1. The problem statement, all variables and given/known data 1. A block mass m is pushed up against a spring, compressing it a distance x, and the block is then released. The spring projects the block along a frictionless horizontal surface, giving the block a speed v. The same spring projects a second block of mass 4m, giving it a speed 3v. What distance was the spring compressed in the second case ?? 2. When the jet powered car went out of control during a test drive, it left skid marks about 9.5km long. (a) If the car was moving initially at a speed of v=708km/h, estimate the coefficient of kinetic friction. (b) what was the kinetic energy K of the car at time t=60s after the brakes were applied ? Take the mass of the car to be 1250kg 2. Relevant equations E=PE +KE +Uel 3. The attempt at a solution 1.I did like this: Initial: E=kx^2/ 2 Final:E=mv^2/2 Then I set them equal. and solve for x. When I did the final energy, I substitute m by 4, and v^2 by 9V^2 However I got a wrong answer. Where did I do wrong ??? 2. I also did the same thing for this problem Initial: E= mv^2/2 Final E=0 Emech= mv^2/2 = -Etherm Then I set E therm =Fx=mg*coeff of kinetic friction*x Then I solved for coeff by substituting x=9500m and m=1250kg. I got coeff= 0.001 It was wrong. Can anyone check my answer ?? For part (b), I use vf^2= Vi^2 + 2ax to find a=-2.036 The v=v0 +at and get 74.54 KE= mv^2/2 I got 46.6 KJ I was wrong again. Anyone know ??