Friction and non-uniform motion
A falling object accelerates because of the force so gravity, but as it speeds up, the frictional forces (Forces that resist one object moving through or over another, such as air resistance on a car) on increase. Eventually, the gravitational force is balanced by the frictional forces and the object falls at a constant speed, called the terminal velocity.
The distance travelled by a car between the time it takes for its drivers to start reacting to a hazard and car stopping depends on two things: the thinking distance and the braking distance. Several factors influence them, and can increase the overall stopping distance.
Falling objects
When an object is dropped, we can identify three stages before it hits the ground.
- At the start, the force of gravity pulls the object down towards the earth, and it accelerates downwards.
- As it gains speed, frictional forces on the object increase and oppose the downwards movement.
- Eventually, the force of gravity acting on the object is balanced by the frictional forces. The object reaches a steady speed call the terminal velocity.
As the feather falls, its drag increases unit it soon balances the weight. Now falls at its terminal velocity. But the coin is much heavier, probably hits the ground before it reaches it terminal velocity.
On the moon: An astronaut dropped a hammer and feather at the same time and landed together. The moon’s gravity is too weak for it to hold onto an atmosphere, so there is no air resistance (A force of friction produced when an object moves through the air). When the hammer and the feather were dropped they fell together with the same acceleration.
The faster the car is travelling, the greater this thinking distance will be.
Thinking distances increase if the driver’s actions are slower because the driver is:
- under the influence of alcohol
- under the influence of drugs
- tired
The braking distance is the distance the car travels from where the brakes are first applied to where the brakes are first applied to where the car stops. The faster the car is travelling, the greater the braking distance will be.
The braking distance will increase if:
- The brakes or tires are worn.
- The weather conditions are poor, such as icy or wet roads.
- The car is move heavily laden.
T dis + B dis= S dis
Seat belts stop you tumbling around if there is a collision. This reduces the deceleration, must be replaced after an accident.
Modern cares are designed with a tough passenger cabin, surrounded by sections that are designed to crush and buckle in a collision. These sections are called crumple zones. During a collision, these sections absorb large amounts of kinetic energy as they bend and crumple, and they reduce the deceleration.