Question

Confused about friction. Why do people put sandbags in their cars in the winter?

Answer 1

Here's my situation: a car accelerates from rest under the force of friction with the roadway. I know that the max force of friction is:\[F_{\max}=\mu_sN=\mu_smg\]So Newton's Second Law gives:\[\mu_smg=ma\]So the maximum acceleration of the car will be:\[a_{\max}=\mu_sg\]Now it seems that adding weight to the car doesn't allow me to do any better. If I add a mass M of sandbags I still get the same maximum acceleration:\[\mu_s(m+M)g=(m+M)a\]\[a=\mu_sg\]So, even though the frictional force increases, the inertia of the car increases by the same amount so my acceleration is the same in both cases. What effect can adding sandbags to my car have? Can anyone show me this?

Answer 2

My best guess is that the increase in pressure from the added weight of the sandbags raises the melting point of ice/snow. So instead of having a ice or snow/tire interface, we have a wet asphalt/tire interface. I looked up the static and kinetic friction coefficients for wet asphalt and got a value of about 0.7. For hard packed snow about 0.15 and about 0.08 for ice. So the added weight effectively must raise the friction coefficient of the situation which indeed would allow the car to do better. Any problem with my analysis? Any comments are appreciated :)

Answer 3

the real use of sandbags in the car in winter is so you can spray the sand under and in front of the tires to provide an increase in traction. The weight is really a secondary characteristic

Answer 4

fair enough. but it seems the weight actually would help someone accelerate from rest due to the effect described above.

Answer 5

there is an effect by putting weight over the drive wheels, so people who drive trucks usually have a few bags of sand, kitty litter. or even cinder blocks placed over their rear wheels for some additional downward force to help keep the back end from sliding all over the place

Answer 6

agreed. but my argument is that the effect is caused by the extra pressure raising the melting point of the snow/ice. If an object were sliding on say a smooth glass table top, adding extra mass to the object would not increase it's traction.

Answer 7

Sand or grit on the ice will increase the coefficient of friction tremendously.

Answer 8

yes, but that's not what I'm discussing.