# Physics: Work

posted by
**Matt** on
.

1st question: I was solving a question and I was asked the work done by friction. It is natural that work done by friction (a retarding force)should be negative and in my answer it was. However, just to be sure, work can be negative?

2nd question: A block of m = 2.00kg is moving a long a frictionless horizontal surface. The block reaches the relaxed spring (x ini. = 0) with an initial velocity of 6 m/s and compresses it to x final = 0.150m. At this value of compression, the speed of the block is reduced to v final = 4.00 m/s.

a) during this process, what is the change in Kinetic energy of the block?

sol) mv2(f)/2 - mv2 (i)/2 (correct?)

B) How much work has been done by the spring force on the block.

W(s)= mv2(i)/2 - mv2(f)/2

c) What is the spring constant of this spring. (hint: use work-kinetic energy theorem)

would it be x * whatever I get for part A?

Do any of the above answers make sense?

work done is negative when work is done on the system.

a correct

b correct

c No. change KE= 1/2 k x^2 solve for k.