Speeding Up and Slowing Down

Forces Between Objects:

• When two objects interact, they always exert equal and opposite forces in each other.
• The unit of force is the newton (N).
• The motive force on a car is the force that makes it move. This force is due to the friction between the ground and the tyre of each drive wheel. Friction acts where the tyre is in contact with the ground.
• When the car moves forward; the force of friction of the ground on the tyre is in the forward direction. And the force of friction of the tyre on the ground is in the reverse direction.

Summary questions:

• The force on a ladder resting against a wall is opposite and equal to the force of the wall on the ladder.
• A book is at rest on a table. The force of the book on the table is downwards. The force of the table on the book is upwards.

Resultant Force:

• We can work out the effect of the forces on an object by replacing them with one single force, the resultant force. This single force has the same effect as all the forces acting on the object.
• When an object is at rest at the start, the resultant force is zero, so the object stays at rest.
• When an object is moving at the start and the resultant force is zero, the velocity stays the same.
• When an object is moving at the start, and the resultant force isn't zero and is in the same direction as the motion of the object, the object accelerates.
• When an object is moving at the start, and the resultant force isn't zero but isn't in the same direction as the motion of the object, the object will decelerate.
• When a car driver applies the brakes, the braking force is the resultant force on the car. It acts in the opposite direction to that in which the car is moving, so it slows the car down.
  

Summary questions:

A car starts from rest and accelerates along a straight…