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Forces around us
Each movement needed a push, a pull or a twist to get it started.
Each force has a direction and magnitude.
Magnitude is the amount of the force.
Pushing, pulling and twisting are forces. We use arrows to show the direction in
which a force is working.
What can forces do?
Forces can:
1- Change the shape of something
2- Change the speed
3- Change the direction that is moving in
We measure forces using a force meter. Force meter has a spring inside it
which can be stretched.
The measuring unit of forces is (Newton)
What is the measuring unit of forces?
The measuring unit of forces is Newton (N).
Friction force:
What is friction force?
Friction is a force between two touching objects.
Friction force acts in the opposite direction to the movement.
Friction makes it harder for things to move or make things stay still.
Gases and liquids can also cause friction.
Air resistance and Water resistance are both kinds of friction force.
1- Surface material:
Rough surfaces have high friction force
Smooth surfaces have low friction force
2- Surface area:
Advantages of friction forces:
1. friction between our shoes and the floor stop us from slipping
2. friction between tyres and the road stop cars from slidding
We can increase friction by using certain materials. Rubber has high friction
force so we use rubber for making tyres of cars. It can stop the car from sliding
off the road.
3. friction between the brakes and wheel help bikes and cars slow down
Friction forces are much smaller on smooth surfaces than on rough surfaces.
Disadvantages of friction forces:
1-Sometimes we want things to move easily/ for example, a bicycle is very
difficult to ride if there is too friction force between the axles. We can
reduce the friction force by using lubricants (such as oil or grease) this is called
What is lubrication?
Lubrication is adding oil or grease to reduce the friction force.
2-Friction produces heat and noise. If a car engine runs without any oil in it,
the high friction force between the moving parts causes heat and stops the
engine from working.
3-Rusty door hinges squeak and make a door difficult to open.
The friction force can be reduced by having smooth surfaces and smooth
Force diagrams
In a force diagram, an arrow represents each force.
The arrow shows:
The magnitude(size of the force), the longer the arrow, the bigger the force.
The direction in which the force acts
Up thrust
Up thrust is the force that pushes an object up and makes it
seem to lose weight in liquids or gases
The weight and the up thrust have the same magnitude but in the opposite direction.
Reaction force
Any object standing on the ground, the reaction force from the ground act in the opposite
direction of its weight.. Without this balancing force you would sink into the
The weight of the book is balanced by the reaction force from the table
Here are other examples:
There are forces acting on this car:
The forward force is the movement or the driving force and in the
opposite direction (the friction force between tyres and the rood + air
The downward force is the weight of the car and in the opposite
direction (the reaction force that acts in the opposite direction of the
In race cars they reduce the surface area of the front body to reduce
the air resistance and increase the speed of the car
2- Moving ship
Up thrust
Air resistance + water resistance
There are four forces acting on it
A is the up thrust force.
C is the weight
B is the movement (thrust)
D is water resistance + air resistance
3- Stopped ship (no movement):
There are two forces only act on the stopped ship up thrust what makes the ship
floats and the weight of the ship downward.
Up thrust
Friction force equal to zero as there is no movement.
4- In airplanes that flying there are four forces acting on
A is the up thrust force (lift).
C is the weight
B is the movement (thrust)
D is the air resistance force or drag
Air resistance in parachutes:
As a parachute is falling down the direction of the movement will be in the same
direction of the gravity and in the opposite direction is the air resistance that
slows down the falling.
Have a look on those parachutes
Does the size of a parachute affect how long it takes to fall?
Yes, as the green one has larger surface area so greater air resistance which
slows down the speed of falling down.
To prove that:
Drop two pieces of paper from the same height, one flat and one crumpled up in
a ball.
Which piece of paper falls faster? Why?
The crumpled one fall faster as it has less surface area and less air resistance so fall
faster than the flat one which has greater air resistance.
Racing cyclists crouch down low on their bikes to reduce the air resistance
on them. This helps them to cycle faster. They also wear streamlined
helmets. These have special, smooth shapes that allow the air to flow over
the cyclist more easily.
Modern vehicles are also streamlined. Their smooth shapes make the air
resistance smaller, which allows them to travel further on the same
amount of fuel.
Streamlining reduces air resistance
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Balanced forces
When two forces acting on an object are equal in size but act in opposite
directions, we say that they are balanced forces.
The overall force on the object is zero.
The resultant force = 5-5 = zero
500 N
500 N
The forces act on the moving car are balanced so the car moves at
constant speed.
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Unbalanced forces
When two forces acting on an object are not equal in size, we say that they are
unbalanced forces. The overall force acting on the object is called the resultant
In the example below, the resultant force is the difference between the two
100 – 60 = 40 N (to the right)
The truck speeds up in the direction of the resultant force
The change in the motion of an object depends upon:
• the size of the resultant force
the direction of the resultant force
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The greater the resultant force, the greater the change in the motion of the
object. Whether a moving object speeds up, or slows down, depends on the
direction of the resultant force:
• the object speeds up if the resultant force acts in the direction of movement
the object slows down if the resultant force acts opposite to the direction of
Move at constant speed
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Calculating the resultant force:
The forces are unbalanced
The resultant force= 6 - 3 = 3N
The direction of the force is in the left direction
The forces are balanced
The resultant force= 5 - 5 = zero
So the object will stay still (stationary)
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MASS measuring unit is gm (gram) , kg (kilogram)or ton
Weight measuring unit is NEWTON.
The weight on earth is 6 times bigger than weight on the moon
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