1. What are forces?
• Forces act on you all the time:
• They are defined as the push or pull or twist
on an object.
• Look at the following pictures and try to work
out where the forces are and in which
direction forces they are acting.

2. 1. 2. 3.
4. 5. 6.

3. The direction of the force is indicated by an arrow
• The size of the arrow indicates the size of the
force a small arrow is less force
• A longer larger arrow is a greater force

4. When all the forces equal each other there is no change in
movement and the object remains stationary or balanced

9. A constant forward motion indicates a balanced force.
If the cyclist started to pedal harder what would happen?

10. What is happening here?

11. Movement
Movement results in one force being bigger
than another force.
We call this an unbalanced force
The object will move in the direction of the
greater force.

14. Measuring forces
• A spring stretches
when a pulling force
acts on it
• It compresses when
a pushing force acts
on it.
• The bigger the force
the more it stretches
or compresses.

15. Measuring forces
• A spring can be used
to measure force by
the needle changing
when the length of
the spring changes.
• A spring balance
measures pull
• Bathroom scales
measure push

16. Weight and mass
• Weight is not the same as Mass
• Mass is the amount of matter an object contains and measured in Kg
• Weightis the force of the gravitational pull on the mass measured in
Newtons
• Weight = mass x gravity ( approx 10)
• Yr 7 student mass = 40 Kg
• Gravity = 10N
• Weight of student is ??

17. Approximate sizes of some forces
• Force to lift and apple 1N
• Force to lift 1Kg bag of sugar 10 N
• Force you exert by sitting on someone 500N
• Force to launch a space rocket 33 000 000N
• Force can also be used in a horizontal direction
eg. Pulling a door open

19. Gravity
• Gravity is a pulling force between things that have
mass.
• The greater the mass of the objects the greater the
force ie Earth has a larger gravitational pull than the
moon which is smaller.
The further the distance
from the force the
weaker it becomes

20. Measuring force
The unit to measure force is the Newton
named after the famous scientist Sir Isaac Newton a professor of
mathematics and known for his theory of gravitational forces

21. Planets in orbit due to gravitational attraction

24. Falling objects
• Things fall because of gravity.
• The objects falls through air
particles thus creating an air
resistance.
• Air resistance opposes gravity
and slows down the falling
object
• The greater the surface area the
greater the air resistance.
• Air resistance is dependent on
the size shape and speed of the
falling object
See video clip

25. Terminal velocity
• As a falling objects speed
increases so does the air
resistance.
• Eventually the air resistance
acting in the object is equal
to its weight force
• The forces are balanced and
the object is falling at a
constant speed or its
terminal velocity

26. Video clip

27. • Gravity is considered a NON CONTACT force because it is not
touching the object to have an effect on it.
• Other non contact forces
• are magnetic forces
• and electrostatic forces.

28. The strength of the gravitational pull is always the same 10N but the larger
the mass of an object the larger the weight force
ie Wt = mass X gravity
.
The greater the mass the
• The more force will be
required to pull
• The more force will be
required to push
Think of an other examples to illustrate this concept.

30. Friction:
A contact force unit 7:2

31. Friction exists whenever 2 surfaces are in contact;
the force that makes most moving objects stop is FRICTION
• Friction provides grip needed by cars, bikes and your shoes to
get moving. Think about running on a smooth or slippery
surface vs a rough surface.
• Friction is needed to change direction and slow down.
• Friction causes heat: rub your hands together quckley.
• Friction can cause problems in machinery when the moving
parts overheat

32. What affects friction?
• How rough the surfaces are:
• How hard the surfaces are pushed together:
• Due to how heavy the object is

33. Reducing Friction
• Rolling surfaces
• Lubricants
• Streamlined smooth
• Reducing area in contact with each other.

34. Inertia
The tendency to resist any form of change in motion.

35. Newtons first law of motion
• An object will remain at rest unless acted on
by a force.
• An object that is moving will continue to move
at the same speed and in the same direction
unless an unbalanced force acts on it

36. Inertia of the coin

37. The resistance of an object to change its state of motion
If the car comes to a sudden stop the passengers keep moving forward until another
force stops them ie windscreen, steering wheel of hopefully seat belt

38. A moving object will stay in that line of motion unless
acted on by a force

39. Newtons second law of motion states
• An object will accelerate in the direction of an
unbalanced force acting upon it.
• The size of the acceleration depends upon the
mass of the object and the size of the force
acting.
• F= m x a
F
m a

40. Same mass

41. Same force different mass =
less acceleration for the larger mass

42. More acceleration more force applied to
the ball

43. Newtons third law of motion
• For every action there is an equal and
opposite reaction

46. Which Law and why?