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CHAPTER 7: DYNAMIC
• Push and pull are types of force.
• We cannot see force but we can see the
effects of force.
• Activities such as pushing a box, climbing,
riding a bicycle, pulling a rope involve the use
of force.
• Force can change the shape, position, speed,
size, stop and direction of the movement of an
object.
Type of force
Not work done
•
•
•
•
•
Push a wall
Sit on the chair
Sleep on a bed
Read a book
Stand up
FORCE
TYPE
OF
FORCE
Magnetic
Force
Electrostatic
force
friction
MEASURING
FORCE
Electrical
Force
Gravitational
Force
Spring
Balance
FRICTIONAL
FORCE
Direction
Magnitude
WORK
Work =
Force X
Distance
Work =
Power X
Time
Frictional Force (friction)
• friction occurs whenever
two surfaces rub against
each other. Friction is a
force that always oppose
motion.
Gravitational Force (gravity)
• gravitational force or
gravity is the force of
attraction that pulls
objects towards the
Earth.
• The force of gravity
acts equally in all
objects. Thus, in a
vacuum, all objects
whether heavy or light
will fall to the Earth
with
the
same
velocity(halaju).
Viscosity - kelikatan
Electrostatic Force
• produced by charged substances.
• Are caused by the presence of static electrical charges.
The electrical charges are produced when two different
objects are rubbed against each other. Like charges
repel while unlike charges attract.
• Example, it can attract small pieces of paper, stream of
water flowing from a tap.
Electrical force
• Is
produced
when electrons
move through
a
conductor
placed in a
magnetic field.
electron flows from negative terminal to
the positive terminal
Magnetic force
• Are the attractive and
repulsive
forces
exerted
• by magnets. Like poles
of magnets repel each
other while unlike
poles of magnets
attract each other.
THE MEASUREMENT OF FORCE
• The unit of force is measured in Newton (N).
• Force is measured using the spring balance.
• 1 N shown on a spring balance is equal to
nearly 100g. (10N = 1kg)
• Example, if
FRICTIONAL FORCE AND ITS
APPLICATION.
• A force which slows down moving
objects is called frictional force or
friction.
• Frictional force can also be used to start
or stop a motion.
• Frictional force acts when two surfaces
come into contact, sliding over each
other.
• Friction can at times be useful but in
other instances it an also be a nuisance
(GANGGUAN).
• Friction is a force that opposes motion. It
acts in the opposite direction to
movement.
• Friction always produces heat.
• Friction has direction and magnitude.
How different types of surface affect
frictional force.
• Friction has magnitude and direction.
• Magnitude depend on:
1. Nature surface – smooth or rough
2. Weight – light or heavy
3. Surface area not influenced magnitude of
force. (big or small)
• Different types of surfaces affect the magnitude
of frictional force.
• Rough surfaces have more friction than smooth.
•
Perlis 2012
ADVANTAGES AND DISADVANTAGES
• Advantages of friction (friction is useful)
– Friction plays an important role in our daily activities.
– Friction between a piece of rough metal and a flint produces sparks.
These sparks ignite the gas in a lighter.
– Friction enables the mechanical belt in car engines and machines to
turn engine parts. This moves the vehicle and machines.
•
• Disadvantages of friction (friction is a nuisance).
– Friction produces heat. For example, machine parts that move
produce heat. This heat can damage certain parts of the machines if
the machine is not carefully controlled.
– Friction opposes and slows down movement. For examples ridding a
bicycle up a slope or paddling a boat.
– Friction wears away materials. For examples, friction wears out the
soles of shoes and the tyres of vehicles.
PMR 10
PMR 2012
FORMULA
Work is done when a force moves an object
through the distance in the direction of the force.
Work (Nm) = Force (N) X Distance (m)
Work (J) = Force (N) x Distance (m)
Joule = Newton metre
1 newton-metre (Nm) = 1 joule (J)
test
• (20 kg + 10 kg) , 2m
• 10 n , 1 km (1000m)
• 30 kg + 20 kg naik tangga 5 anak tangga, tinggi
5 anak tangga ialah 8 m, kira WD
8m
15m
20 kg
10 m
test
8m
15m
20 kg
10 m
TRY
•
•
•
•
•
•
WD (Nm) = force (N) x distance (m)
= (40kg + 5 kg ) x 10N x 4m
= (45 kg x 10N) x 4 m
= 450 N x 4 m
= 1800 Nm
= 1800 J
Power is the rate of doing work, that is
the work done in one second (or the
energy used per second).
Check ?
Selangor 2011
TEST 1
45 kg
7m
10 m
a. Calculate work done?
b. Calculate Power if time taken is 5 s
8m
TEST 2
45 kg
20 cm
8m
20 cm
20 cm
20 cm
a. Calculate work done?
b. Calculate Power if time taken is 5 s
10 m
TEST 3
20 cm
45 kg
20 cm
8m
20 cm
20 cm
20 cm
a. Calculate work done?
b. Calculate Power if time taken is 5 minute
10 m
Test 4
45 kg
12 m
7m
10 m
a. Calculate work done?
b. Calculate Power if time taken is 5 s
5m
Test 5
45 kg
12 m
7m
10 kg
10 m
a. Calculate work done?
b. Calculate Power if time taken is 5 s
5m
Test 5
45 kg
10 kg
10 m
a. Calculate work done?
b. Calculate Power if time taken is 5 s
5m
wall
Test 6
45 kg
Ali tolak dinding
a. Calculate work done?
b. Calculate Power if time taken is 5 s
Test 7
Siti panjat dinding setinggi 5 m. Kira kerja yang
Dilakukan?
wall
45 kg
a. Calculate work done?
b. Calculate Power if time taken is 5 s
5m
The chart below shows situations when work is done
and when work is not done.
work
Work Done
Work not done
Pushing a wheelbarrow
Climbing the stairs/ladder
Throwing a ball upwards
Brushing teeth
Sitting on a chair
Standing on a ladder
Sleeping on a bed
swimming
Moment of force
N 9 2011
Moment of force - turning effect of a force.
Moment of force (Nm) = perpendicular distance from the fulcrum to the force (m) X force (N)
To increase the moment of force
1.
Increase the perpendicular distance
2.
Increase the magnitude of applied force
Moment of force (Nm) = perpendicular distance from the fulcrum to the force (m) x force (N)
Moment either clockwise or anticlockwise
a. Tightening a nut (clockwise )
Perpendicular distance = 15 cm = 0.15 m
The moment of force = 20 N x 0.15 m = 3 Nm
b. Loosening a nut (anticlockwise )
Perpendicular distance = 15 cm = 0.15 m
The moment of force = 20 N x 0.15 m = 3 Nm
Penutup
1. Baca Surah Al-`Ashr
2. Tasbih Kifarah.