# Physics Revision: Forces and their effects

## Motion

• Scalar quantities, e.g. speed, time, have only size. Vector quantities e.g. force, momentum, have size and direction.
• The speed of an object can be found by looking at the gradient of a distance-time graph.
• The acceleration of an object can be found by looking at the gradient of a velocity-time graph.
• Distance travelled by an object is the area under a velocity-time graph.

Acceleration of an object=     Final Velocity-Initial Velocity

Time

Velocity of an object=    Displacement

Time Taken

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## Resultant Forces

When 2 objects interact, the forces exerted on each other are equal and opposite

The resultant force has the same effect on motion as all other forces acting on the object combined.

When an object is stationary:

If the resultant force is zero= Object remains stationary

If it's not zero= Object accelerates in the direction of the resultant force

When an object is moving:

If the resultant force is zero= The object will continue moving at the same speed and same direction

If it's not zero= The object will accelerate in the direction of the resultant force

FORCE=MASS X ACCELERATION

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## Momentum

MOMENTUM= MASS X VELOCITY

In a closed system, the total momentum before an event= the total momentum after an event

This is 'conservation of momentum'

FORCE= CHANGE IN MOMENTUM

TIME

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## Forces and Braking

When a vehicle is at a steady speed, driving force=total resistive forces

The greater the speed of the vehicle, the greater the braking force needed to stop it

Stopping distance= Distance travelled during the driver's reaction time+distance travelled under the braking force

Reaction Time: Affected by tiredness, drugs and alcohol

Braking Distance: Affected by icy/wet road conditions and damaged tyres/brakes

When brakes are applied, kinetic energy of the car decreases and temperature of the brakes increases

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## Forces and Terminal Velocity

The faster an object falls, the greater the frictional force acting on it

An object falling will accelerate due to gravity, then eventually weight and resistive forces will balance, meaning the resultant force=0 and the object will move at its terminal velocity.

Parachutists use parachutes to increase the effect of air resistance and significantly reduce their terminal velocity

WEIGHT=MASS X 10

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## Forces and Elasticity

Forces acting on an object can change the shape of the object

Elasticity is when the object returns to its original shape when the force is removed

Applying a force to e.g. a spring will result in the object stretching and storing Elastic Potential Energy

Extension is proportional to the force applied (provided the limit of proportionality is not exceeded)

FORCE= CONSTANT X EXTENSION

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## Forces and Energy

Work is when a force causes an object to move through a distance

WORK= FORCE X DISTANCE MOVED

Energy is transferred when work is done

Work against frictional forces causes energy transfer by heating

POWER= WORK DONE/ENERGY TRANSFERRED

TIME

GPE= MASS X 10 X HEIGHT

KINETIC ENERGY = 0.5 X MASS X SPEED(SQUARED)

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## Centre of Mass

Centre of mass is the point where the mass of an object is considered concentrated

If freely suspended, the object will come to rest with the centre of mass directly below the point of suspension.

For a symmetrical object, the centre of mass is along the axis of symmetry.

TIME PERIOD= 1

FREQUENCY

The time period for a pendulum depnds on the length

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## Moments

MOMENT=TURNING FORCE X DISTANCE FROM THE PIVOT

If an object isn't turning, the clockwise moment is equal to the anticlockwise moment

Simple levers can be used as force multipliers

If the line of action of the weight of an object is outside the base of the object, the resultant force (imbalance) will cause the body to topple

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## Circular Motion

When an object moves in a circle, it accelerates towards the centre of the circle. This changes the direction of the motion of the body, not the speed.

This resultant force is known as the centripetal force and always is directed towards the centre of the circle.

The centripetal force needed to make an object perform circular motion increases as:

• The mass of the object increases
• The speed of the object increses
• The radius of the circle decreases
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## Hydraulics

Liquids are virtually incompressible, and the pressure in a liquid is transmitted equally in all directions.

This means a force exerted at one point on a liquid will be transmitted to other points in the liquid

PRESSURE= FORCE

CROSS-SECTIONAL AREA

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