1. Centre of Mass/Gravity

2. Learning Outcomes
On completion of this presentation you should be able to:
Define mass, weight and centre of gravity/mass.
Differentiate between the various concepts above.

3. What is Mass?
Mass is the same everywhere in the universe regardless of gravity and is related to the amount of matter a body has and inertia.
Inertia is the property of mass which means that it is
hard to get a massive body moving
also hard to stop it once it is moving
Mass is measured in kilogrammes kg

4. What is weight? WEIGHT is a force It depends on gravity
therefore zero in outer space
one sixth of Earth value on the moon
measured in newtons N
calculated using W = m x g
g = 10 newtons per kg mass

5. What is the difference between mass and weight?
Weight and mass are often confused.
Mass is a measure of inertia, an objects resistance to a change in motion.
Weight is the force an object feels due to gravitational attraction and depends on location.
Weight = mass x gravity

6. Centre of gravity
When examining the motion of objects the total weight of a body is said to be concentrated at its centre of gravity.
Every body or object has a centre of gravity.
It is not necessarily within the body – it may be situated in space outside the body.

7. Tasks Where do you think the centre of gravity would be for:
A rectangular object
A circle

8. Centre of Mass
This is the scientific term for centre of gravity - since the concept is not dependent on gravity
CoM is the single point in a body which represents all the spread out mass of a body
the weight acts at the CoM since gravity acts on mass to produce weight

9. Centre of Mass

10. Where is the COM?
The position of centre of mass depends on the shape of body
This is how the high jumper can have his CoM pass under the bar but he could still clear the bar.

11. Where is the COM?

12. What is the difference between COG & COM?
The term Centre of Mass is often confused with the Centre of Gravity.
The two terms are so similar that they can be used interchangeably.
The COG of an object coincides with its COM if the object is in a completely uniform gravitational field.
If it is not in a uniform gravitational field, then the COG and the COM will be at too different locations.

13. In motion
In linear motion, each point on a body undergoes the same displacement as any other point.
The motion of a body can be described as the motion of its centre of gravity/mass

14. Pictures of motion of COM