part b and c Ullllllul BdLin Date: 6 4/2016 12:00:00 AM --Due Date: 6/11/2016 1:

Question

part b and c Ullllllul BdLin Date: 6 4/2016 12:00:00 AM --Due Date: 6/11/2016 1:00:00 AM End Date: 6/18/2016 12:00:00 AM (S%) Problem 4: Suppose a car approaches a hill and has an initial speed of 118 km/h at the bottom of the hill. The driver takes her foot off of the gas pedal and allows the car to coast up the hill. id 33% Part (a) If the car has the initial speed stated at a height of h 0, high the car coast up a hil if work done by fricti is negligible? how in m) can its 33% Part c) If in actuality. a 750. kg car with coast up a hill and stops ar a height starting point, how much thermal energy was an initial s of ziskmh is observed to generated by friction in J? 21 m abo 33% Part (c) What is the magnitude ofthe average force in Newtons of friction if the hill has a slope 21 above the horizontal?

Explanation / Answer

Part b):

Convert given quantities to a consistent set of units. Usually SI.
mass m = 750 kg, Initial speed, v = 118 km/h = 32.8 m/s
If the vehicle was at a speed of 32.8 m/s
Ek = 1/2 m v2 = 0.5 x 750 x (32.8)2 = 402894 J

At a height of 21 m energy has gone into gravitational potential energy,

mgh = 750 x 9.8 x 21 = 154350 J

The missing energy (402894J - 154350J) = 248544J has gone to friction.
Hence, 248544J thermal energy was generated by friction.

Part c):

We can find the distance (D) travelled by the car up the hill is given by:
sin(2.1°) = opposit / hypotenuse = 21/D.

So,    D = 21/sin(2.1°) = 573 m
So we use
Work lost to friction, W = FD
F = W/D = 248544J/573m = 433.8 N = 434N

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