Friction

Classical Mechanics Level 3

Consider the scenario of a gardener cutting grass with a lawnmower.

Would he experience more friction when pushing the lawnmower or pulling it?

Push Pull They are the same

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Saraswati Sharma by Saraswati Sharma , 23, India

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2 solutions

Because of the way humans tend to push and pull, when they push something, they tend to push it closer to the ground in some sense.

Now because friction is understood as the interaction of the two surfaces, there is more scope for such interaction when the roller is pressed onto the ground instead of pulling it away.

Quantitatively, like Rakshit said, F f r i c t i o n ∝ N F_{friction} \propto N . And when the effective weight is increased, the Normal reaction increases, so as to balance it.

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The force to push and pull the weight of the lawn mower would be the same , the friction would be the same , the leverage would not be the same , making it seem harder to push than pull , the friction would be the same in both directions , imagine putting the lawn mower on a concrete slab and tying a string to the front and the back , pulling one and then the other , you get the same results, the same effort, its only harder to push in the grass than pull because of the change of leverage

David English - 4 years, 6 months ago

This should be level 3 or higher.

William Nathanael Supriadi - 4 years, 6 months ago

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Why do you think so?

Agnishom Chattopadhyay - 4 years, 6 months ago

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Quite deceiving. In fact, the pass rate is only a third.

William Nathanael Supriadi - 4 years, 6 months ago
Rakshit Joshi
Nov 23, 2016

Friction depends on normal as it is u×N and when we push a body the component of force acting downwards increased the normal so does the friction. Though in the case of pull the upward force decreases the Normal acting in the block, so does the friction is less.

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The problem should probably mention that we are not pulling or pushing purely parallel to the surface.

Steven Chase - 4 years, 6 months ago

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Hmm. Even I thought of that too but when I put the answer it got right. @Saraswati sharma, can you update your question telling that force is not parallel to the surface, it must be fine.

Rakshit Joshi - 4 years, 6 months ago

Yes, I agree with Steven. I thought about what he said before I answered, but as usually asked in school textbooks, the purely parallel direction is always assumed. And if you're pulling things from a lower ground, is it not the same as usual pushing?

Saya Suka - 4 years, 6 months ago

Even when the push/pull is parallel to the ground the friction still increases/decreases in my opinion. Consider the back wheels of the mower to be the center of rotation then pushing the handle horizontally will result in the front of the mower being pressed to the ground whereas pulling would kinds lift it. But please correct me if what I said is wrong.

Kai Ott - 4 years, 6 months ago

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Actually when the question was first posed, it did not say anything about a lawnmower, only mentioned as push and pull without any context whatsoever. If it was a little kid shorter than the point where a rope was fixed to a cube of load tried to pull it, then there would be more friction that way than him doing it the other way round, by parallel-pushing it forward.

Saya Suka - 4 years, 6 months ago

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As I recall, the original question had only about five words and zero contextual detail.

Steven Chase - 4 years, 6 months ago

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