Unbalanced Forces


A force is a push or a pull that causes an object to either:
- increase or decrease its speed
- change its direction
- change its shape.
If any of these things happen then a net force must be acting on the object in question.


A net force is the overall force acting on an object. If the net force acting on an object is not zero then we say that unbalanced forces are acting on the object. If the net force is zero then there may be no force acting on the object or all the forces acting are balanced.

When a rocket accelerates skyward a net force is acting to push it skyward. The forces of gravity and thrust, which act in opposite directions, are not balanced.

A hovering stationary aircraft is experiencing zero net force, hence its speed, shape and direction remain unchanged. The forces acting on this aircraft are said to be balanced.

This F111 Fighter plane is accelerating skyward. Since it is accelerating, its speed is increasing, a force must be acting on it. Click to find out more about acceleration. The forces acting on it include drag, lift, gravity and thrust. Obviously the plane is acted on by unbalanced forces. Since the plane is climbing, the force of lift is greater than the force of gravity and thrust is greater than drag.
The racing car on the right accelerates along a horizontal road surface. Once again unbalanced forces are acting on this vehicle. The force acting on this vehicle include drag, friction, thrust from the engine, gravity and the push upwards by the road surface. Since the car accelerates the thrust produced by the engine is greater than the drag and friction forces acting on the vehicle. The road acts to push the car upwards with a force equal and opposite to the force of gravity pushing down.

The Tomcat pictured on the left is accelerating horizontally close to the surface of the water.

What forces are acting on this plane?

What are the balanced forces?

What are the unbalanced forces?

What happens to the force of drag as the plane slows down?

The skydiver is descending at a slow constant speed. The drag forces are maximised by the parachute and oppose the force of gravity pushing down.

In what direction are the drag and gravitational forces acting?
Are they balanced?
What would happen if both of these forces were equal and acted in opposite directions?


Click to see a 120kb movie of a goldfish swimming around in a bowl. Explain why it is obvious that unbalanced forces are acting

The vertical take-off jet shown on the right accelerates skyward from a standing start and then falls back to earth again.

What type of forces are acting on the aircraft?

Which force can be controlled by the pilot to lower the plane or raise it off the ground?

What forces are acting on the plane when it is on the ground with its engines shut down? . What can you say about these forces? .





Consider the jet pictured on the right taking off down a runway. Which statement is true?


A car is travelling on a straight road, its speed vs time graph is shown on the right. When answering the questions below keep in mind that a net force is acting when the speed of an object changes.

a) Over which period of time is a force applied to the car to increase its speed?

b) Over which period of time is there no net force acting on the car?

c) Over which period of time are frictional forces larger than thrust created by the engine?

d) When is the force of thrust created by the engine greater than the drag and frictional forces acting on the car?


What force is maximised by the parachute?

The Shuttle is slowing down. Which statement is true?


During the Olympics, on windy days, records that are smashed may not be recorded. The wind can assist athletes to jump to record heights or lengths. The net force acting on athletes is increased by wind, as shown on the right.

a) How does a runner generate thrust?

b) Why is drag acting in the opposite direction to thrust?

c) If an athlete is able to complete a Long Jump which of the following is true?

d) It is important for an athlete performing the Long Jump not to carry too much weight . This is to

e) When is wind capable of increasing the net force ?

f) When is wind capable of decreasing the net force ?

g) What is the net force acting on the athlete as he jumps off the runway, if he jumps with a force of 15N and with a tail wind of 3N acting while the drag force as he moves through the air is 1N? Solution.


What are the forces acting on the rocket as it ascends skyward?

Are there any forces that impede the rocket from ascending?

A rocket that is orbiting Earth but out of the atmosphere will most likely not experience


What other force should be included in this diagram ?

In what direction should it be acting?

What force is capable of overcoming this force?


How do we know that a net force acted upon this vehicle?
Continue with the forces of yachting