1. The problem statement, all variables and given/known data A block of mass 1.60 kg is pulled up a rough 30 degree incline at constant speed by a string parallel to the surface. The friction force on the block is 7.00 N. The incline is 4.00 m long and 2.00 m high. (1) How much gravitational potential energy did the block gain? (2) How much work was done by friction? (3) How much work was done by the string? 2. Relevant equations PEG = m*g*y W = F(parallel)*d 3. The attempt at a solution (1) [using equation 1] The gravitational potential energy gained was just (1.6 kg)(9.8 m/s2)(2 m) = 31.4 kg*m2/s2, which is 31.4 J. (2) [using equation 2] The work done by friction was (7 N)(4 m) = 28 J. BUT, the answer key says -28 J, so my best reasoning for why is that, the friction force of 7 N is opposite the direction of motion, which makes this value negative. Can someone please tell me if my line of thinking is correct, and add an explanation that will allow me to (in the future) understand when the value should be negative and when it should not? (3) For the amount of work done by the string, I really have no idea. It would need to be greater than the 28 J of work done by friction in order to pull the object up the incline (forgive me if this is a gross oversimplification), and it would also be combatting gravity. And surely the mass must be figured in as well. And, I believe, the distance traveled. Can anyone help me in setting up an equation that would correctly integrate everything that must be included and lead me in the right direction? Thank you all in advance.