GRADE 8 MATTERIALS OF PHYSICS

1st semester

I. FORCE

II. WORK

III. ENERGY

IV.PRESSURE

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Standard CompetenceStudents are able to understand the

concept of motion, the role of effort, force, and energy in the daily life

Basic CompetenceStudents are able to analyst experiment’s

data of contact and non contact force, resultant force, and the application in daily life

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Key words:1. Force = gaya2. Contact force = gaya sentuh3. Non- Contact force = gaya tak sentuh4. Frictional force = gaya gesek5. Muscular force = gaya otot6. Spring force = gaya pegas7. Static frictional force= gaya gesek statis8. Kinetic frictional force= gaya gesek kenetik9. Newton’s Law = Hukum Newton10. Inertia = kelembaman11. 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 beeasily seen, like when a running car strikes a tree.Force effects on the tree can be observed. But not allforce effects are observable.

How can you measure force? Force is measuredusing a spring balance, as shown in Figure 5.1. The unit of Force is Newton (N).

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force

Contact force Non –Contact force

Muscular forceSpring force

Frictional force, etc

Magnetism forceGravitational force

Electric force

The Effect of Force on an Object

Forces given to the ball change the speed of the ball.

•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 on an object that are equal in size and have opposite are called balanced forces.

Balanced Forces and un Balanced Force ( RESULTANT FORCE )

•Unbalanced forces always change the speed and directionof an object.

Figure 5.5

Two forces can be added up (a), can be balanced (b), or can be subtracted (c).

+ =

+ = 0

+ =

a).

b).

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 accelerationOr

F = m x aThis equation is known as the second of Newton's laws of motion.

With : F = Force ( N )m = mass ( kg )a = acceleration ( m/s2)

1 N = 1 kgm/s2

Figure 5.15(a) The acceleration of the toy car depends on the force givenby the child.(b) To move at the same acceleration, the child must pullmore 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 theforce 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 kgforce, F = 75 N2. What is not known?acceleration, a3. Choose the equation, F = m x a4. Solution:F = m x a, so a = 0.3 m/s2

Problems

1. What is the force used to accelerate a motor- cycle 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 Reaction1.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 oppositedirection.

Air Resistance

gravitation

The example OF Action force and reaction

Reaction Force

“Action force” of the gas pushes the rocket up. “Reaction force” of the rocket pushes the gas out.

1. Rocket Launching

2.

When the feet and the hands of the swimmer pushthe water, the water pushes the swimmer back. This Action force and reaction

Sir Isaac Newton

Born: 4 Jan 1643 in Woolsthorpe, Lincolnshire, EnglandDied: 31 March 1727 in London, England

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FRICTION Friction is a force to oppose motion

between two touching surfaces. Kinds of friction:

1. Static friction : on bodies at rest

2. Kinetic friction: on bodies in motion Friction between two surfaces depend on:

1. The nature of surfaces in contact. Friction is greater if the surfaces are rough.

2. The force pressing on the surface together

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.

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Gravitational Force

• The gravitational force of objects is determined by two things: the mass of the objects and the distance between the objects.

•Gravitation is a pulling force of an object to another object

Figure 5.11

The coconut is falling down andaccelerated to the groundbecause 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 = m x g

EXAMPLE

1. There are two bodies, which each has a mass of 2 kg and 5 kgrespectively. The two bodies are at the same place. If acceleration due to gravity in that place is 9.8 m/s2, determinethe weight of each body.

2. An Astronaut has a weight of 490 N on the earth. If on the moonacceleration 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 = m x gWith : W = weight ( N)

m = mass ( kg)

g = acceleration of earth’s gravitation ( m/s2)

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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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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

EVALUATION

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5. Calculate weight of these two parachutist 5. Calculate weight of these two parachutist below, with Earth gravitational acceleration below, with Earth gravitational acceleration is 9.8 m/sis 9.8 m/s22! ! 

  

A=56 kgA=56 kg B= 60kgB= 60kg

6. Weight of a body around equator is 1960 6. Weight of a body around equator is 1960 N. While weight of the body in north pole N. While weight of the body in north pole is 2000N. If gravitational acceleration is 2000N. If gravitational acceleration around equator is 9.8 m/s around equator is 9.8 m/s 2 2 , what is , what is gravitational acceleration in the north gravitational acceleration in the north pole?pole?

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7. A car of mass 200 kg is pushed by two men. 7. A car of mass 200 kg is pushed by two men. The first man push with force of 500 N, while the The first man push with force of 500 N, while the second man of 400N. Calculate the acceleration second man of 400N. Calculate the acceleration of the body.of the body.

8.Two8.Two children push a cupboard to the right. children push a cupboard to the right. Resultant of those forces is 90N. If ratio of Resultant of those forces is 90N. If ratio of those two forces is 7:8, calculate value of those two forces is 7:8, calculate value of each forceeach force

1 and 21 and 2

Each of the diagrams shows the Each of the diagrams shows the forces on an objetc of mass 2.0 kg. forces on an objetc of mass 2.0 kg. Find the acceleration of the object.Find the acceleration of the object.

1 1

22

10 N

20 N10 N

24N

8N

11. Please explain the harmful and the beneficial of friction!