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GCSE Physics P2 revision
Frictional Force and Terminal Velocity
Friction is always there to slow things down. If an object has no force propelling it along, it
will always slow down and stop because of friction (unless you're in space where there's
nothing to rub against). Friction always acts in the opposite direction movement. To travel at
a steady speed, the driving force needs to balance the frictional forces. You get friction
between two surfaces in contact, or when an object passes through a fluid (drag).
Resistance or `drag' from fluids (air or liquid)
Most of the resistive forces are caused by air resistance or `drag'. The most important
factor by far in reducing drag in fluids is keeping the shape of the object streamlined. The
opposite extreme is a parachute which is about as high drag as you can get.
Drag increases as the speed increases. Frictional forces from fluids always increase with
speed. A car has much more friction to work against when travelling at 70 mph compared to
30 mph. So at 70 mph, the engine has to work much harder just to maintain a steady speed.
Objects falling through fluids reach a terminal velocity. When falling objects first set off, the
force of gravity is much more than the frictional force slowing them down, so they
accelerate. As the speed increases the friction builds up. This gradually reduces the
acceleration until eventually the frictional force is equal to the accelerating force, and then it
won't accelerate anymore. It will have reached its maximum speed or terminal velocity, and
it will fall at a steady speed.
The terminal velocity of falling objects depends on their shape and area. The accelerating
force acting on all falling objects is gravity and it would make them all fall at the same rate, if
it wasn't for air resistance. This means that on the moon, where there's no air, bricks and
feathers dropped simultaneously will hit the ground together. However on Earth, air
resistance causes things to fall at different speeds, and the terminal velocity of any object is
determined by its drag in comparison to its weight. An important example is the human
skydiver. Without his parachute open, he has quite a small area and a force of `W = mg'
pulling him down. He reaches a terminal velocity of about 120 mph; but with the parachute
open, there's much more air resistance (at any given speed), and still only the same force of
`W = mg' pulling him down. This means his terminal velocity comes right down to about 15
mph, which is a safe speed to hit the ground at.