Terminal Velocity

Friction is always there to slow things down

  • If an object has no forces propelling it along it will always slow down and stop because of friction
  • Friction always acts in the opposite direction to 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 increases as speed increases

  • Drag is the resistance you get in a fluid. Air resistance is a type of drag.
  • The most important factor by far in reducing drag is keeping the shape of the object streamlined. This is where the object is designed to allow fluid to flow easily across it, reducing drag. Parachutes work in the opposite way-they want as much drag as they can get
  • Frictional forces from fluids always increase with speed. A car has much more friction to work against when travelling at 70mph compared to 30mph. So at 70mph the engine has to work much harder just to maintain a steady speed.

Objects falling through fluids reach a terminal velocity

When a falling object first sets off, the force of gravity is much more than the frictional force slowing it down, so it accelerates. As the speed increases the friction builds up. This gradually reduces the acceleration until eventually the frictional force is equal to the accelerating force. It will have reached its maximum speed or terminal velocity and will fall at a steady speed

Terminal velocity depends on 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, hamsters and feathers dropped simultaneously will hit the ground together. However, on Earth, air resistance causes things to fall at different speed, and the terminal velocity of any object is determined by its drag in comparison to its weight. The frictional force depends on its shape and area.

The most 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.