Download FORCE What is force?

GRADE 8 MATTERIALS OF PHYSICS
1st semester
I. FORCE
II. WORK
III. ENERGY
IV.PRESSURE
2

Standard Competence


Students are able to understand the
concept of motion, the role of effort, force,
and energy in the daily life
Basic Competence

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Students are able to analyst experiment’s
data of contact and non contact force,
resultant force, and the application in daily
life
SMP N 2 KENDAL
3
Key words:
1. Force
= gaya
2. Contact force
= gaya sentuh
3. Non- Contact force = gaya tak sentuh
4. Frictional force
= gaya gesek
5. Muscular force
= gaya otot
6. Spring force
= gaya pegas
7. Static frictional force= gaya gesek statis
8. Kinetic frictional force= gaya gesek kenetik
9. Newton’s Law
= Hukum Newton
10. Inertia
= kelembaman
11. Electric force
= gaya listrik
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FORCE
What is force?
Push a door open. Pull a rubber band. Lift your books.
In each case, you give some force.
Force is a push or pull given by an object to
another object.
Sometimes the effects of force can be
easily seen, like when a running car strikes a tree.
Force effects on the tree can be observed. But not all
force effects are observable.
How can you measure force? Force is measured
using a spring balance, as shown in Figure 5.1. The
unit of Force is Newton (N).
force
Contact force
Muscular force
Spring force
Frictional force, etc
Non –Contact force
Magnetism force
Gravitational force
Electric force
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The Effect of Force on an Object
•The objects moves.
•The objects at rest.
•Force changes the speed of an object.
•Force changes the shape
•Forces can change the direction
Figure 5.2
Forces given to the ball
change the speed of the
ball.
Balanced Forces and un Balanced Force
( RESULTANT FORCE )
•Forces on an object that are equal in size and have
opposite are called balanced forces.
•Unbalanced forces always change the speed and direction
of an object.
a).
+
=
b).
+
Figure 5.5
=0
c).
Two forces can be added
up (a), can be balanced (b),
or can be subtracted (c).
+
=
Newton’s First Laws of Motion
•Sir Isaac Newton (1642-1727) was finally able to
formulate the laws explaining the effects of those forces
on an object.
•Newton's first laws of motion states that an
object moving at a constant velocity will keep moving at
the same velocity, unless there is a resultant force acting
on the object.
If an object is at rest, it stays at rest, unless there is a
resultant force causing it to move.
•This law is sometimes called the inertia law.
Newton's Second Laws of Motion
( Accelerated motion )
•If you pull a toy-car, it will start to move.
•The more strongly you pull the car, the faster it moves.
•The bigger the force, the faster the acceleration.
•If your toy-car is loaded, you must pull more strongly to
move it.
•The force acting on the toy-car is proportional to the mass
and acceleration of the toy-car.
•This statement can be written mathematically as follows:
Force = mass x acceleration
Or
F = m x a
This equation is known as the second of
Newton's laws of motion.
With : F = Force ( N )
m = mass ( kg )
a = acceleration ( m/s2)
1N
= 1 kgm/s2
Figure 5.15
(a) The acceleration of the toy
car depends on the force given
by the child.
(b) To move at the same
acceleration, the child must pull
more strongly.
The second of Newton's laws of
motion states that a force working on an
object causes the object to be
accelerated in the same direction as the
direction of the force.
The acceleration is dependent upon the
force and the mass of an object.
Example
Someone is pushing a 250-kg table using the force of 75 N.
Measure the acceleration of the table.
Steps in solving the problem:
1. What is known?
mass of the table, m = 250 kg
force, F = 75 N
2. What is not known?
acceleration, a
3. Choose the equation, F = m x a
4. Solution:
F = m x a, so a = 0.3 m/s2
Problems
1. What is the force used to accelerate a motorcycle having a mass of 200 kg while the rider
has a mass of 70 kg and the acceleration is 4
m/s2 ?
2. A child is pulling a toy-car having a mass of
2.5 kg with a 4N force. How much is the
acceleration of the toy-car?
Newton's Third Laws of Motion ( Action and
Reaction)
Action and Reaction
1. Action and reaction forces always work in pair.
2. Action-reaction forces always have the same
amount of force and are in the opposite directions.
3. When the first object gives a force to the second object,
the second object gives a force to the first. Action force
and reaction
Newton’s third laws of motion states a
relationship between action and reaction
forces :When the first object gives a
force to the second object, the second
object gives a force to the first object of
the same quantity but of an opposite
direction.
Air Resistance
gravitation
The example OF Action force and reaction
1. Rocket Launching
Reaction Force
“Action force” of the gas pushes the rocket up. “Reaction
force” of the rocket pushes the gas out.
2.
When the feet and the hands of the swimmer push
the water, the water pushes the swimmer back. This
Action force and reaction
Sir Isaac Newton
Born: 4 Jan 1643 in Woolsthorpe, Lincolnshire, England
Died: 31 March 1727 in London, England
FRICTION


1.
2.

