3 IGCSE-CAM -CHP-3-FORCES AND MOTION.ppt

caie igcse forces and motion note

1. 3. Forces and Motion
3.1 Balanced and unbalanced forces
3.2 Friction
3.3 Circular motion

2. Learning outcomes
•Candidates should be able to:
•(a) state Newton’s third law.
•(b) describe the effect of balanced and unbalanced forces on a body.
•(c) describe the ways in which a force may change the motion of a body.
•(d) recall and use the equation force = mass × acceleration.
•(e) explain that friction is a force that impedes motion and produces heating.
•(f) discuss the effect of friction on the motion of a vehicle in the context of
tyre surface, road conditions
•(including skidding), braking force, braking distance, thinking distance and
stopping distance.
•(g) describe qualitatively motion in a circular path due to a constant
perpendicular force, including
•electrostatic forces on an electron in an atom and gravitational forces on a
satellite. (F = mv 2 / r is not
•required.)
•(h) discuss how ideas of circular motion are related to the motion of planets
in the solar system.

3. Learning outcomes
•Candidates should be able to:
•(i) state that a force may produce a change in size and shape of a
body.
•(j) *plot, draw and interpret extension-load graphs for an elastic solid
and describe the associated
•experimental procedure.
•(k) *recognise the significance of the term “limit of proportionality” for
an elastic solid (an understanding of
•the elastic limit is not required).
•(l) calculate extensions for an elastic solid using proportionality.

4. Force
A force is a push or a pull of
one object on another.
Force can not be seen but
its effect can be seen
A force can cause an object
to:
–speed up
–slow down
–change direction
–change shape
Force is measured in
newtons (N).
Force is measured with a
newtonmeter.
newtonmeters

5. Some types of force
1. Gravitational Force
This is the attractive force exerted
between bodies because of their
masses.
This force increases if either or both
of the masses is increased and
decreases if they are moved further
apart.
Weight is the gravitational force of
the Earth on an object.
Bathroom scales measure
weight.
A mass of 1kg weighs
about 10N
1 stone is about 63N.

6. 2. Normal reaction or contact
This is the repulsive force that
stops two touching bodies
moving into each other.
The word ’normal’ means that
this force acts at 90° to the
surfaces of the bodies.
It is caused by repulsive
molecular forces.
normal reaction forces
weight
The two upward reaction forces
on the tyres balance the
downward weight of the car

7. 3. Friction
This is the force that opposes motion.
The kinetic energy of the moving object is converted
to heat energy by the force of friction.
Friction is needed for racing
cars to grip the road
Friction is needed for
walking!

8. 4. Air resistance or drag
This is the force that opposes the movement of objects
through air.
Drag is a more general term used for the opposition
force in any gas or liquid.
Objects are often streamlined to reduce this force.
streamlined car
a parachute
maximises drag force

9. 5. Upthrust
This is the force
experienced by objects
when they are placed into
a fluid (liquid or gas).
An object will float on a
liquid if the upthrust force
equals its weight.
A hot air balloon rises when the
upthrust from the surrounding
air is greater than the balloon’s
weight.

10. 6. Magnetic
Between magnets
but also the force that
allows electric motors
to work.
7. Electrostatic
Attractive and
repulsive forces due
to bodies being
charged.
Electrostatic force causes the
girls’ hair to rise when they
touch the Van der Graaff
generator.

11. Choose appropriate words to fill in the gaps below:
A _____ is a push or a pull. A force can cause an object to
___________ or change shape.
Force is measured in _______ (N) with a newtonmeter.
There are many types of force. ________ force occurs when
two bodies touch each other.
Friction is a force that _______ the _______ of one body
relative to another. It is caused by the _________ forces
between ___________.
attractive
motioncontact
newtons
opposes
forceaccelerate
WORD SELECTION:
molecules
attractive
motion
contact
newtons
opposes
force
accelerate
molecules

12. Force mass and acceleration
The force, mass and acceleration of an object
are related by the equation:
force = mass × acceleration
F = m x a
force is measured in N
mass is measured in kg
acceleration is measured in m/s
2

13. also:
acceleration =
and:
mass =
ma
F
force
mass
force
acceleration

15. Checking the equation

16. Question 1
Calculate the force required to cause a car
of mass 1200 kg to accelerate by 5 m/s
2
.
F = m x a
= 1200 kg x 5 m/s
2
Force = 6000 N

17. Question 2
Calculate the acceleration produced by a
force of 200N on a mass of 4kg.
F = m x a
becomes: a = F ÷ m
= 200N ÷ 4kg
acceleration = 50 m/s
2

18. Question 3
Calculate the force that accelerates a mass of
300kg from rest to 6 m/s over a time of 3 seconds.
acceleration = change in velocity ÷ time
= (6 – 0)m/s ÷ 3s
acceleration = 2 m/s
2

F = m x a
= 300kg x 2 m/s
2

force = 600N

19. Answers
F m a
24 N 4 kg 6 m/s
2
200 N 40 kg 5 m/s
2
600 N 30 kg 20 m/s
2
2 N 5 g 400 m/s
2
5 N 100 g 50 cm/s
2
24 N
40 kg
20
2 N
5000
Complete:

20. Car forces
When a vehicle travels at a steady speed the frictional forces balance the
driving force.
To slow the car the engine force is reduced by releasing the throttle and the
frictional force is increased by applying the brakes.

