# Physics

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(a) How high a hill can a car coast up (engine disengaged) if work done by friction is negligible
and its initial speed is 110 km/h? (b) If, in actuality, a 750-kg car with an initial speed of 110 km/h
is observed to coast up a hill to a height 22.0 m above its starting point, how much thermal energy
was generated by friction? (c) What is the average force of friction if the hill has a slope 2.5°
above the horizontal?

• Physics -

110 km/hr = 110,000 m/3600 s = 30.6 m/s

m g h = (1/2) m v^2

9.81 h = .5 (30.6)^2

h = 47.7 meters

(b) loss of energy = m g (47.7-22)
= 750(9.81)(25.7) = 189,000 Joules lost

(c) work done = force in direction of motion * distance moved

189,000 = F (22/sin2.5)

F = 375 N

• Physics -

Thank You

• Physics -

You are welcome.

• Physics -

I disagree with the answer to (c). The distance traveled (d) is not the same as the height the car has reached at the end of its coasting. d is actually the hypotenuse. so h=dsin(2.5) thus d=h/sin(2.5), so d=22/0.424 which is 52.1, the distance traveled. Plugging this into the work done by friction,(Wf), Wf= Fdcos(2.5), F= Wf / dcos(2.5),F = 189,000/52.06cos2.5 = 3634N.

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