Why can you use energy conservation for a dropped tennis ball?

[RachelT]

Homework Statement

Using the formulae for potential energy and kinetic energy, find the speed that a tennis ball hits the ground when dropped from a height of 20m.

With reference to the principle of conservation of energy, explain why you could use these equations and what assumptions you have made.

Homework Equations

KE = 1/2mv^2
PE = mgh

The Attempt at a Solution

The first part I think I'm ok with. The tennis ball is dropped therefore no KE only PE.
PE = 9.8 x 20 = 196 Joules.

It is the second part that I don't really understand. What is it asking me? Can somebody maybe re-frase this part of the question. Thank you.

 

[RachelT]

sorry. I forgot to turn that PE into KE by making 196 = 1/2v^2
This would give 19.79m/s

 

[NascentOxygen]

Homework Statement

Using the formulae for potential energy and kinetic energy, find the speed that a tennis ball hits the ground when dropped from a height of 20m.

With reference to the principle of conservation of energy, explain why you could use these equations and what assumptions you have made.

Homework Equations

KE = 1/2mv^2
PE = mgh

The Attempt at a Solution

The first part I think I'm ok with. The tennis ball is dropped therefore no KE only PE.
PE = 9.8 x 20 = 196 Joules.

You can't just drop mass from your equation because the ball's mass is not stated.
Keep mass where it belongs, but represent it by the symbol m.
PE = m x 9.8 x 20 = 196m Joules

 

[CWatters]

Also best to avoid plugging in the number until the end.


PE = mgh
KE = 0.5mv²

0.5mv² = mgh

Mass cancels.

Rearrange to give

v = SQRT(2gh)

Then put the numbers in.

As for part 2...

You need to explain why the KE the ball has when it hits the ground will be equal to the PE it had when dropped. What assumptions does that statement rely on?

Hint: Why would it be more reasonable to make that ssumption on the moon than on earth?

Make sure your answer mentions conservation of energy.