RESUME CHAPTER 1 FORCE

RESUME SEMESTER GASAL PHYSIC VIII
FORCE
The definition of force
Force is a push or pull.
The force can causes change as follows :
1. an object at rest become moving
2. the moving an object to be at rest
3. the directions of an object
4. the shape and size
5. the velocity
Types of force
1.Contact forces
Contact forces is the force which happen because of the presence of a touch between two or more things



Example : muscle or muscular force, friction force, machine
force, spring force.
2.Non contact forces
Non contact force is force which happen without contact with the another thing
Example : gravitational force, electric force, magnetic force






DRAWING AND MEASURING A FORCE

The device used to measure force is spring balance or dynamometer
In SI, Force is measured in unit of Newton( N ) and in CGS system, force is measured in unit dyne.

1 N = 105 dyne

N = kgm/s2

Dyne = gr/cm2


The force is vector quantity because force has value and direction.
The force can be drawn by using a diagram vector in the form of an arrow, as the figure below:


The direction of force is represented by direction of arrow
The magnitude of force is represented by the length of arrow

RESULTANT FORCE
1.

R= F1 + F2, if two force the same direction

2.



R= F1- F2 , if two force the opposite direction

3. F1




F2

If two force the perpendicularly direction

R =

Friction force
Do you ever walk on the slippery floor then you walk on the rough floor ?do you feel the difference ?
Friction force is the force produced because of direct contact between two bodies surface.
The direction of friction force is opposite direction with direction of bodies motion.
fs F

Friction force is divided into two kinds ,those are :
1. static friction force ( fs ) ,namely a friction force between two bodies that will start to move or at rest.
2. kinetic friction ( fk ),namely a friction force between two bodies which one of the body moves.
Beneficial friction force:
1.friction force between feet and surface of floor cause we can walk
2.friction force between parachute and air cause the parachute can fly on the air and fall slowly.
3. braking system in a vehicle to retard/stop the vehicle.
4. friction force between car tires which is made serrated so the car is not slipped
Harmful friction force :
1. friction force between car /motorcycle tires and road cause car /motorcycle tires are quickly thin
2. friction force piston and cylinder in car engine so the engine hot quickly which cause it timeworn quickly.
3. friction force between air and the body of motorcycle rider, causes the motion of
motorcycle is retarded
4. the base of sandal become thin because often in friction with the road.


WEIGHT

The weight of an object is defined as the gravitational force acting on the mass of an object.
W = m.g
Note : w = weight (N )
m = mass ( kg )
g = gravitational acceleration ( m/s2 = N/kg )
weight of an object can change depends on the magnitude of gravitational force.
Weight can be measured with a spring balance
Mass is the amount of materials in the substance. It is constants everywhere in the universe.

Exercise :
A boy has a mass of 45 kg.what is his weight (a) on earth ( b) on the moon ? take g= 10 ms-2 and the acceleration due to gravity on the moon to be one sixth its value on earth .




NEWTON’S LAWS

Newton’s I Law explaines :
Σ F = 0Every body will be at rest or moves in uniform rectilinear if there is no force change that condition.
Formula Newton’s I law =
The property to stand self condition is called as inertia. So Newton’s I law or as inertia law.
Newton’s II Law explaines :
The acceleration of object which is caused by force is directly proportional to the magnitude of force and inversely proportional to its mass.

Formula Newton’s II law =
a =

a = acceleration ( m/s2 )
F = force ( N )
m = mass (kg )
Newton’s III Law explaines :
If the first object makes force ( making action) to second object, appears reaction force from the second object to first object which the directions are in opposite.
Formula Newton’s III Law = F action = - F reaction or F1 = -F2


Archimedes’ Law
“ an object which is immersed partly or entirely into liquid, will get lift force which the magnitude is equal to the weight of liquid moved by the object.
Mathematically ,Archimedes law is formulated as follows

Fa = ρ. g.V Fa = wu - waNote : Fa = Lift force ( N )ρ = Density of liquid ( kg/m3 )
g = gravitational acceleration (m/s2)
V = volume of object ( m3 )
wu = weight in air ( N )
wa = weight in liquid ( N )

Floating , suspending , Sinking
Fa Fa Fa




W W W

Energy and Work

a. Energy
Energy is the ability to make changes.
Energy is the ability to do work.
Forms of energy :
1. potential energy
2. kinetic energy
3. mechanic energy
4. heat energy
5. sound energy
6. light energy
7.
Energy always changes from one form to another.