The document discusses different types of forces including magnetic, weight, tension, contact, and friction. It explains that a net force is the sum of all individual forces acting on an object and can cause the object to accelerate. Friction opposes motion and is an example of a contact force that must be overcome for an object to move.

1. FORCES
Sec 4
http://www.uq.edu.au/_School_Science_Lessons/PushPull0.GIF

2. BTEC, you should be able to:
 Describe the effects of a force
 Recall the relationship resultant force =
mass x acceleration (F = m x a)
 Solve related problems with above
relationships
 Understand mass, weight and acceleration
 Understand weight and able to differentiate
weight and mass
 Recall the relationship weight = mass x
gravity Understand Inertia

3. Force
• Think of force as a PUSH or PULL
PUSH PULL
• The block was pushed/pulled by tractor
• Both blocks experience a force to the right.
• Force felt by block is provided by tractor

4. Types of Forces
• Magnetic force
– force due to magnetic attraction and repulsion
• Weight
– force acting on a body due to gravitational pull
• Tension
– force at the ends of a stretched string, spring or
rope
• Contact force
– force that appears only when there is contact
between two bodies
• Friction
– force due to roughness of surfaces

5. Force
• When a force acts on an object, it tends to
cause the object to accelerate.
• Other effects of force are:
– Decelerate an object
– Change the direction of motion of a body
– Change the shape and size of a body

6. Effect of force
• Make a stationary object move

7. Effect of force
– Change the speed of a body

8. Effect of force
– Change the direction of motion of a
body without its speed

9. Effect ofand size of a body
• Change the shape
force

10. Understanding Net Force
5N
15 N
10 N =
• What happens if an object is pushed by
two set of forces as shown above?
• The result is that the object
experiences a SUM of the forces, which
is 10 + 5 = 15 N.
• In this case, we say that the net force
is 15 N.

11. Understanding Net Force
5N
5N
10 N
=
• What happens if an object is pushed by two
set of opposite forces as shown above?
• The result is that the object experiences a
SUM of the forces, which is 10 - 5 = 5 N.
• In this case, we say that the net force is 5 N.

12. Net force
• What is the net force acting on objects below?
2N A 8N A 6N
10N
B B 12N
2N
No net
8N C 8N C force
5N
D 2N D
10N 3N

13. Net force
• What is the net force acting on objects below?
2N A 8N A 6N
10N
B B 12N
2N
No net
8N C 8N C force
5N
D 2N D
10N 3N

14. Resolving Forces in 2D

15. Force
• Force can be calculated with this
simple equation:
F = m×a
Net Force Mass Acceleration
(N) (kg) (m/s 2)
SI Unit: newton ; symbol : N

16. Quick Check
• A car of mass 1500 kg accelerates at
2 m/s2. Calculate the forward force
provided by the engine.
F=mxa
= 1500 x 2
= 3000 N

17. Quick Check
• A 1800 kg van is reversing.
Backward force provided by the
engine is 2400 N. Calculate the
backward acceleration of the van.
F=mxa
2400 = 1800 x a
a = 2400 / 1800
= 1.33 m/s2

18. Quick Check
• A boat of mass 500kg is moving against the flow of a
river. Current of river produces a force of 1000 N.
Engine of the boat produces a forward force of 3000
N. What is the net force acting on the boat?
Net force = 3000 – 1000 = 2000 N
3000 N
1000 N
• What is the acceleration of the boat?
F=mxa
2000 = 500 x a
a = 2000 / 500 = 4 m/s2

19. Quick Check
10,000 N
• A small rocket with mass 800
kg has a weight of 8000 N. At
take-off, the rocket engine
produces an upward force of
10 000 N.
• What is the net upward force
acting on the rocket during
take-off?
Net force, F = 10,000 – 8000
= 2,000 N 8000 N

20. Quick Check 10,000 N
• What is the acceleration of the
rocket?
F=mxa 
a = F/m
= 2000 N / 800 kg
= 2.5 m/s2
8000 N

21. Mass and Weight
• In the study of mass and weight, you first
need to forget the day-to-day understanding
of weight.
• For example, “the weight of a pupil is 50 kg”
is WRONG! FORGET THIS, at least
temporarily.
• Mass of an object can be
understood as the amount of atoms
that makes up the object.*
* This is not strictly correct, but as a beginner
it is useful to think of it this way

