In IBDP Physics, Forces act on objects.
When a force causes an object to move, it is the force that does the work.
The distance moved by the force is the same as the distance moved by the object.
The work done by a forceFmoving through a distancedis given by:
W = Fd
If the force is overcoming frictional forces, all or some of the work done by the force is converted to heat energy.
In IBDP Physics, The work done by the force may also be converted to kinetic energy or potential energy of the object.
When work is converted to different forms of energy you can use the work done relationship to calculate energy gained or lost, and energy relationships to calculate work done.
In IBDP Physics, You should be able to describe the forces affecting a falling object at different stages of its fall. Usually, you need to think about two forces:
- The weight of the object. This is a force acting downwards, caused by the object’s mass the Earth’s gravitational field.
- Air resistance. This is a frictional force acting in the opposite direction to the movement of the object.
Three stages of falling
When an object is dropped, we can identify three stages before it hits the ground:
- At the start, the object accelerates downwards because of its weight. There is no air resistance. There is a resultant force acting downwards.
- As it gains speed, the object’s weight stays the same, but the air resistance on it increases. There is a resultant force acting downwards.
- Eventually, the object’s weight is balanced by the air resistance. There is no resultant force and the object reaches a steady speed, called the terminal velocity.
In IBDP Physics, What happens if you drop a feather and a coin together? The feather and the coin have roughly the same surface area, so when they begin to fall they have about the same air resistance.
As the feather falls, its air resistance increases until it soon balances the weight of the feather. The feather now falls at its terminal velocity. But the coin is much heavier, so it has to travel quite fast before air resistance is large enough to balance its weight. In fact, it probably hits the ground before it reaches its terminal velocity.
This is the end of this topic.