1.
2.
Friction is a force to oppose motion
between two touching surfaces.
Kinds of friction:
Static friction : on bodies at rest
Kinetic friction: on bodies in motion
Friction between two surfaces depend on:
The nature of surfaces in contact. Friction is
greater if the surfaces are rough.
The force pressing on the surface together
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THE HARMFUL AND BENEFICIAL FRICTION
The following are the harmful frictions ( The Disadvantageous):
1. Friction between car engine and its clutch can produce exceed
heat, which can break down engine.
2. Friction between air and car can resist the car movement.
3. Friction between the tire of a vehicle and the road causes the
tire become thin
The following are the beneficial frictions (The advantageous) :
1. Friction between our feet and the surface of a road makes us
able to walk.
2. Friction in the braking system can slow down the vehicle.
3. Friction between a tire and the road surface so that vehicle is
not slip.
Gravitational Force
•Gravitation is a pulling force of
an object to another object
• The gravitational force of objects
is determined by two things: the
mass of the objects and the
distance between the objects.
Figure 5.11
The coconut is falling down and
accelerated to the ground
because of the gravitation.
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Weight
• Weight
of an object arises because of gravitational force.
• Weight of an object on or above the earth is the
gravitational force that the earth exerts on the object.
• The weight always acts downwards towards the center of
the earth.
• On any other astronomical body , weight is the
gravitational force exerted on the object by that body.
• Unit of weight is Newton (N), mathematically:
W=mxg
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EXAMPLE
1. There are two bodies, which each has a mass of 2 kg and 5 kg
respectively. The two bodies are at the same place.
If acceleration due to gravity in that place is 9.8 m/s2, determine
the weight of each body.
2. An Astronaut has a weight of 490 N on the earth. If on the moon
acceleration due to gravity is 1.6 m/s2, what is the astronaut’s
weight on the moon. g EARTH = 9.8 m/s2
Relation between Mass and Weight
• Mass and Weight are not the same quantity !
• Mass is a quantitative measure of inertia
• Mass is an intrinsic property
• Weight varies depending on the location
(height from earths surface etc.
•The Unit of weight is Newton (N), mathematically:
W=mxg
With : W = weight ( N)
m = mass ( kg)
g = acceleration of earth’s gravitation ( m/s2)
DRAW CONCLUSION
1. Force acting on a body can cause the change of shape
and size of the body, Change of body motion’s direction,
and change of body condition (the body at rest becomes
moving or moving body becomes at rest)
2. Based on its property, force grouped into touchable
(contact) and untouchable force (non contact) force.
3. Based on its cause, force grouped into muscular force,
spring force, magnetic force, electric force, machine force,
etc.
4. A force can be drawn using a vector diagram which has
shape of arrow.
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5. The unit of force is Newton (N)
1 N = 1 kg m/s2
1 N = 105 dyne
6. Resultant of forces acting on a body have the same
direction, opposite direction.
7. The formula of weight is w = m x g
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EVALUATION
1. Explain the definition of Force and Acceleration .
2. Mention changes which can produced by a Force
3. Mention kinds of Forces based on their cause.
Give each one example.
4. Draw vector diagram of the following forces, with
length of each 1 N is equal to 1 cm. F1= 2N to the
right, F2= 3N to the left, F3= 5N to the right. Then
calculate the resultant of those three forces
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5. Calculate weight of these two parachutist
below, with Earth gravitational acceleration is
9.8 m/s2!
A=56 kg
B= 60kg
6. Weight of a body around equator is 1960 N.
While weight of the body in north pole is 2000N.
If gravitational acceleration around equator is
9.8 m/s 2 , what is gravitational acceleration in
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the north pole?
7. A car of mass 200 kg is pushed by two men.
The first man push with force of 500 N, while the
second man of 400N. Calculate the acceleration
of the body.
8.Two children push a cupboard to the right.
Resultant of those forces is 90N. If ratio of
those two forces is 7:8, calculate value of
each force
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1 and 2
Each of the diagrams shows the forces on
an objetc of mass 2.0 kg. Find the
acceleration of the object.
1
24N
10 N
2
20 N
10 N
8N
11. Please explain the harmful and the
beneficial of friction!