A block is pushing a spring with spring constant k=80N/m. The spring is compressed by 10cm. The block is released and stops 40 cm away from the point of compression.


What is the mass of the block if the kinetic friction coefficient between the block and the ground is .15?

As with your other post - please show us your attempt at solving this and we'll help if you get stuck. But here's a hint to get you started: equate the PE of the compressed spring with the work done by friction on the block (force of friction times distance traveled).

Thank you!
Using your method I got the answer:)

I still don't understand though - why we can equate work of friction to the potential energy of the spring?

There is a principle that the energy expended to do work is equal to the work itself. When the spring is compressed the object is at rest, so its energy is equal to the PE of the compressed spring. When the spring is released all the PE is converted into kinetic energy (KE). Then as the object slides along the surface it loses KE due to having to oppose the force of friction, and the amount of work it does is equal to the force of friction times the distance it distance it sides. When the block comes to rest it has no KE left. Hence the amount of work performed by the block equals the KE it had after being releases by the spring, which equals the PE that the spring started with.