# A discussion with a friend

1. Sep 18, 2006

### electrocake

Hello, I'm not much of a physics entrepeneur, but I was just discussing this with my friend, and hunger for a more difinitive answer came upon me:

Say you design a car, and it does 0-60km/h in 2 seconds.

Now say you want to redesign the car to do exactly the same results on planet Q, which has 50% the gravity of Earth.

Basically, we agreed that you would need to make sure it had the same amount of traction on Q as it did on Earth, so you would need to increase the weight. So to do that you increase its mass.

I then stipulated that the Q Design of the car would need a more powerful engine in order to move the more massive body of the car. My friend disagreed, he said the engine could be equal or less powerful to still get the 0-60km/h in 2 seconds.

Probably an elementry question, but what are your thoughts?

Last edited: Sep 18, 2006
2. Sep 18, 2006

### indoforce

Im only in high school taking pretty low level physics but if you increase the mass, the car's inertia will be higher meaning you will need a more powerful engine.

I think that sounds sensible...

3. Sep 18, 2006

### Hootenanny

Staff Emeritus
A larger force is required to accelerate a larger mass at the same rate as a smaller mass. Since P = Fv, a more powerful engine will be required.

Last edited: Sep 18, 2006
4. Sep 18, 2006

### electrocake

Ahh excellent, thank you!

5. Sep 18, 2006

### Danger

Although your primary question has been answered, I will point out that there are other ways to increase the traction that wouldn't require more power. This would involve better tires and suspension set-up, alteration of weight-transfer characteristics, and various surface modifications to increase aerodynamic downforce. (That is assuming that such are not already optimized.)

6. Sep 18, 2006

### electrocake

Oh brilliant. Yeah we were considering if there were exactly the same car in shape, size, appearance, etc. Which essentially is pretty stupid, I guess, hehe.

7. Sep 18, 2006

### Epicurus

You don't even need to increase the traction! As long as the coefficient of friction is large enough to stop slipping in either case they would perform the same. You could also simply deflate the tyres a little on the lower g planet.

8. Sep 18, 2006

### Danger

Not at all. I assumed that you meant that, which is why I specified that such had not already been optimized. It's just that you have to point out things like that when posting a question. In science, all variables have to be considered.

9. Sep 22, 2006

### subaru

actually, you are all wrong guys

you cant get the same 0-60 by making the weight twice and making the power twice also

f=m.a , and f=c.N , c being the coefficient of friction and N is basically the weight=gravitational acceleration*mass

since grav. acc will be halved and the mass will be doubled, the f in the formula f=c.N will be the same (which is the max. possible traction)

and from f=m.a, you will get an acceleration which is half of the original one since m is doubled in this formula..

so your 0-60 time will be at least doubled(4 seconds), no matter how much power or mass you add to the vehicle( i assumed that the car constantly spins its wheels up to 60 mph,or its just at the edge of slipping, which is a remarkable assumption)

the only way to keep its 0-60 time 2 seconds is to increase the coef. of friction, by changing the tires, or put some kind of rocket like propulsion:surprised

10. Oct 5, 2006

Following the equation $$P=F.v$$ the "power" needed to accelerate the car if the gravity in planet Q is g/2 then the P needed will be 1/2- Power needed to run the car on earth, this is related to the fact that there will be a "friction" for a "frictionless" road then the power to run the car to 60 km/h in two seconds will be the same, no matter the gravity (considering that the car can't fly so z=0 as a constraint.