A bodybuilder configures a leg-press apparatus (see figure) to a resistance of 1

Question

A bodybuilder configures a leg-press apparatus (see figure) to a resistance of 1080 N (about 243 lb). She pushes the weight to her full extension and comes to rest.

a. What is the normal force on each of her feet?

b. Assume she is able to move the push-plate of the leg-press machine at an acceleration of 0.50 m/s2 for the first half of the displacement. What is the normal force on each foot during this period of acceleration? Assume the resistive force is due to a mass with weight of 1080 N hanging on the cable of the leg-press apparatus.

A hockey puck is sliding down an inclined plane with angle = 12.2° as shown in the figure below. If the puck is moving with a constant speed, what is the coefficient of kinetic friction between the puck and the plane?

The person shown below is pushing on a refrigerator that sits on a level floor. Assume the section of the floor underneath the refrigerator is very slippery so that the only horizontal force on the refrigerator is due to the person. If the force exerted by the person has a magnitude of 134 N and the refrigerator has a mass of 196 kg, what is the magnitude of the refrigerator's acceleration? (m/s2)

Explanation / Answer

First Question -

(a) Normal force on each feet = 1080 / 2 = 54-0 N

(b) 243 lb = 110 kg.

Normal force on each feet in this condition = [1080 + (110*0.5)] / 2 = 567.5 N

Second Question -

Here the puck moves at constant speed, there is no NET force in the direction of motion.

Further -

1) the weight of the puck = mg,
and
2) the friction force.

a diagram of the forces & components of mg (weight of puck) on incline reveals:
mg sin 12.2 = FF {where FF = friction force acts parallel to and up plane"
mg cos 12.2 = Normal Force = N
µ = coefficient of kinetic friction

by definition
µ = FF/N = mg(sin 12.2)/mg(cos 12.2)
cancel "mg"s
µ = sin 12.2/cos 12.2 = tan 12.2 = 0.22

Third Question -

Magnitude of refrigerator's acceleration = F / m = 134 / 196 = 0.68 m/s^2.

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