# Physics P2

Main Facts expected for Physics Unit 2. (2014)

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## Resultant Forces

Whenever two objects interact, the forces they exert on each other are equal and opposite.

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## Resultant Forces

A number of forces acting at a point may be replaced by a single force that has the same effect on the motion as the original forces all acting together. This single force is called the resultant force. E.g. Force A + Force B = Force C

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## Resultant Forces

A Resultant Force is the sum of two or more forces. E.g. Force A (10N) + Force B (5N) = 15N

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## Resultant Forces

A resultant force acting on an object may cause a change in its state of rest or motion. E.g. A force that is exerted on a stationary object will make the object accelerate.

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## Resultant Forces

If the resultant force acting on a stationary object is:

■ zero, the object will remain stationary

■ not zero, the object will accelerate in the

direction of the resultant force.

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## Resultant Forces

If the resultant force acting on a moving object is:

■ zero, the object will continue to move at the same

speed and in the same direction

■ not zero, the object will accelerate in the

direction of the resultant force.

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## Forces and Motion

The acceleration of an object is determined by:

• The resultant force acting on the object
• The mass of the object.

(F=M * A)

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## Forces and Motion

The gradient of a distance–time graph represents speed.

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## Forces and Motion

You can calculate the speed of an object from the gradient of a distance–time graph. To calculate the gradient of the line on a graph, divide the change in the vertical axis by the change in the horizontal axis.

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## Forces and Motion

The velocity of an object is its speed in a given direction. E.g. a Car moving 10m/s to the LEFT.

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## Forces and Motion

The gradient of a velocity–time graph represents acceleration.

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## Forces and Motion

You can calculate the acceleration of an object from the gradient of a velocity–time graph.

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## Forces and Motion

You can calculate the distance travelled by an object from a velocity–time graph. You can do this by finding the area of the light blue triangle and the dark blue rectangle; Add the two together.

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## Forces and Braking

When a vehicle travels at a steady speed, the resistive forces balance the driving force. Most of the resistive forces are caused by air resistance.

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## Forces and Braking

The greater the speed of a vehicle, the greater the braking force needed to stop it in a certain distance.

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## Forces and Braking

Stopping Distance = Thinking Distance (driver’s reaction time) + Braking Distance (braking force).

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## Forces and Braking

A driver’s reaction time can be affected by tiredness, drugs and alcohol.

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## Forces and Braking

A vehicle’s braking distance can be affected by adverse road, poor weather conditions and poor condition of the vehicle.

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## Terminal Velocity

The faster an object falls, the greater the frictional force that acts on it.

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## Terminal Velocity

An object falling will initially accelerate due to the force of gravity.

Eventually the resultant force will be zero and the object will move at its terminal velocity (steady speed).

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## Elasticity

A force acting on an object may cause a change in shape of the object.

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## Elasticity

A force applied to an elastic object (such as a spring) will result in the object stretching and storing elastic potential energy.

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## Elasticity

For an object that is able to recover its original shape, elastic potential energy is stored in the object when work is done on the object to change its shape.

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## Momentum

Momentum is a property of moving objects. A moving object has momentum. This is the tendency of the object to keep moving in the same direction. It is difficult to change the direction of movement of an object with a lot of momentum.

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## Work Done

Energy is transferred when work is done. Work is done against frictional forces. Power is the work done or energy transferred in a given time. Gravitational potential energy is the energy that an object has by virtue of its position in a gravitational field.

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## Static Electricity

When certain insulating materials are rubbed against each other, they become electrically charged.

When this happens, negatively charged electrons are rubbed off one material and onto the other.

The material that gains electrons becomes negatively charged.

The material that loses electrons is left with a positive charge.

When two electrically charged objects are brought together, they exert a force on each other.

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## Static Electricity

Two objects that carry the same type of charge repel.

Two objects that carry different types of charge attract.

Electrical charges can move easily through some substances, eg metals.

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## Electrical Circuits

Electric current is a flow of electric charge. The size of the electric current is the rate of flow of electric charge.

The potential difference (voltage) between two points in an electric circuit is the work done (energy transferred) per coulomb of charge that passes between the points.

following standard symbols should be known:

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## Electrical Circuits

Current–potential difference graphs are used to show how the current through a component varies with the potential difference across it.

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