Why 2 men can't lift a 2 ton car but can push it for even 1000 meters

[shaks]

Hi guys,

 

I am thinking what science is behind this case.

 

Why 2 men can't lift a 2 ton car but same men with same force/energy can push same mass car for even 1000 meters.

 

Second question is why at start pushing a car requires more energy but later once moved it requires less energy to push same car.

 

Can anyone throw some light on this?

 

Which scientific forces are involved in this and any other example of same forces in our common life?

 

Shaks

Edited November 25, 2014 by shaks

[swansont]

What force must you exert to lift a 2 ton car? What force must you exert to push it?

 

As to the second question, static friction and kinetic (moving) friction are different; once something starts moving the friction decreases.

[pears]

When you lift a car you are fighting against gravity and bearing its whole weight yourself. When you push it you are just moving it. Its weight is mainly supported by the ground.

[derek w]

wheel axles of cars contain roller bearings packed in grease,these offer little resistance to the wheels turning.

[Greg H.]

wheel axles of cars contain roller bearings packed in grease,these offer little resistance to the wheels turning.

 

Not only that, but cars are designed to roll as efficiently as engineering allows - anything less would be a waste of fuel. It's reallky not that surprising that it takes far less energy to push a car forward than it does to lift one off the ground.

[Robittybob1]

wheel axles of cars contain roller bearings packed in grease,these offer little resistance to the wheels turning.

That is right. Take the air out of the tires and try and push it 1000 meters then! Or worse still take the 4 wheels off and put them in the boot and now try and budge it! Don't release the handbrake and have it in reverse gear if you still want to make it even more difficult to push.

[Greg H.]

That is right. Take the air out of the tires and try and push it 1000 meters then! Or worse still take the 4 wheels off and put them in the boot and now try and budge it! Don't release the handbrake and have it in reverse gear if you still want to make it even more difficult to push.

Crush the car into a cube. Try and push the cube.

[StringJunky]

Hi guys,

 

I am thinking what science is behind this case.

 

Why 2 men can't lift a 2 ton car but same men with same force/energy can push same mass car for even 1000 meters.

 

Second question is why at start pushing a car requires more energy but later once moved it requires less energy to push same car.

 

They can't lift it because the force of gravity is too high but they can push it because that force is mediated by the wheel assembly so all they have left to overcome to move perpendicular to the gravitational force is the inertia. Once this is overcome that effort is stored as momentum and that's what keeps it going and makes it feel easy; in gravity-free space that stored momentum would keep the car going indefinitely without any more pushing because there aren't any frictional forces to slow it down.

Edited November 26, 2014 by StringJunky

[Robittybob1]

Crush the car into a cube. Try and push the cube.

Would it still be a car after it was cubed?

[StringJunky]

Would it still be a car after it was cubed?

Yes, it becomes a car³

[swansont]

Would it still be a car after it was cubed?

 

More like a minivan.

http://en.wikipedia.org/wiki/Nissan_Cube

[shaks]

They can't lift it because the force of gravity is too high but they can push it because that force is mediated by the wheel assembly so all they have left to overcome to move perpendicular to the gravitational force is the inertia. Once this is overcome that effort is stored as momentum and that's what keeps it going and makes it feel easy; in gravity-free space that stored momentum would keep the car going indefinitely without any more pushing because there aren't any frictional forces to slow it down.

 

This is the easiest answer to understand for a beginner like me.

 

Thank you everybody.

 

Shaks

[Robittybob1]

 

This is the easiest answer to understand for a beginner like me.

 

Thank you everybody.

 

Shaks

I don't like the answer myself because he says "the force of gravity is too high" and it isn't.

[StringJunky]

I don't like the answer myself because he says "the force of gravity is too high" and it isn't.

If the two people were very strong, they could probably lift a car of 2000kg (Earth weight) on the moon since the weight - which is due to gravity - would only be 1/6th of that on Earth ie about 300kg each.

Edited November 29, 2014 by StringJunky

[Robittybob1]

If the two people were very strong, they could probably lift a car of 2000kg (Earth weight) on the moon since the weight - which is due to gravity - would only be 1/6th of that on Earth ie about 300kg each.

How are you going to get the car on the Moon?

[StringJunky]

How are you going to get the car on the Moon?

What's that got to do with anything? It was a thought experiment. You disagreed that the force of gravity was too high on Earth and I illustrated that it is. Weight is function of gravity. Note that I didn't say 'mass'.

[Robittybob1]

What's that got to do with anything? It was a thought experiment. You disagreed that the force of gravity was too high on Earth and I illustrated that it is. Weight is function of gravity. Note that I didn't say 'mass'.

Exactly, but don't say gravity is too strong. If it is a thought experiment, think about how you'd get a 2 tonne car onto the Moon?

Gravity on the Earth is exactly the strength it is supposed to be. It is not too strong or too weak.

[StringJunky]

Gravity on the Earth is exactly the strength it is supposed to be. It is not too strong or too weak.

It's too strong for the scenario given.

[Robittybob1]

It's too strong for the scenario given.

No its not, and that is my reason for my objection. Gravity's strength makes it difficult to lift but it is not too strong. Use some other way of saying it.

[swansont]

I don't like the answer myself because he says "the force of gravity is too high" and it isn't.

 

OK, then the men are too weak. It's a relative comparison, not an absolute one. F_exert < F_gravity