Gravity or Weight
Weight is the force of gravity acting on an object. It is measured in N.
Mass is how much stuff an object has in it. It is measured in kg. It will never change, unless the object goes on a diet.
Weight can change if the gravitational field strength changes so if you want to lose weight, go to the moon.
The gravitational field strength on Earth =9.81 or 10m/s/s.
Weight always acts from the centre of mass, shown in the diagram on the right, and can be calculated using the following formula:
Newton's 3rd Law tells us that every force has a reaction force. It must always be acting against a force and so be in the opposite direction with an arrow the same size.
Air Resistance or Drag
Friction is a force that acts against movement. The arrow will always be in the opposite direction to the way the object is travelling and will run along a surface. The size of the force depends on the speed on the object.
These are examples of frictional forces. They will act in the opposite direction to the motion of the object and will increase with speed.