IB Physics Mechanics help? : Bus on an incline

  • #1

Homework Statement


A bus is travelling at a constant speed of 6.2ms-1 along a section of road that is inclined at an angle of 6.0degrees to the horizontal.

a) the total output power of the engine of the bus is 70kW and the efficiency of the engine is 35%. calculate the power to the engine

b) the mass of the bus is 8.5 x 10^3. determine the rate of increase of GPE of the bus
c) using your answer to b and the data in a, estimate the magnitude of the resistive forces acting on the bus
d) determine the magnitude of the net force opposing the motion of the bus at the instant at which the engine stops
e) discuss, with reference to the air resistance, the change in the net force as the bus slows down
2. The attempt at a solution
a) 70 000 / 0.35 = 200 000W
b) vertical distance: 6.2sin6 = 0.648
8.5 * 10^3 * 9.81 * 0.648 = 54 033 Js-1
c) 70 000 - 54 033 = 15967
15967/6.2 = 2575.3N
is this right ???
d) some suggestions would be useful for this one!
e) does the air resistance decrease proportionally to equal the thrust so that the net force decreases too?
 

Answers and Replies

  • #2
haruspex
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is this right ???
d) some suggestions would be useful for this one!
Looks good so far.
What forces act on the bus if the engine is suddenly turned off?
 
  • #3
Looks good so far.
What forces act on the bus if the engine is suddenly turned off?
the only force would be air resistance? so would the net force be the force due to air resistance only and, therefore, it decelerates to a stop?
 
  • #4
haruspex
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the only force would be air resistance? so would the net force be the force due to air resistance only and, therefore, it decelerates to a stop?
It is still on the slope, no? And still moving.
 
  • #5
so there would also be frictional force as well as air resistance. but surely not thrust because the engine is off
in the other directions, there would be weight and normal contact force
 
  • #6
haruspex
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so there would also be frictional force as well as air resistance. but surely not thrust because the engine is off
in the other directions, there would be weight and normal contact force
Right, so which of those have at least some component opposing motion of the bus?
(Alternatively, which forces are different from when the engine was running?)
 
  • #7
just friction and air resistance.
so the net force would be in the opposite direction to motion?
 
  • #8
haruspex
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just friction and air resistance.
What about the slope?
As I hinted, it is easier to answer the question by considering what change to the forces resulted from turning off the engine.
 

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