Shaun and a friend are pushing Shirley (m = 70.0 kg) with a force F. The coeffic

Question

Shaun and a friend are pushing Shirley (m = 70.0 kg) with a force F. The coefficients of static and kinetic friction between Shirley's feet and the turf are 0.800 and 0.650 respectively. What is the value of the normal force between Shirley and the ground? If Shirley has no velocity, what it the minimum force that Shaun and his friend must apply in order to get Shirley to start sliding? Suppose that Shaun and his friend give Shirley a big shove all at once, applying a force of F = 1030 N. What is Shirley's acceleration during this shove? (Take the positive values to be toward the right.)

Explanation / Answer

m = shirley mass = 70 kg

us = cofficient of static friction = 0.800

uk = cofficient of kinetic friction = 0.650

i) Normal force, N = mg

N = 70*9.81

N = 686.7 Newton

ii) velocity = 0

F_min = f ( f = friction force = u*N)

= us*mg

= 0.800*70*9.81

= 549.36 N

iii) F ( applied force ) = 1030 N

F - f = ma

F - uk*mg = ma ( a = acceleration)

1030 - 0.650*70*9.8 = 70*a

a = 8.34 m/s2

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