Hints on Set 4

Hint on Set 4, problem 3:

The masses are given in pounds (lb); 1 kg = 2.2 lb. You can either convert lb to kg and give the answer in Newtons (N); or you can use the (confusing!) English system of units, in which 1 lb-force = (the weight of a 1 lb mass) = 4.45 N. Example: the gravitational force on a 10 lb mass is just 10 lb-force.

Hint on Set 4, problem 6 = KK 2.16:

The block slips and slides on the slanted face of the wedge. The slanted face of the wedge can be described as a straight line

      y = - x + At²/2

with slope -1, and with an intercept that depends on time. Then take the second derivative in time to get

      d²y/dt² = - d²x/dt² + A

so that the components of the block's acceleration are related by

      a_y = - a_x + A

You should now be able to simultaneously solve this equation together with the two components of Newton's 2nd law for the block

      F=ma

where as usual, the total force on the block is the sum of gravity and an unknown normal force from the wedge.