Terminal velocity

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Key Points

When an object is falling, the two main forces acting on it are and .

When an object falls it can reach terminal velocity.

This is the maximum velocity it can reach when all the forces are balanced.

Review of forces

A is a push or a pull that acts on an object.

There are two types of forces:

  • a contact force - objects touch each other and exert a force

or

  • non-contact force - objects do not have to touch each other to exert a force.

Force is measured in newtons (N).

When two or more forces act on an object a can be calculated.

The size and direction of the resultant force enables us to describe how the motion of the object changes. For example, it might speed up, slow down or change direction.

Click here to revise balanced and unbalanced forces.

A diagram of two arrows. One arrow points left and the other points right. Both arrows are labelled 500 N. The resultant force is 0 N.
The forces acting on the object are balanced and there is no resultant force. The object is stationary or at a constant speed.

If an object is moving upwards and the forces acting on the object are unbalanced, there is a resultant force. The resultant force is acting in the same direction as the motion.

In this diagram, the object is accelerating, or speeding up.

A diagram of two arrows. One arrow points upwards and is labelled 700 N. The other points downwards and is labelled 500 N. The resultant force is 200 N.

If an object is moving downwards and the forces acting on the object are unbalanced, there is a resultant force. The resultant force is acting in the opposite direction to the motion.

In this diagram, the object is decelerating, or slowing down.

A diagram of two arrows. One arrow points upwards and is labelled 50,000 N. The other points downwards and is labelled 10,000 N. The resultant force is 40,000 N.

Free fall

When an object is falling, the two main forces acting on it are and .

A diagram of two arrows of equal length. The arrow pointing up is labelled Air resistance and the arrow pointing down is labelled Weight.

The weight of the object remains constant throughout the fall. The weight of the object is the force due to the mass being acted on by Earth's gravity.

The air resistance is a frictional force which acts against the movement of the object. However, if an object is falling in a vacuum there is no air resistance.

As an object falls it accelerates to begin with. The faster the object is travelling the greater the air resistance acting against it.

Terminal Velocity

Terminal velocity is the maximum speed achieved by an object freely falling through a gas or liquid.

At terminal velocity, the forces acting on the object are balanced so it is no longer accelerating.

For example, when a skydiver jumps from an aeroplane, they speed up because to begin with the is less than their weight. As they speed up, the air resistance increases until the two forces are balanced, so they eventually reach a maximum speed and don’t go any faster. This is their terminal velocity.

A skydiver with two arrows of equal length coming from the skydiver. One arrow points upwards and is labelled Air resistance. The other arrow points downwards and is labelled Weight.

The two main factors which affect the terminal velocity of an object falling through a fluid are the mass and the shape of the object. The larger the mass of the object, the greater the weight.

Objects with large surface areas will often experience a large amount of air resistance when they move. We describe these objects as being less .

Higher terminal velocity

Smaller surface area = less air resistance.

For example, when a person falls, their surface area is relatively small which produces only a small amount of air resistance. The person is able to reach a high terminal velocity.

A skydiver jumping from a plane

Lower terminal velocity

Large surface area = more air resistance.

For example, when a parachutist opens their parachute, the surface area increases. This produces a large amount of air resistance. The parachutist is able to reach a low terminal velocity.

A parachutist heading towards the ground

Describing the motion of a falling object

These diagrams show what happens as a parachutist jumps out of a plane, freefalls, and then opens her parachute.

  1. The skydiver jumps out of the plane and accelerates.
A skydiver has jumped from a plane. The parachute is closed. An arrow labelled Air resistance points up. A longer arrow points down and is labelled Weight indicating a large resultant force.
  1. The skydiver continues to accelerate. As she speeds up the air resistance increases.
A skydiver has jumped from a plane. The parachute is closed. An arrow labelled Air resistance points up. A shorter arrow points down and is labelled Weight indicating smaller resultant force.
  1. The skydiver has reached terminal velocity - weight and air resistance are balanced.
A skydiver has jumped from a plane. The parachute is closed. An arrow labelled Air resistance points up. An arrow of the same length points down and is labelled Weight indicating resultant force is 0.
  1. The skydiver pulls her parachute. This drastically increases her surface area which increases the air resistance and slows her down.
A skydiver has jumped from a plane. The parachute is open. An arrow labelled Air resistance points up. A shorter arrow points down and is labelled Weight indicating a large resultant force.
  1. The skydiver is decelerating. As she slows down the air resistance decreases.
A skydiver has jumped from a plane. The parachute is open. An arrow labelled Air resistance points up. A shorter arrow points down and is labelled Weight indicating smaller resultant force.
  1. The skydiver has reached a new lower terminal velocity. The terminal velocity here is much slower than the terminal velocity of the skydiver before she opened her parachute.
A skydiver has jumped from a plane. The parachute is open. An arrow labelled Air resistance points up. An arrow of the same length points down and is labelled Weight indicating resultant force is 0.