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Gravity -

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Gravity

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Dr Derek Muller takes to the streets with a medicine ball and a basketball and poses the question:
when dropped from the same height, which one will hit the ground first? You may know the correct
answer, but can you explain why? This bouncy segment has Derek playing with his hapless subjects
before delivering the goods and putting them out of their bemused puzzlement.

Dr Derek Muller

Now I want you to make a prediction. In my left hand I have a standard sized basketball and in my
right hand a 5kg medicine ball. If I drop them both at exactly the same time, which one will hit
the ground first?

Man 1

This is a trick one isn't it

Man 2

The heavier one will go down first

Man 3

Yeah, it'll drop faster

Man 4

They'll both hit the ground at the same time

Dr Derek Muller

Give me your prediction

Woman 1

I actually would have thought this one would go faster because it's heavier

Boys

Maybe it's that one. Maybe this one yeah

Man 1

So you reckon this one will be faster?

Woman 2

I reckon that one will be faster

Dr Derek Muller

So why does that make it go faster?

Woman 1

Because the weight pulls it down quicker?

Dr Derek Muller

What are we measuring when we say it is heavier, what are we feeling?

Man 1 and Woman 1

Gravity?

Woman 1

Objects being pulled to the earth I guess.

Dr Derek Muller

OK, so which of these objects is being more pulled to the earth?

Man 1

That one, the black one

Dr Derek Muller

Well, here's what we're going to do. I want you to hold those above your head. On the count of
three, ready, 3,2,1.

Man 1

Exactly the same

Woman 1

They fell at the same time

Woman 3

Exactly the same time

Man 5

They hit at the same time

Dr Derek Muller

Was that what you expected to see?

Man 5

No

Dr Derek Muller

So what do you make of that?

Woman 1

I need to go read more books.

Man 5

Magic?

Woman 3

Gravity is going to pull at the same rate no matter how heavy or how light it is.

Dr Derek Muller

But gravity is how light or heavy something is?

Woman 3

It's just one big massive mind (beep)

Man 4

There must be some force that's dragging them down and the actual weight of it doesn't matter

Woman 2

Something to do with mass or something

Man 1

I can't remember why

Woman 1

It's almost like the weight holds it back in a way, even though that doesn't seem to make sense.

Dr Derek Muller

I think you might be hitting on an interesting point there, weight holds it back

Dr Derek Muller

Now it's not actually weight that holds it back, but inertia. That's the tendency of matter to
maintain its state of motion, so to remain at rest if stationary or to continue with constant
velocity when in motion

Man 1

What's the big idea?

Dr Derek Muller

The big idea is this one has more mass, so it's got more weight which you can clearly feel, but
it's also got more inertia which means it's also more sluggish, it tends to resist acceleration, so
that greater force is required to accelerate it at the same rate as this ball.

Woman 1

Like a heavy car trying to accelerate

Dr Derek Muller

Like a heavy car trying to accelerate you need more force to get it going, exactly. So what does
all that mean? Well the force on the medicine ball is greater than the force on the basketball, but
it has more inertia and what's really important is that the ratio of force to inertia is the same
for all objects, so everything accelerates at the same rate and lands at the same time.