1 of 32© boardworks ltd 2009. 2 of 32 3 of 32© boardworks ltd 2009 balanced and unbalanced forces...

1 of 32 © Boardworks Ltd 2009


Balanced and unbalanced forces

When forces in opposite directions have the same value, they cancel each other out. The forces are balanced. The object is either stationary or moves at a constant speed.

If the forces on an object are not balanced, there is a resultant force, and the object accelerates in the direction of this force.


Turning effects from balanced forces


Examples of turning effects

These are some examples of everyday turning effects:

Can you think of any more?


What is a moment?

A moment is a turning effect caused by a force.

The size of the turning effect caused by a force depends on two factors. What are they?

The bigger the force, the larger the turning effect.

The larger the perpendicular distance from the pivot to the point where the force is applied, the larger the turning effect.


In this example, the forces are constant, but the larger the perpendicular distance, the faster the roundabout accelerates:

The moment equation

moment (Nm) = force (N) × perpendicular distance from pivot (m)

The size of a moment obeys the moment equation:

perpendicular distance

force


Perpendicular distances


When Mark and Tim lift their feet off the ground, what will happen to the see-saw?

Clockwise and anticlockwise moments

The force of gravity creates a turning effect on each person. On the left-hand side, it creates an anticlockwise moment.On the right-hand side, it creates a clockwise moment.

The clockwise moment is larger than the anticlockwise moment, so the see-saw will turn in a clockwise direction.

Tim Mark


The principle of moments

If an object is balanced about a pivot, then we can say that:

sum of anticlockwise moments

This is the principle of moments.

So, the forces on an object must obey two conditions to keep the object completely stationary:

sum of clockwise moments=

1. The forces must be balanced.

2. The moments must be balanced.


Using the principle of moments


What is a centre of mass?

Can you balance a ruler on the flat end of a pencil? If so, where do you put the pencil to make it balance?

The point at which all the mass of an object seems to be concentrated is called its centre of mass.

The ruler is stationary when the moments from the weights of its entire length balance out.

This is equivalent to one weight force acting through the centre of the ruler.


Forces and centres of mass

When forces on an object are balanced, its centre of mass is stationary, or moves at a constant speed.

“When the forces on an object are balanced, it is either stationary or

moves at a constant speed.”

How can the idea of centre of mass help us to understand this statement?

What does ‘stationary’ mean if an object is turning?


Centre of mass or centre of gravity?

Imagine that the gravitational field on the left of this picture suddenly becomes much stronger than the gravitational field on the right. Would the see-saw still balance?

The see-saw’s centre of mass is still in the centre of the picture. But its centre of gravity – the point where all its weight appears to act – has moved to the left.

In a uniform gravitational field, such as on the Earth’s surface, centre of gravity is equivalent to centre of mass.


Finding a centre of mass


Understanding centres of mass


When is an object stable?

An object is said to be in stable equilibrium if it returns to its original position when it is disturbed.

This happens because its centre of mass moves upwards when pushed, so gravity tends to pull the object back down towards its equilibrium position.

An object is in stable equilibrium if its centre of mass is suspended below a pivot, or balanced above a wide base.

C.O.M.


When is an object unstable?

An object is said to be in unstable equilibrium if it falls or topples when it is disturbed.

This happens because its centre of mass moves downwards when pushed, so gravity tends to pull the object further away from its equilibrium position.

An object is in unstable equilibrium if its centre of mass is balanced above a pivot point or a narrow base.

C.O.M.


Stable or unstable?


Neutral equilibrium

An object is said to be in neutral equilibrium if it moves sideways when it is disturbed.

This happens because its centre of mass stays at the same height when pushed, so gravity does not tend to pull the object towards or away from its original position.

Spheres and cylinders are examples of objects that can be in neutral equilibrium when resting on flat ground.

C.O.M.


Recognizing states of equilibrium


Losing equilibrium


Making things more stable

Look at the picture to the right. Can you explain why this situation is dangerous, using ideas about centre of mass and stable and unstable equilibrium?

How could you make the bus more stable?

Think of two solutions.


Understanding stability


Glossary


Anagrams slide


Multiple-choice quiz

1 of 32© boardworks ltd 2009. 2 of 32 3 of 32© boardworks ltd 2009 balanced and unbalanced forces...

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