Forces

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FORCES BETWEEN OBJECTS

Forces are measured in newtons, N.  Objects always exert equal and opposite forces on each other.

If a car hits a barrier, it exerts a force on the barrier.  The barrier also 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.  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.

RESULTANT FORCE

Most objects have more than one force acting on them.  The resultant force is the single force that would have the same effect as all the original forces acting together.  When the resultant force on an object is zero, a stationary object will stay at rest.  If the object is moving, it will carry on moving with the same velocity.

When the resultant force on an object is not zero, there will be an acceleration in the direction of the force.  Therefore 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 will accelerate in that direction.  If the object is moving in the opposite direction to the resultant force, it will decelerate.

FORCE AND ACCELERATION

If there is a resultant force, it will always cause an acceleration.  If there is no acceleration (or deceleration), the resultant force must be zero.  An object accelerates if its velocity changes. Therefore, a resultant force is needed to change direction.  We find the resultant force on an object by using the equation:

F = m x a

F - resultant force in newtons, m - mass in kg, a - acceleration in m/s2

The greater the resultant force on an object, the greater the acceleration.  The greater the mass of an object, the greater the force needed to make it accelerate.

ON THE ROAD

When a vehicle is travelling at a steady speed, the resultant force on it is zero.  The driving forces are equal and opposite to the frictional forces.

The greater the speed of a vehicle, the greater the deceleration needed to stop it in a given

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