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

• karabiner98k
In summary, the conversation discusses the effect of weight on the speed and acceleration of two identical cars rolling downhill with the engine off. While the force of gravity and rolling resistance increase with weight, the force of air resistance remains constant. Therefore, the heavier car will accelerate slightly faster and reach a higher terminal speed compared to the lighter car. However, the exact effect on the terminal velocity depends on factors such as the slope angle, tire quality, and road conditions.
karabiner98k
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?

welcome to pf!

hi karabiner98k! welcome to pf!

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

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

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
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.

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.

Last edited:

## 1. Can a heavier car reach a higher top speed than a lighter car when rolling down a steep hill?

Yes, a heavier car can reach a higher top speed than a lighter car when rolling down a steep hill. This is because the force of gravity acting on the car is directly proportional to its mass. The heavier the car, the greater the force of gravity and the faster it will accelerate down the hill.

## 2. Does the incline of the hill affect the top speed of a car rolling down it?

Yes, the incline of the hill does affect the top speed of a car rolling down it. The steeper the incline, the greater the force of gravity acting on the car, resulting in a higher top speed. However, other factors such as air resistance and the friction between the wheels and the ground can also impact the top speed.

## 3. Are there any limitations to a heavier car reaching a higher top speed when rolling down a steep hill?

Yes, there are limitations to a heavier car reaching a higher top speed when rolling down a steep hill. The weight of the car alone is not the only determining factor, as factors such as the car's aerodynamics, engine power, and tire grip also play a role. Additionally, the steepness and length of the hill will also impact the car's top speed.

## 4. Can a heavier car reach a higher top speed than a lighter car on a flat surface?

No, a heavier car cannot reach a higher top speed than a lighter car on a flat surface. On a flat surface, other factors such as air resistance and the car's power and aerodynamics have a greater influence on its top speed, making the weight of the car less significant. In this case, a lighter car may have a higher top speed due to its better aerodynamics.

## 5. How does the weight of a car affect its acceleration when rolling down a steep hill?

The weight of a car directly affects its acceleration when rolling down a steep hill. The heavier the car, the greater the force of gravity acting on it, resulting in a faster acceleration. However, there are other factors such as air resistance and friction that can also impact the car's acceleration, making weight just one factor in the overall equation.

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