newton law problem?

Question

Two blocks with masses of 45 kg and 23.5 kg are stacked on a table with the heavier block on top. The coefficient of static friction is 0.6 between the 2 blocks and 0.3 between the bottom block and the table. A horizontal force is slowly allpied to the top block until one of the block moves.
a) Where does slippage occur first, between the 2 blocks or between the bottom block and the table? Explain
b) What value of the coefficient of static friction between the bottom block and the table would change the answer to part (a)?

A hockey puck is hit on a frozen lake and starts moving with a speed of 12.0 m/s. Exactly 5.0 s later, its speed is 6.0 m/s.
a) What is its acerage acceleration?
b) What is the coefficient of kinetic friction between the puck and the ice?
c) How far does the puck travel during this 5.0 sinterval?

Could someone please help me?

Answer 1

For the first question:

when working with static friction, I use the equation that the maximum force to cause slippage is
Fmax=u*m*g
where u is the coefficient of static friction
m is the mass
g is the gravitational constant

for the interface between the blocks
Fmax=.6*45*9.8
=264N
for the interface at the table
Fmax=.3*(45+23.5)*9.8
=201.39N

That shows that the first slippage will occur at the table interface.

Part b:
This requires that the Fmax at the table be greater than the Fmax at the block interfaces.
so
.3*68.5*9.8 .3*68.5/45 .4567
For the second question, this is solved using conservation of energy, where the loss in kinetic energy is caused by the frictional loss of the puck sliding in the ice.


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