are hull shaped to
reduce water resistance .
Some boats are equipped with
a device called a hydrofoil
which reduces the area of
contact between the boat and
the water to the minimum
making the boat move quickly
over the water surface .