Balanced and Unbalanced Forces
• Introduction

For an object to move the forces on it must be unbalanced.

In the diagram below the duck, is floating on the water. It is not moving up or down because the force acting downwards caused by gravity is equal to the force of the water pushing up onto the duck (upthrust).

The force of gravity (red arrow) is equal to the upthrust (green arrow) from the water.

The car shown below is accelerating because the driving force (green arrow) is greater than the drag caused by air resistance (red arrow).

The car is accelerating left to right becasue the driver force is greater than the air resistance

When the drag is greater than the driving force the car will slow down, but if they are balanced and the car is already moving, it will continue to move at a constant speed. This is described in more detail in Isaac Newton's 'Laws of Motion'.

There are many forces acting on a skydiver when he leaves the plane:

As a skydiver leaves the plane he accelerates downwards due to gravity, but as his speed increases, so does the air resistance. This continues until both forces are balanced and the skydiver will fall at a constant speed, called terminal velocity.

• Question 1

Complete the sentence below by typing the missing word into the text box.

When an object is stationary then the forces on it are _____________.

• Question 2

Complete the sentence below by typing the missing word into the text box:

When an object is moving then the forces on it are _____________.

• Question 3

Complete the following sentence by selecting one of the options below:

If a ball falls a short distance, the downwards force is........

smaller than the air resistance

greater than the air resistance

equal to the air resistance

• Question 4

Complete the following sentence by selecting one of the options below:

At terminal velocity the downwards force is.....

smaller than the air resistance

greater than the air resistance

equal to the air resistance

• Question 5

Complete the following sentence by selecting one of the options below:

A cyclist peddles to 10 m/s and then stops peddling, which results in his speed decreasing because the driving force is....

smaller than the air resistance

greater than the air resistance

equal to the air resistance

• Question 6

A harrier Jump-jet is capable of hovering as well as performing many manoeuvers which require forces in all directions.

Complete the table below to give the direction of the force when the plane is moving

 forces balanced downwards force high upthrust high moving downwards moving upwards hovering
• Question 7

Complete the table below showing the forces on a skydiver at various points during the jump.

 forces balanced downwards force high upthrust high accelerating downwards terminal velocity parachute deployed
• Question 8