Forces that oppose Motion
In our everyday experience we do not come across perpetual motion - a phenomena which would allow objects to move continuously. Our experience is that when things are moving they eventually stop, unless there is something to keep it going such as an engine.
On the ground we are familiar with friction and how friction acts upon a moving object. We have probably all experienced friction when we rub our hands together. We are very glad of friction when the car in front of us suddenly brakes. Think of the problems we might encounter when we walked or drove if there was no friction force.
However, there are other forces which act in a similar way to friction. In air and water there is resistance to objects passing through them. Again it is useful to use a model.
The most obvious example in thinking about air resistance is that of the parachute. The parachutist is pulled towards the ground by the force of gravity but the air resists the parachute and slows the rate of fall. This is truce of any object falling through the air although the effect is not always so apparent.
In water the effect is similar. The object passes through water and as it does so the upthrust acts against the weight of the object (as you now know - this is caused by gravity). In some cases the upthrust is equal to the force of gravity and so the object floats. If the object sinks the upthrust slows the rate of descent.
ContentMeasuring Force, Mass, Speed