21. Stopping a car
The total distance required to stop a car, the
stopping distance, is equal to the thinking
distance plus the braking distance.

22. Factors affecting stopping distance
1. The reaction time of the driver
This will increase if the driver is tired, distracted or
has consumed alcohol or drugs. Increasing
reaction time increases the thinking distance.
2. The speed of the car
The greater the speed the greater will be both the
thinking and braking distances.
Doubling the speed increases the overall stopping
distance by about four times.

23. 3. The mass of the car and its contents
The greater the mass the greater will be the
braking distance.

4. The condition of the road
Wet and icy roads will cause the braking distance
to increase.
5. The condition of the vehicle
Worn brakes or worn tyres will both increase the
braking distance.

24. Choose appropriate words to fill in the gaps below:
When a car is moving at a _______ speed the engine force is
equal to the resistive force.
The __________ distance of a car is equal to the thinking
distance plus the _________ distance.
Tiredness, ________ and drugs are all likely to increase the
__________ distance.
A car travelling at 60 m.p.h. will require roughly ______
times stopping __________ of a car travelling at 30 m.p.h..
alcoholfour distancesteady stoppingthinking
WORD SELECTION:
braking
alcohol
four
distance
steady
stopping
thinking
braking

25. Mass and weight
Mass is the amount of matter in an
object.
Mass is measured in kilograms.
The mass of an object is the same on
the Moon as on the Earth.
Weight is the force of gravity on an
object.
Weight is measured in newtons.
The weight of an object on the Moon is
about one sixth that on the Earth.
A newtonmeter is used to determine
the weight of the parcel

26. The acceleration due to gravity (g)
The acceleration due to gravity (g) varies with planet, moon and star
and depends on the height of an object.
Some examples of the acceleration due to gravity:
Location m/s
2
Location m/s
2
Earth 10 Jupiter 24
Moon 1.6 Pluto 0.7
Mars 3.7 The Sun 270

27. Gravitational field strength (g)
This is an alternative way of measuring the strength of gravity.
The gravitational field strength is equal to the gravitational force exerted per kilogram.
Near the Earth’s surface, g = 10 N/kg
In most cases gravitational field strength in N/kg is numerically equal to the acceleration due to gravity in m/s
2
, hence they both use the same symbol ‘g’.

28. Calculating weight
weight = mass × gravitational acceleration
W = m x g
weight is measured in N
mass is measured in kg
gravitational acceleration is measured in m/s
2
On the Earth’s surface a mass of 1kg
has a weight of 10N.

29. Falling objects
When an object falls through air
or some other fluid initially the
only significant force acting on it
is the downward pull of gravity.
On Earth, it will initially accelerate
downwards at 10 m/s
2
.

30. As the object speeds up
frictional forces such as air
resistance become greater the
faster the object moves.
Eventually the resultant force
on the object will be zero when
the frictional forces equal the
weight of the object.
The object then moves at a
constant speed called terminal
velocity.

31. Velocity-time graphs for a falling object

32. Parachuting
A parachutist will have two
different terminal velocities.
Before opening the parachute it
is about 60 m/s (140 m.p.h..).
Afterwards, due the much greater
drag force, the terminal velocity is
about 5 m/s (12 m.p.h.)

33. velocity
time
first terminal
velocity
initial
acceleration
= 10 m/s
2
Velocity-time graph of a parachutist
second terminal
velocity
parachute
opened
ground
reached

34. Vectors and Scalars
All physical quantities (e.g. speed and force) are
described by a magnitude and a unit.
VECTORS – also need to have their direction
specified
examples: displacement, velocity, acceleration, force,
Moment, Momentum
SCALARS – do not have a direction
examples: distance, speed, mass, work, energy.

35. Addition of vectors
The original vectors are called COMPONENT vectors.
The final overall vector is called the RESULTANT vector.
4N
6N
object
4N
6N
object
resultant = 10N
object
4N 6N
object
4N6N
object
resultant = 2N
object

36. Resultant force
A number of forces acting
on a body may be
replaced by a single force
which has the same effect
on the body as the original
forces all acting together.
This overall force is called
resultant force.
In the example opposite,
5N is the resultant force of
the 3N and 2N forces.
3N
2N
5N

37. Determine the resultant force in the cases below:
4N
6N
1.
4N3N
2.
7N
3N
3.
2N
6N
4.
4N
4N
5.
10N 1N
4N
4N
There is no resultant
force in this case

38. Resultant force and motion
Resultant force Effect on the motion
of an object
Zero
Object’s velocity stays
the same including
staying stationary
In the direction the
object is moving
Object accelerates
In the opposite
direction in which the
object is moving
Object decelerates

39. Examples 1 & 2
The box will move
when the man’s push
force is greater than
the friction force.
The plane will
accelerate provided
that the engine force is
greater than the drag
force.

40. Examples 3 & 4
The brakes exert a
resultant force in
the opposite
direction to the
car’s motion
causing the car to
decelerate.
Once released, the
glider moves at a near
constant velocity as it
experiences a nearly
zero horizontal
resultant force.