The stopping distance of a vehicle depends on....
- the thinking distance (the distance travelled during the driver's reaction time)
- the braking distance distance (the distance travelled under the breaking force)
The overall stopping distance is increased if ...
- the vehicle is travelling at greater speeds
- there are adverse weather conditions e.g wet roads, poor visability
- the driver is tired or under the influence of drugs or alcohol or is distracted and can't react as quickly as normal
- the vehicle is in poor condition.
Friction forces between the brakes and the wheel and the wheel and the road surface reduce the kinetic energy of the vehicle. This kinetic energy is transformed into heating the brakes resulting in an increase in brake temperature. If a vehicles wheels lock when braking, a skid results. Overheating can result in brake failure
Forces and Weight
All falling objects experience two forces:
- A downward force, called weight (W).
- An upward frictional force
Although weight always remains the same, the faster an object moves through the air or fluid the greater the frictional force that acts on it.
The weigth of an object is the force exerted on it's mass by gravitiy (sometimes called gravitiational field strength). Weight is measured in newtons. To calculate the weight of an object the following equation is used :
W = m x g
An object falling through the air or a fluid will initially accelerate because of the force due to gravity. Eventually the resultant force will be zero as the weight and resistive forces balance. At this point the object will move at a steady speed, called it's terminal velocity.
If a skydiver jumps out of an areoplane, the speed of their descent can be considered in two seperate parts:
- Before the parachute opens.
- After the parachute.
Forces and Elasticity
A force acting on an object may cause the obejct to change it's shape.
A force applied to an object that's able to recover it's original shape when the force is removed is said to be elastic, e.g a spring
When a force is applied to a spring, work is done in strectching the spring. The energy stored is called elastic potential energy. When the force is removed, the energy stored is used to bring the object back to it's original shape.
For elastic objects, like spring, th extension is directly proportional to the force applied, provided that the limit of proportionality is not exceeded. The equation is as follows:
F = k x e
resultant forces are