# Force and Kinetic Energy

Now, suppose that Zak's younger cousin, Greta, sees him sliding and takes off her shoes so that she can slide as well (assume her socks have the same coefficient of kinetic friction as Zak's). Instead of getting a running start, she asks Zak to give her a push. So, Zak pushes her with a force of 125 \rm N over a distance of 1.00 \rm m. If her mass is 20.0 \rm kg, what distance d_2 does she slide after Zak's push ends? Remember that the frictional force acts on Greta during Zak's push and while she is sliding after the push. F= Fp-Fr

E= F*Lp= (Fp-Fr)*Lp= Fr*Lr

Lr= Lp*((Fp/Fr)-1)

Lr= 1*((125/(20*9.8*0.25))-1)= 1.6 m

Mark pushes his broken car 150 m down the block to his friend’s house. He has to exert a 110 N horizontal force to push the car at a constant speed. How much thermal energy is created in the tires and road during this short trip? thermal energy is created in the tires and road

= 110 * 150

=16500 J

A 30 kg child slides down a playground slide at a constant speed. The slide has a height of 4.0 m and is 7.0 m long. Using energy considerations, find the magnitude of the kinetic friction force acting on the child. The friction force F is parallel to the slope and is constant in magnitude, so its work is W = - F d

with d = length of the slide.

ΔU = m g Δh

Therefore:

- F d = m g Δh

F = - m g Δh / d = - 30 x 9.8 x (- 4.0) / 7.0 = 168N

A block of weight w = 15.0 N sits on a frictionless inclined plane, which makes an angle θ = 23.0° with respect to the horizontal, as shown in the figure. A force of magnitude F = 5.86 N, applied parallel to the incline, is just sufficient to pull the block up the plane at constant speed.

Part A

The block moves up an incline with constant speed. What is the total work WTotal done on the block by all forces as the block moves a distance L = 3.40 m up the incline? Include only the work done after the block has started moving at constant speed, not the work needed to start the block moving from rest....

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