# Does friction depend on mass?

## Main Question or Discussion Point

Walter Lewin in his lecture says, a truck and a car on a road will start to slide at a same angle if their tyres are made out of identical material. I know that max. frictional force is proportional to normal force. and if normal force is dependent on mass. shouldn't the frictional force depend on mass?

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Svein
if normal force is dependent on mass. shouldn't the frictional force depend on mass?
It does. So the frictional force is $R=k\cdot m\cdot g\cdot \cos(\alpha)$ and the force needed to accelerate the car/truck is...

So, you are saying we need more force to move truck(high mass) than a car(low mass) and hence they don't start sliding at same angle on an inclined plane?

Merlin3189
Homework Helper
Gold Member
Professor Lewin is, unsurprisingly, correct. So why are they the same?
Just look at a block on an inclined plane and work out what has to happen for it to start sliding. What makes it move at all? Is that friction???

I am assuming the more massive object needs more frictional force(k.M.g.sin α) to overcome and also it has large component of force(M.g.cos α) downhill. Object with low mass needs less Frictional force to overcome and has lower component downhill. Both the cases it is the proportionate value that decides when the bodies needs to slide and it happens at same angle independent of mass

On a flat road with identical tires the object with more mass (and therefore more weight) will be harder to slide. The maximum static friction value is a product of the coefficient of friction and the normal force (weight). The greater the weight (all other things being equal) the greater the maximum static friction. This can easily be proven with a flat board, a fishing scale, and some bricks.

On an inclined plane the weight of the truck and the weight of the car contribute to force along the incline, so it's a different situation. The different weights contribute forces along the inclined plane proportional to their contribution to the normal force.

But that's theory. In reality there are other things happening that would keep the truck sliding when the car did slide. For example, the heavier truck will squish the tires deeper into the surface of the road increasing their effective coefficient of static friction. Surface effects are why dragsters can accelerate at rates well above G.