## What is at the centre of gravity? V.

When an object is in motion, it can be very complicated. Just think about the way a car starts moving or stops moving – it behaves a whole group of bits and pieces – it does not behaves as a single unit.

But if we have to deal with every object and figure out what is happening with each piece, physics would be insanely complicated. It is necessary to do these complex calculations if you are an engineer, but as a student, you are trying to learn the underlying principles, not guarantee that a bridge is going to stay up!

So how do we simplify these complex problems? We use a concept called “Centre of Mass” (note that this is similar to centre of gravity – it is the same physical point in an object). The centre of mass of an object is the point at which, if all the mass of the object where concentrated there, it would behave identically when in motion.

This means that an object in motion can be treated as a single point – and that makes your work a lot easier – all the calculations can be simplified down to a single application of each needed formula, rather than having to repeat it for each part (or even particle!) of the item.

So how do you find the centre of mass? Well, remember that any object’s centre of mass is the point at which you can treat all the mass of the object being concentrated. Support the object from any point, and that centre of mass will be suspended below (or above) the point of support.

This does lead to some cool tricks you can do that can do (like the tilted coke can). You may want to try some of them out! (1, 2, 3).  But the important relationship, as we experimented with when you tried to stand on one foot, is between centre of mass and balance. If your centre of mass is not above your support, you will tip over – check this video:

So, that’s it. You need to understand how centre of mass allows us to use the XUVAT equations and what the relationship is to stability (balance). You may also want to play this game…

You have completed a practical to identify the centre of mass of two separate objects and those two objects combined. Your work requirement is to explain how you found the centre of mass of an object, and how the centre of mass of the two parts compare the centre of mass of the whole object.

See you in class!

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### 11 Comments on “What is at the centre of gravity? V.”

1. JACK is awesome. Says:

We should do something on the Gömböc

2. Ashraf Says:

Sir!!!! when is this work reqirement DUE??

3. darrren10 Says:

Hey guys, if there’s anyone out there who was a little confused about turning x-t to v-t to a-t graphs or wants to consolidate their knowledge, i found that this video was pretty useful.

4. Lovleen Says:

Mr. G,
Is the motion test we did in class up on the blog??

5. Brendan Says:

Hi sir!

Could you please put up the tests for us to redo so we can hand in our 4 work requirements on Monday.

Thanks

6. […] Motion: XUVAT equations, Centre of mass, Energy transformations, Basic […]