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Forces & Terminal Velocity


Mass (m) is measured in kilogram (kg). It is the amount of matter an object is made from. An objects mass is the same on all planets.


Weight (W) is measured in newtons (N) it is a force. It depends on the mass of the objects and the gravitational field strength on the planet. An objects weight can be different on different planets because gravity can be different strength on different planets.

On the earth the gravitational field strength is 10 newtons per kilogram (N/kg). So a 60 kg person has a weight of 600N.

weight         =              mass            x     gravitational field strength

(newtons, N)        (kilograms, kg)       (newtons per kilogram, N/Kg)

Objects falling through fluids (liquids and gases)

  • when the object starts to fall it is travelling slowly so air resistance is small compared to the weight of the object. There is a large resultant force and the  object accelerated quickly.
  • As the object accelerates the air resistance increases but the weight stays the same, so the resultant force is not as great as before but the object is still accelerating.
  • Eventually the air resistance force is the same size as the weight of the object, this makes the resultant force zero. Therefore the object falls at a steady speed it is no longer accelerating. This is called terminal velocity.
terminal velocity


terminal vel graph

a. air resistance is much smaller than weight as object is travelling slowly so large resultant force giving a large acceleration.

b. object has increased speed up so air resistance is bigger making resultant force smaller, object still accelerating but at slower rate.

c. air resistance = weight so there is a zero resultant force. Object not accelerating but at terminal velocity.

d. parachute deployed giving a large increase in surface area and therefore a large resultant force upwards now against the direction of motion this causes the object to decelerate (it does NOT go up).

e. object has now slowed so air resistance gets less until it again equals the weight of the object a new lower terminal velocity is achieved which makes it safe for the person to land.