Physics help anyone??

Question

A 91.9kg man stands in a 109kg rowboat at rest in still water. He faces the back of the boat & throws a 6.7kg rock horizontally at a speed of 18m/s. The boat recoils forward and comes to rest 2.42m from its original position. The acceleration of gravity is 9.8m/s^2. What is the initial recoil speed of the boat? What is the loss in mechanical energy due to the frictional force exerted by the water? What is the effective coefficient of friction between the boat & the water?

Ok, I know this question is really long, so any help at all would be greatly appreciated. Like just starting me off with the first part would be fine..thanks!

Answer 1

Equate the forward momentum of the boat to the backward momentum of the rock.

If you can explain what happened to the last answer I provided for you, I will consider helping you further with this one.

Your Question:

Physics help???????????

A 114g ball is dropped from a height of 52.3cm above a spring of negligible mass. The ball compresses the spring to a max displacement of 4.04225cm. The acceleration of gravity is 9.8mxs^2. Calculate the spring force constant.



Ok, so I began by finding how much gravitational energy the ball had when it first dropped by using Eg = mass x 9.8 x .523m. Then I set my answer (.5842956) equal to the elastic energy equation: Eel = 1/2k x deltax^2, and I ended up getting k to = 715.1815006 N/m. I think this is way too big, so could anyone tell me if I missed a step or did something wrong in my solution process?



My Answer:

When the spring is fully compressed, the ball has fallen:

(52.3 + 4.04225) / 100

= 56.34225 cm

= 0.5634225 m.



At the instant it reverses, all its energy

0.114 * 9.81 * 0.5634225

= 0.6301 J

is stored in the spring.



The energy in a spring of spring constant k compressed distance x is kx^2 / 2.



k * 0.0404225^2 / 2 = 0.6301

k = 2 * 0.6301 / 0.0404225^2

= 771 N/m.