22. Mass
• Mass of a handphone is always 0.2
kg, in Singapore, or US or Brazil or
even on the Moon or Mars.
• A 1.5 liter PET bottle of Coke will
always have the same mass even if
the drink is poured out into a
container. (shape of liquid has
changed but not the mass)

23. Mass
• In Physics is defined as (this means that if
asked in exams, please write this statement
down)
A measure of the amount of matter in
a body
• Symbol: m
• SI Unit: kilogram (kg)
Eraser
• Mass of an object cannot be changed by its:
– Shape
– Location

24. Weight
• Weight of an object is the effect of gravity
on it.
• For instance, a box has a mass of 1 kg.
(this means that the box has a great
number of atoms worth 1 kg)
• For a 1kg box on Earth, it weighs 10 N.
However if the box is on the Moon, where
the gravity is only 1/6 of Earth, the box
weighs only 10/6 N.

25. Weight
• In other words, you might not be
able to lift a 60kg dumb-bell on Earth
but on the Moon, you possibly can
because it is now appears lighter.
• On Earth, the dumb-bell weighs 600
N while on the Moon is only 100 N.

26. Weight
• Weight of an object can be measured
by simply multiplying the mass of a
object with the gravity of its location.
W = m× g
Weight = Mass x Gravitational
of object of object strength
(N) (kg) (N/kg)

27. Quick check
• A book has a mass of 1.5 kg. What is its
weight on Earth?
W=mxg
= 1.5 x 10
= 15 N
• If gravitational acceleration of Moon is 1/6 of
Earth’s, how much is the book’s weight on
the Moon.
W=mxg Ask yourself, what is
= 1.5 x (1/6 x 10) the mass of the book
on the Moon? Has it
= 2.5 N changed? Can it
change?

28. Quick check
• What is the mass of a boy who
weighs 450N on the surface of the
Earth?
W = m x g  m = W/g
= 450/10
= 45 kg

29. Friction
Friction Force, F
• A force that opposes motion
• Block is pulled to the right by force F
• So, block tends to move to the right
• But friction acts on the block towards
the left

30. Friction
Friction Force, F
• Consider that the friction is constant.
This means for two particular surfaces
in contact, the friction force between is
fixed. For other surfaces, the value of
frictional force could be different. *
* not strictly true, but for N-levels,
consider it so

31. Friction
Friction Force, F
• When force applied is more than friction, the
box will start moving. In fact it will
accelerate.
• If the force applied is removed, the box will
stop moving due to friction, since it opposes
motion.
• However if the force is reduced such that it is
equal to friction, the result is that the box will
move at constant speed.

32. Friction – What’s Good about
it?
• Allows us to walk
• Allows a car to move
• Allows us to hold a chopstick

33. Friction – What’s bad about it?
• Causes wear and tear – brake pads of
vehicles, soles of shoes
• Reduces efficiency of performance –
traveling

34. Solutions to overcome
friction
• Moving on air cushion or
electromagnetic fields
• Use rollers or wheels
• Lubricate surface
• Smoothen surface
• Using materials with very low frictional
resistance

35. Quick check
• A 5kg box is pushed with a forward force of
10N on a rough surface with friction of 4N,
what will be the acceleration of the box?
5 kg 10 N
4N
Net force on box = 10 – 4 = 6N
F = m x a  a = F/m
= 6/5
= 1.2 m/s2

36. Quick check
• Tom pushes a crate of mass 20kg
across a floor at constant velocity of
0.2m/s by exerting a horizontal force of
50N. 0.2m/s
Friction 20 kg 50 N
?N
• What is the resultant force acting on
crate?
Since velocity is constant, there is no
acceleration, so (by Newton’s 2nd Law)
resultant force is 0.

37. Quick check
0.2m/s
Friction 20 kg 50 N
?N
• What is the frictional force acting on
crate?
Since resultant force = 0 N, frictional
force must also be 50 N, but in
opposite direction.

38. Quick check
Friction 20 kg 80 N
?N
• Tom now increases his force to 80N.
• What is the new resultant force on
crate?
Frictional force remains at 50 N.
Resultant force = 80 – 50 = 30 N

39. Quick check
Friction 20 kg 80 N
?N
• What is the acceleration of the case?
F = m x a  a = F/m
= 30 / 20
= 1.5 m/s2