Buoyancy and mechanical energy

[future_vet]

1. The problem statement, all variables and given/known data
A block of unknown material weighs 50 N in air and 20 N in water. What is the buoyant force of water?

2. Relevant equations
I know how to calculate density etc. I thought the formula to use was -density x V x g but it's not correct.


1. The problem statement, all variables and given/known data
A x Kg mass is attached to a horizontal spring of constant 20 N/m and set into harmonic motion with an amplitude = 0.2m. What is the total mechanical ernergy of the system?

I have no idea what to do... Some help would be greatly appreciated. Thanks!

[Mentz114]

The first problem is rather easy. You're only given two weights ( weight is force) which show the thing weighs less in water. So what's holding it up ?

For the second problem - look here

http://hyperphysics.phy-astr.gsu.edu/hbase/shm.html#c1

[future_vet]

I was thinking it would be 50 - 20 = 30 but it seems a bit too simple to be true...

[future_vet]

So for the second:
0.5 * 25 * 0.2^2 = 0.50J ?

THANK YOU!

[Mentz114]

It's simple and it's true.

I can't check your second answer because I can't see what the mass is.

[edit] E = 1/2ka^2 - whoops, you don't need the mass, so you're right there also.