P2aL6 Terminal Velocity

Key Words

Accelerating - changing in speed over a time interval.

Air resistance - drag experienced by moving through air.

Friction - force opposing movement caused by surfaces rubbing against each other.

Gravitational field strength - acceleration due to gravity, or force per unit mass.

Resultant force - force that occurs on an object when two or more forces are applied.

Terminal velocity - the maximum downwards velocity of an object falling through a fluid.

Velocity time graph - a graph in which velocity (vertical axis) is plotted against time (horizontal axis).

Weight - a force acting on an object caused by gravity.

Test Yourself

Homework
Physics GCSE
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Grade E

When a car first drives off, the friction and air resistance are very low.  The resultant accelerating force is high, so the acceleration is high.   As it speeds up, the air resistance increases, so the resultant force gets less, and the acceleration gets less.

Eventually a point is reached where the driving force is balanced by the air resistance and friction.  The car has reached is maximum speed.

This can be changed by:

  • Having a more powerful engine to give more of a force.

  • Having a more streamlined shape to cut down air resistance.

This car will have a higher maximum speed.

The same is true of objects falling through the air.  When a parachutist leaps out of an aeroplane, the only force acting on her is her weight.  Weight is calculated by this equation:

weight (N) = mass (kg) × gravitational field strength (N/kg)

The Earth's gravitational field strength is 9.8 N/kg.  At GCSE level, we usually say 10 N/kg.  Look at this animation:

As she speeds up, air resistance increases, so the acceleration gets less.  Eventually the force from the drag balances the force due to gravity.  Then she falls at terminal velocity, which for a parachutist is about 60 m/s (220 km/h).

Without a parachute, she would hit the ground at this speed, which will be pretty terminal.  So she pulls open her parachute, which has a huge area, and slows her terminal velocity down to about 5 m/s.  Modern parachutes allow the parachutist to "land" like an aeroplane.

 

Grade C

The velocity time graph for a parachutist descent is like this:

The graph shows us:

  • At A the parachutist accelerates at 10 m/s2.  Actually the acceleration would not be constant.

  • At B the parachutist is falling at a constant 60 m/s (terminal velocity).  The forces are balanced.

  • At C the parachute has been opened and the parachutist is slowing down to 5 m/s.

  • At D she is drifting down at 5 m/s.

  • At E, she lands.

Terminal velocity can be affected by the weight, or the area offered to the air resistance.  By diving head first, the terminal velocity can be increased.

Grade A

Many people have tried to be bird-men, making wings and jumping off tall towers.  For 72 out of 75 who have attempted, the experience was terminal.  Bird-brains!

It is now possible to get a bird suit, and jump from a plane at 4000 m, gliding down at a terminal velocity of 10 m/s.