The document discusses forces and friction. It begins by stating the learning objectives of identifying forces on objects using free body diagrams and explaining the effects of friction. It then defines different types of forces - contact forces like normal reaction and tension, and non-contact forces like gravitational, electric, and magnetic. It also discusses how to draw free body diagrams and examples like a crate held by a string. Finally, it defines friction as the opposing contact force and discusses how friction affects motion, giving examples like braking a car. It ends by stating some factors that affect the frictional force between surfaces.

2. At the end of the lesson, you should be able to:
a) Identify forces acting on an object and draw
free body diagram(s) representing the forces
acting on the object (for cases involving
forces acting in at most 2 dimensions)
b) Explain the effects of friction on the
motion of a body

3. • A push or a pull – represented by arrows
• S.I. Unit: newton (N)
Contact Forces – force touching the object
Normal Reaction Friction Tension
Push exerted by a
surface on an object
pressing on it – always
perpendicular to the
surface.
Force that opposes or
tends to oppose motion
between surfaces in
contact
Pull exerted by a
stretched spring, string
or rope on an object
attached to it.
N
T
f

4. Non-Contact Forces – force not touching the object
Gravitational Force Electric Force Magnetic Force
Pull exerted by Earth’s
gravity on any object
(i.e. weight)
The attractive (pull) or
repulsive (push) forces
between electric
charges.
The attractive (pull) or
repulsive (push) forces
between magnets.
W
+ -
Attractive electric forces
between unlike charges.
+ +
Repulsive electric forces
between like charges.
S N S N
Attractive magnetic
forces between unlike
poles.
S NN S
Repulsive magnetic
forces between like
charges.

5. Effects of a force
Move a body at
rest
Increase the
speed of a moving
body
Decrease the
speed of a moving
body
Change direction
of a moving body
Throwing a ball
Kicking a rolling
ball to make it
move faster
Using your feet to
cushion a rolling
ball slowly to a
stop
Kicking a rolling
ball back to its
original position

7. At the end of the lesson, you should be able to:
• Identify forces acting on an object and draw
free body diagram(s) representing the forces
acting on the object (for cases involving
forces acting in at most 2 dimensions)

8. • A diagram which shows all the forces acting
on a body
• Consists of a sketch of a body and arrows
representing the forces acting on it.
Weight
Normal reaction force
Tension
Friction

9. • Weight of a body is the gravitational force
exerted by the Earth on the body.
• Always acts at the centre of gravity of a
body.
• Directed towards the centre of the Earth
(always vertically downwards)
W

10. • Normal reaction force is the force exerted by
the contact surface on the body
Acts perpendicularly to the contact surface
• Two forces act on the body:
the Normal reaction force and the weight.
W
N

11. • The tension is directed along the length of the
string and pulls equally on the support and
the body.
• But this is not a free body diagram!
There are 3 bodies – rod, string, object
T
T

12. • Isolate the body as a free-body
• Free-body diagram of the body consists of the
tension and weight only.
T
W

13. • The free-body diagram of the crate would consist
of
• The weight of the crate,
• The tension force exerted on the crate,
• The contact force of the floor of the crate.

14. • Weight and contact force are equal. No
resultant force acting vertically.
• Resultant force of crate is due to the tension
force.

15. Useful or Useless?

16. At the end of the lesson, you should be able to:
• Explain the effects of friction on the motion of
a body

17. • Friction is the contact force that oppose or
tends to oppose motion between surfaces in
contact.
• Represented by small f
• Result from irregularities of surfaces
• Must be drawn along the surface of contact
• Slows down or stops an object in motion
F
f

18. • Air resistance, or drag, is the friction
between a moving object and the air it is
moving through.
• Slows vehicles down and uses more fuel
• Streamlining helps overcome air resistance

19. • To move forward, you place one foot in front
of you and push you other foot backwards on
the ground.
• The friction between your shoe and the
ground prevents slipping.

20. • As the wheel turns, friction
is generated between the
wheel and the road.
• At point of contact, wheel is
moving backwards.
• Friction acts in the opposite
direction, providing the
forward force that drives
the car.

21. • Without friction, the wheels will spin without
helping the car to move forward.
E.g. Car stuck in mud

22. • Wear and tear of surfaces
e.g. Soles of shoes

23. The frictional force between two surfaces on a horizontal
plane changes with several factors.
• Nature of the surfaces in contact
• the rougher the surface the greater is the friction
• Proportional to the force pressing the surfaces
together
• in the case of horizontal movement, the pressing force is the
weight.
• Independent of the area of contact!
• A larger area does not mean greater friction!

24. Complete Worksheet 1in pencil.
We will discuss the answer in class.