Principle of Moments for a Body in Equilibrium

(b) state the principle of moments for a body in equilibrium.

We know that when a body is in equilibrium, the sum of all the forces acting on that body is equal to zero. For a body in equilibrium, the principle of moments state that the clockwise moment would be equal to the anticlockwise moment.

Clockwise moment is a moment which is in clockwise direction around the pivot. Anticlockwise moment is a moment which is in anticlockwise direction around the pivot.

Look at the above picture. We have to find out whether the above body is in equilibrium or not according to the principle of moments. How can we do that?

It is simple enough. We have to find out whether the clockwise moment around the pivot is equal to the anticlockwise moment. We know that moment = force * distance from pivot. In the above picture we can see that the clockwise moment due to the block of weight 20N is equal to 20*1 = 20 Nm.

Now to the anticlockwise moment. Moment = 10 * 2 = 20Nm. The anticlockwise moment is 20 Nm too.

As both the clockwise and the anticlockwise moments are equal, we can therefore say that the above body is in equilibrium according to the principle of moments.

What are moments

5a) describe the moment of a force in terms of its turning effect and relate this to everyday examples.

The moment of a force, is the measure of the turning effect of a force about a fixed point. This fixed point is usually referred to as axis or pivot. The pivot is a fixed point, around which the turning effect is produced

The moment of a force = force * perpendicular distance from the pivot . Sounds difficult, right?

Let us take a daily life example where moment of a force is used.

The above is a diagram of a door viewed from above. Whenever we open the door, we apply a force

Density

(h) describe how to determine the density of a liquid, of a regularly shaped solid and of an irregularly shaped solid which sinks in water (volume by displacement)

Density

Density is the amount of matter of a body in a fixed volume. The formula to collect density is mass/volume. Its unit is therefore kg/m^3. An object of 1 kg/m^3 density can be said to contain 1Kg of mass in 1m^3 of volume. Different elements and things have specific densities. Therefore,like melting point and boiling point, density can also be used to check the purity of a substance e-g the density of pure water is 1000kg/m^3. The tale of how Archimedes used density to determine whether the king's crown was made of pure gold is well known.

Density of a Liquid

How to Meaure Volume

(g) describe how to use a graduated cylinder to measure the volume of a liquid or solid.


A graduated cylinder can be used to measure the volume of liquids and gases. The following is a diagram of a graduated cylinder.

 

A graduated cylinder gives readings in cm^3, centimeter cube. 

Volume of liquid

Measuring the volume of a liquid with a graduated cylinder is simple. The liquid is poured into the cylinder,

How to measure weight

4(f) describe how to measure mass and weight by using appropriate balances.

 As covered in the previous post, mass and weight are two different quantities. Therefore, we use different equipment for the measurement of mass and weight. To measure the mass of an object, we use a beam balance. The use of a beam balance is covered in the previous post.

To measure the weight of an object, we use a Spring Balance. The following is a picture of a spring balance.

The object to be weighed is attached to the hook at the bottom. The spring stretches due to the weight of the object, and lowers down in the spring balance. The extension of the spring is directly proportional to the weight of the object, so that a heavier object would cause more extension. The spring balance gives readings in N, Newtons.


As the spring balance relies on the weight of an object to give readings, the readings would be different in different gravitational fields. An object which gives a reading of 10N on Earth would give much lower reading on the moon, due to lower gravitational strength of the moon.

Balance Scale

4 (e) explain that weights, and therefore masses, may be compared using a balance.

A balance is a measuring instrument used to measure the mass and weight of a body. It works on the principle of moments, which will be elaborated later. The beam is balanced, or let us say it becomes straight when the masses in both pans are of the same value. The following image is the simplest form of a balance.

In one pan of the balance, we put an object of a known mass. In the other pan, the object with the

What is Weight?

(d) calculate weight from the equation weight = mass × gravitational field strength.

 What is Weight?

Weight is a force that acts on a body when it is in a gravitational field. The magnitude of weight is the same on Earth, that is 10 N. The direction of this force acting on a body is downwards. As weight has a magnitude as well as a direction, it is a vector quantity.

How to Calculate Weight?