Balanced Forces

No change = No net force acting.

Forces act in pairs and many forces can act on an object at any one time. It is the sum of these forces that determines the net force applied to an object.

We often watch our gold fish at home and notice that they can float motionless for a few seconds. Since there is no acceleration, deceleration, change of direction or shape we can say that there is no net force acting on them. But to say there are no forces acting on them is not quite true. There are forces acting on them such as gravity and buoyancy. These forces are equal in size and act in opposite directions.

Right about now we should mention Newton's first law of motion. It states that an object at rest will remain at rest and an object in motion will remain in motion unless acted upon by an unbalanced force.


Consider a car cruising at a constant speed of 100km/hr along a country road. There does not seem to be any change. Is there a force acting on it? Yes ofcourse there is but because there is no change in its shape speed or direction there is no net force.

Net force will be discussed under unbalanced forces.

That is, when all forces acting on the vehicle are added the result is zero force. Gravity is always acting to push the vehicle down but the road is pushing up with the same force. The engine is producing a force pushing the car forwards while drag and friction together oppose this force in the opposite direction.

Notice how the forces exerted on the vehicle act in pairs. Drag and friction act to oppose the forward thrust by the engine while the road pushes upwards to oppose the downward push of gravity.
This toy boat is floating on the water. Buoyancy and gravity are acting on it. What can be said about the two forces acting on the boat?
Bridges are structures that are under extreme gravitational forces. Beams that form the structure of the bridge are either under compression or tension. Compressive forces squeeze the beams inwards while tensile forces stretch the beams outwards. The bridge is not moving so all forces acting on the bridge are balanced.


A toy globe spins, with constant speed, in mid air, as shown on the right. An electro magnet is used to make this toy work.

a) What can you say about the forces acting on the globe?

b) What are the forces acting on the globe?

c) What force acting on the globe is a non-contact force?



What forces act on this toy aluminium boat?

As more weight is put into the toy boat: