How do we make things speed up or slow down?

The weight of a body is calculated using the equation:

       weight = mass × gravitational field strength   or    W = mg
           (N)           (kg)                      (N/kg)

a mass of 1 kg on Earth has a weight of 1 N A mass of 70 kg on Earth has a weight of
70 x 10 = 700N		 A mass of 1 kg on the moon has a weight of 1.6N A mass of 70 kg on the Moon has a weight of
70 x 1.6 = 112N

Forces and Motion

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


A number of forces acting on a body may be replaced by a single force which has the same effect on the body as the original forces
all acting together. The force is called the resultant force.


• If the resultant force acting on a stationary body is zero the body will remain stationary. (pic 1)
• If the resultant force acting on a stationary body is not zero the body will accelerate in the direction of the resultant force. (pic 2)
• If the resultant force acting on a moving body is zero the body will continue to move at the same speed and in the same direction. (pic 3)
• If the resultant force acting on a moving body is not zero the body will accelerate in the direction of the resultant force. (pic 4)

When a vehicle travels at a steady speed (as in pic 3) the frictional forces balance the driving force.

Force and Acceleration

The greater the resultant force which acts on an object the greater its acceleration will be.
• Force, mass and acceleration are related by the equation:

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

Speed and safety

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

The stopping distance of a vehicle depends on the distance the vehicle travels during the drivers reaction time and the distance it
travels under the braking force.

A drivers reaction time can be affected by tiredness, drugs and alcohol.

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


Terminal Velocity

The faster a body moves through a fluid the greater the frictional force which acts on it. (e.g. air resistance)
A body falling through a fluid will initially accelerate due to the force of gravity.
Eventually the resultant force on the body will be zero and it will fall at its terminal velocity.

Here is a speed time graph for a skydiver. Notice how the slope of the graph (the acceleration) changes as the size of the air resistance force increases.