144 The Laws of Motion(c)From above, Q=800.N and P=14 0. N.(d)The 3-kg block models the heavy block of wood. The contact force on your back isrepresented by Q, which is much less than the force F. The difference between Fand Qisthe net force causing acceleration of the 5-kg pair of objects. The acceleration is real andnonzero, but lasts for so short a time that it never is associated with a large velocity. Theframe of the building and your legs exert forces, small relative to the hammer blow, to bringthe partition, block, and you to rest again over a time large relative to the hammer blow.This problem lends itself to interesting lecture demonstrations. One person can hold a leadbrick in one hand while another hits the brick with a hammer.P5.55(a)First, we note that FT=1. Next, we focus on themass Mand write TMg5=. Next, we focus on the
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