# Why is it hard to accelerate forwards in a car under low gravity?

I saw a physics simulation where a dude was driving a car under normal vs low gravity, and it was notably harder to accelerate forwards/change direction under lower gravity. any explanation for this?

• this is why race cars have wings (inverted wings, that is).
– JEB
Oct 7, 2021 at 12:41

Assuming that the car is a wheel-driven vehicle, then the horizontal force that accelerates the car is actually the friction between the car's tyres and the road surface. The maximum friction force is proportional to the vertical normal force exerted by the road on the car, which is in turn (on a level road) equal to the car's weight.

In lower gravity the car's weight is reduced so the maximum friction force between the car and the road surface is reduced. This reduces the maximum acceleration of the car.

In higher gravity the car's weight is increased so the maximum friction force between the car and the road surface is increased, and the car can in theory accelerate more quickly (remember that although the car weighs more in high gravity, its mass is unchanged). However, there is another limiting factor on the car's maximum acceleration - the maximum torque that the engine can produce on the car's wheels. This will depend on the power of the engine and the ratio of the car's lowest gear.

• I believe that in a super high gravity situation the car would accelerate slower. By super high gravity I mean that the wheels would deform and the friction would be greatly increased which would require the car to produce a greater force to overcome the friction. If the gravity is just a bit over the normal (earth) then it has the $\textbf{possibility}$ to accelerate faster if the engine is able to produce a suitable torque for the new friction that is required.
– ludz
Oct 7, 2021 at 10:37
• @ludz The car is not trying to overcome friction between its tyres and the road surface. To make maximum use of the engine's power, the bottom of the cars tyres should be stationary on the road surface. If the car tyres overcome road surface friction then the wheels are spinning instead of driving the car forward. If overcoming friction was desirable, cars would accelerate fastest when driving on ice. Oct 7, 2021 at 10:46
• That is true, I totally agree with that. I'm not talking about sliding friction. I'm talking about rolling resistance (also known as rolling friction, see wikipedia en.wikipedia.org/wiki/Rolling_resistance).
– ludz
Oct 7, 2021 at 12:16

This is because the typical car accelerates by exerting a force on the ground. The reaction force the ground exerts on the car then propels the car forward. In a low gravity situation, the car is not in solid contact with the ground, there is no reaction force, and hence it cannot accelerate.

Note this doesn't apply if the car uses a jet engine, which works by expelling air backwards. In that situation the car will still be able to accelerate under low gravity.

• I agree with your answer. Would super high gravity also make the car accelerate slower? I believe so since then the friction would be much higher than usual hence the car would need to produce a higher force to overcome the friction.
– ludz
Oct 7, 2021 at 10:15
• To be a bit more clear, A car accelerates by exerting a force parallel to the surface. The only way it can do that is if the tires stick to the surface. The "stickiness" is provided by static friction, which is proportional to the weight of the car. Oct 7, 2021 at 14:47
• @ludz, More friction between tires and the road is a good thing. It gives better ability to accelerate. Sometimes people say that the forward motion of a car is opposed by friction, but that is incorrect. It is opposed by (among other things) rolling resistance which is not the same thing as simple friction between the tires and the road. Oct 7, 2021 at 14:50
• I agree (see my comment on gandalf61). I'm not talking about sliding friction I'm talking about rolling resistance (also known as rolling friction) as you mentioned in your previous answer. And I believe that in a higher gravity situation this will oppose the motion of the car more than before hence having a lower acceleration.
– ludz
Oct 7, 2021 at 16:40

I am assuming "notably harder to accelerate forwards" means that the car loses traction sooner at low gravity vs normal gravity. The reason that is the case is traction is lost when the static friction force, which is responsible for acceleration, reaches the maximum possible static friction force sooner, which is

$$F_{f-max}=\mu_{s}mg$$

Where $$mg$$ is the portion of the weight of the car on the drive wheel and $$\mu_{s}$$ is the coefficient of static friction between the tire and road. Assuming $$m$$ and $$\mu_s$$ are constant, $$F_{f-max}$$ is lower if $$g$$ is lower.

Hope this helps.