Pushing a car and Newton’s third law
Hope you have seen people pushing their car when it’s not starting due to some issue.
When someone is pushing a car or something like that, he first pushes the ground or floor backwards with his feet. Following the ‘3rd law of Newton‘ the ground also exerts an equal and opposite force on the feet of the person i.e. the person is pushed forward. A component of this Reaction Force is obtained horizontally in the forward direction. It’s kind of a forward push which is generated by Friction only. Detailed free body diagram is here, if you want to read the details of this frictional forward push you can read this post.This person now transfers that forward push on the vehicle through his hands. We’ll also discuss the involvement of friction while pushing the car.
Friction you face while pushing a car
Types of Friction you face
A resistive force called Friction acts opposite to this applied force and is generated between the tires of the vehicle and the road.
As the person starts pushing harder( i.e. the applied force by him starts rising), this Friction also starts rising equally (of course in the opposite direction) and the car is yet to move.
But this friction(of static type) has an upper limit beyond which it can’t rise anymore. This max value of the static friction is called Limiting Friction.
As the applied force goes beyond this limit, the vehicle starts rolling.
The friction which was resisting the applied force when the vehicle was not moving is called static friction. When the car starts rolling there is still a friction between the tires and the road. It is called Rolling friction.
Remember that as the vehicle has tires it can move so easily defeating the friction. Apart from Rolling friction and Static friction there is another type of friction and it is sliding friction. If you drag something (not rolling) then this type of friction arises.
Out of these three types, rolling friction is the smallest in value and static friction is the largest. Now you know why the tires are circular. Yes, to face the rolling friction which is the easiest to defeat.
Friction factors you need to know
Friction between 2 surfaces depends on 2 things:
1) Roughness of the surface makes the friction value high.
2) Friction value depends on the normal reaction i.e. the Reaction force given by the surface on the object, in the perpendicular direction. Again this Normal Reaction depends on the weight and in turn on the mass of the object. So simply we can say, heavier bodies with large mass will face greater friction while moving.
If friction is high because of these reasons, you need to apply more force to overcome this opposing force.
More detailed discussion on friction with free body diagram is here
Friction: How it helps in walking, useful friction
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