A football player at practice pushes a 70 kg blocking sled across the field at a

Question

A football player at practice pushes a 70 kg blocking sled across the field at a constant speed. The coefficient of kinetic friction between the grass and the sled is 0.30.
Part A
How much force must he apply to the sled?
ANSWER:


Problem 9
Zach, whose mass is 73 kg, is in an elevator descending at 12 m/s . The elevator takes 4.0 s to brake to a stop at the first floor.
Part A
What is Zach's apparent weight before the elevator starts braking?
ANSWER:
=
Part B
What is Zach's apparent weight while the elevator is braking?

Explanation / Answer

since its constant speed , so applied force = friction force

Fapp = F friction

F app = N ;

F =     mg ;

F = 0.3 * 70 * 9.8 ;

F = 205.8 N <--------------ans to A .

problem 9 ;

A.

since it descends with constant speed ,

a = 0 ;

N - mg = ma ;

N - mg = m* 0 ;

N = mg ;

N = 70 * 9.8 ;

N = 686 N <------------ans to Part A

B .

v= u + at ;

0 = 12 + a * 4 ;

a = 12 / 4 = 3 m / s^2 ;

N - mg = ma ;

N = ma + mg ;

N = m ( a + g ) ;

N = 70 ( 3 + 9.8 ) ;

N = 896 N <------------ans to Part B

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