Can a heavier car reach higher top speed rolling down a steep?

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karabiner98k
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Hi everybody,
The question is that we have two cars with similar specifications except that one of them is heavier than the other.
I was told that the heavier car can go faster on a steep because of more weight.
Is that true?
 
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welcome to pf!

hi karabiner98k! welcome to pf! :smile:

the force of gravity, and the force of rolling resistance (and i think also the friction in the bearings), are both proportional to weight (or mass)

but the force of air resistance is proportional to surface area

so two identical cars rolling downhill with the engine off, one heavily laden, will feel the same air resistance, but the other forces will be proportional to weight, so yes, the heavier car will accelerate slightly faster :wink:
 


tiny-tim said:
hi karabiner98k! welcome to pf! :smile:

the force of gravity, and the force of rolling resistance (and i think also the friction in the bearings), are both proportional to weight (or mass)

but the force of air resistance is proportional to surface area

so two identical cars rolling downhill with the engine off, one heavily laden, will feel the same air resistance, but the other forces will be proportional to weight, so yes, the heavier car will accelerate slightly faster :wink:
Thanks for your answer.
What about the speed? Can the heavier car reach a higher speed than the lighter one?
 
Since its acceleration is always slightly more, its speed will be also.

And its terminal speed (when gravity equals air resistance) will be slightly higher too, for the same reason. :smile:
 
Huh, it depends on the angle of the slope, the quality of the bearings, the pressure, size, and quality of the tires, and the quality (roughness) of the road.

Check terminal velocity Vt =...):
http://en.wikipedia.org/wiki/Terminal_velocity

But if you include all forms of friction in Cd above, (not just air drag), Cd becomes a function of mass m i.e. increased mass means increased rolling resistance (tire and road dependent) and bearing friction. Both increase Cd, which may in turn decreases Vt. It will depend on the ratio m/Cd(m). The effect of the mass on Cd(m) will depend on the angle of the slope.
 
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