# P2 Forces

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• Created by: Phoebe
• Created on: 29-12-12 13:07

## 2.1 Forces between objects

The unit of force is the newton (N)

Two objects interacting always exert equal and opposite forces on each other

A force can change the shape of an object or change its motion or state of rest

If a car hits a barrier, it exerts a force on the barrier. The barrier exerts a force on the car that is equal in size and in the opposite direction

If you place a book on a table, the weight of the book will act vertically downwards on the table. The table will exert an equal and opposite reaction force upwards on the book

The force of weight always acts downwards, towards the Earth's core

When a car is being driven forwards there is a force from the tyre on the ground pushing backwards. There is an equal and opposite force from the ground on the tyre which pushes the car forwards

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## 2.2 Resultant force

Most objects have more than one force acting on them. The resultant force = the single force that has the same effect on the object as all the original forces together.

When the resultant force on an object is zero:

• if the object is at rest, it will stay at rest
• if the object is moving, it will carry on moving at the same speed and direction

When the resultant force on an object is NOT zero, there will be an acceleration in the direction on the force:

• if the object is at rest, it will accelerate in the direction of the resultant force
• if the object is moving in the same direction as the resultant force, it willaccelerate in that direction
• if the object is moving in the opposite direction to the resultant force, it will decelerate
• e.g. the resultant force of a 3N force and a 4N force acting in the same direction is 7N

If an object is accelerating it can be speeding upslowing down or changing direction.

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## 2.3 Force and Acceleration

A resultant force always causes an acceleration. If there is no acceleration, the resultant force must be zero.

Acceleration is a change in velocity. An object can accelerate by changing its direction even if it is going at a constant speed.

A resultant force is needed to make an object change direction.

F = m x a              F=resultant force (N)   m=mass (kg)   a=acceleration (m/s²)

The greater the resultant force on an object, the greater its acceleration.

The bigger the mass of an object, the bigger the force needed to give it a particular acceleration.

If the velocity of an object changes, it must be acted on by a resultant force. Its acceleration is always in the same direction as the resultant force

• the velocity of an object increases if the RF is in the same direction as its velocity
• the velocity of an object decreases if the RF is opposite in direction to its velocity
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For a car travelling at constant velocity, the resultant force on it is zero. The resistive forces (friction and air resistance) are balanced.

The faster the speed of the vehicle, the bigger the deceleration needed to stop it, so the bigger the braking force needed.

The greater the mass of the vehicle, the greater the braking force needed for a given deceleration.

Stopping distance: the shortest distance a vehicle can safely stop in =

• the thinking distance: the distance travelled by the vehicle in the time it takes the driver to react PLUS
• the braking distance: the distance travelled by the vehicle during the time the braking force acts

Factors affecting stopping distances:

• tiredness, alcohol and phones increase reaction time and the thinking distance
• high speed, poor weather and maintenance all increase the braking distance
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## 2.5 Falling objects

The weight of an object is the force of gravity on it. Its mass is the quantity of matter in it.

An object acted on only by gravity is in free fall, and accelerates at about 10 m/s².

W = m x g       W=weight (N)   m=mass (kg)   g=acceleration due to gravity (m/s²)

When an object falls through a fluid, the fluid exerts a drag force on the object, resisting its motion.

The faster an object falls, the bigger the drag force becomes, until eventually it will be equal to the weight of the object.

The resultant force is now zero, so the body stops accelerating. It moves at a constant velocity called the terminal velocity.

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## 2.6 Stretching and squashing

An elastic object regains its original shape when the forces deforming it are removed.

The extension is the difference between the length of the spring and its original length.

Hooke's Law: the extension of a spring is directly proportional to the force applied provided the limit of proportionality is not exceeded.

Force applied (N) = Spring Constant (N/M) x Extension (m)

The spring constant of a spring is the force per unit extension needed to stretch.

The stiffer a spring is, the greater its spring constant.

When an elastic object is stretched, elastic potential energy is stored in the object. When the stretching force is removed, this energy is released.

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## 2.7 Force and Speed issues

Fuel economy of road vehicles can be improved by reducing the speed or fitting a wind deflector.

Average speed cameras are linked in pairs and they measure the average speed of a vehicle.

Anti-skid surfaces increase the friction between a car tyre and the road surface. This reduces skids, or even prevents skids altogether.

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