Forces can cause objects to move, stop, change speed or direction. There are two main types of forces - contact forces which act when objects touch, and non-contact forces like gravity and magnetism which act over a distance. Friction is a contact force that opposes motion between surfaces in contact. Reducing friction, through lubricants or streamlining, can reduce resistance to motion. Air and water resistance are non-contact forces caused by fluids pushing back on objects moving through them. The shape of objects can be designed to minimize resistance and maximize speed.

1. FORCES AND
MOTION
Physics
Chapter 14.
(Week 1)

2. What is a force?
■ Forces are what makes things move,
like a push or a pull.

3. Forces can:
1) Make
things
move
2) Make a
moving
object stop.
3) Change
the speed
of a moving
object.
4) Change
the direction
of a moving
object.
5) Change
the shape
of an
object.

4. Force is measured in
Newton (N). The more
Newton, the bigger the
force.
How do we
measure
force?
Force can be measured
using a Newton meter
(Force meter/spring
balance).

5. Different types of
forces
Contact forces
Non-Contact
forces
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.
Examples:
- Impact force
- Strain force
- Friction
- Air resistance
- Water resistance
Examples:
- Magnetic force
- Gravitational force
- Weight

6. Contact forces
1) Impact
force- When a moving object
collides with a stationary
object.
- Impact force can be large
ex. Hammer striking nail.
- OR small ex. Air particles
striking the skin.

7. 2) Strain
force- When some materials are
squashed, stretched,
twisted or bent, they exert a
force opposite the direction
of the force acting on them.
- These materials are called
elastic materials.
- The force they undergo
when deformed is called the
strain force.

8. Look at the racket and the tennis
ball.
Draw arrows to represent the

9. - Tension is a strain
force.
- Exerted by a stretched
rope, spring or string.
Pulling
force
Tension
force

10. 3) Friction
- Occurs when an object
(being pushed or pulled)
moves over the surface of
another object.
- As the push or pull on the
object increases, friction
increases.
- The friction that exits
between two objects when
there is no movement is

11. - Push or pull = static friction (no movement)
- Push or pull , than friction the object starts to
move.
- When the object moves there is still friction, it is
called sliding friction.
- The force of the sliding friction is than the

12. A closer look at friction
- The surfaces of objects are not
completely smooth.
- Under the microscope it can be
seen that they have tiny
projections with hollows between
them.

13. Metal objects appear smooth to our
eyes.
Have a look what they look like under
the microscope.

14. - 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.

15. Reducing friction
- If a liquid is placed between the
two surfaces, the projections are
forced apart a little and the
number of connections is
reduced.

16. - Reducing connections means
reducing friction.
- 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.

17. - But, oil between moving metal
parts of an engine reduces
friction and reduces wear on the
metal parts.

18. Increasing friction
- 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.

19. Friction AND ROAD SAFETY
Driver in a
moving car
sees a hazard
Driver reacts
and applies
Brakes .
Car stops
Thinking
distance
Braking
distance
Speed Thinking distance /m Braking distance /m Total stopping
distance /m
48 km /h
(30 mph)
9 14 23
80 km/h
(50 mph)
15 38 53
112 km /h
(70 mph)
21 75 96

20. 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.

21. Other forces affecting speed
Friction Air resistance Water resistance

22. Air resistance
When an object moves through air it pushes air out of the
way.
Air moves over the object sides and pushes back on the
object.
This push on the object is called air resistance or DRAG.

23. The value of air resistance depends on the size and shape of an
object .
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
streamlined shapes.

24. ■ 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.’

25. The air resistance
produced by a parachute
is also used to bring sky
divers safely to the
ground.
The resistance of the gases
in the atmosphere of other
planets in the solar system is
used to land space probes
safely.

26. Water resistance
■ 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.

27. ■ Object that move through water quickly have a streamlined
shapes .
■ A barracuda which moves quickly through water has a much
more streamlined shape than the slow-moving sun fish.
Fast-moving
Barracuda fish
Slow-moving
Sun fish

28. ■ 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 .
Hydrofoil boat
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 .