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**1. Homework Statement**

1) If a planet moves in a circle about the sun, how much work does the force of gravity do on the earth? Give reasons for your answer.

2) A force of 120N pushes a crate mass 15kg along the ground at a constant speed of 3.2 m/s for a distance of 6.2m. What amount of heat energy was generated at this time?

3)The earth has a mass of 5.98 x10^24 kg and a speed of 2.97 m/s. The sun, which is 1.49 x10^11m away, exerts 3.56 X10^11 N on it. If the earth orbited the sun in a perfect circle, how much work would the sun do on the earth in one second?

4) A car of mass 1350kg rolls from rest down a hill 235m long and 25m high. If its speed at the bottom of the hill is 12.0m/s, what's the force of friction on the car?

**2. Homework Equations**

Fnet=ma

pi*diameter

Ep=mgh

Ek=(0.5)mv^2

**3. The Attempt at a Solution**

1) I said none because the disatnce in the equation force times distance has to be a parallel distance to the force exerted on it, and its not in this case.

2)6.2m/3.2m/s=1.9375 sec

120N-Ff=m(0)

Ff=120N

W=Fd

W=120x6.2

W=744J

3)2(1.49 x10^11m)pi

d=2.97m

(3.56 x10^11 N)(2.97m)=1.06 x10^12 J

4)(1350kg)(9.8)(25m)

Ep'=330750 J

Ek2=(0.5)1350(144)=97200 J

W= 330750-97200=233550 J

233550J=235F

F=994N