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				   pls explain how can one decide the direction of friction in case of translational,rotational and traslation+rotational combined on rough plane and rough inclined plane? 

7 years ago


Answers : (1)


Dear sanket sit

Many people are confused about the expected direction in which a friction reaction will act. You have to realise that there is no magic answer to this question, and that you will need to consider what is physically occurring in each problem

I find it best to look at the body in question, and think about the direction in which I expect it to slip, relative to the body it is in contact with. In some cases you may later on prove that the object won't slip, but you should consider the direction it would slip if the applied forces were great enough. The friction reaction will act on the body in the opposite direction to this potential motion.


1.  Chair pulled along a surface

  a rope is tied to the chair, and it is to be dragged to the right. If it is going to move, I would expect the chair to move to the right. The friction reaction will therefore be in the opposite direction to the motion.

2 A box sitting on a slope

   a box sits on a slope, and is acted on by a weight force. If it is going to slip you would expect it to move down the slope. Therefore the friction reaction will be in the opposite direction to this motion.

Note that even if the box does not slip, there must be a friction reaction in the same direction.

3 A hoop rolling down a slope

This is quite a complex example  The metal hoop is placed on the slope, and we are unsure whether the hoop will roll without slip, or roll and slip at the same time. You need to consider that if it did slip, in which direction would it slip? You might imagine that if the surface was very smooth, that the point on the hoop in contact with the slope would slide down the slope. Therefore, the friction reaction will act in the opposite direction to this motion.

4 A car accelerating to the right:

  There is quite a lot of confusion caused by problems involving vehicles with powered wheels. Consider the car , with rear wheel drive.

If the car is accelerating, the driver must be putting their foot down on the accelerator pedal. If the driver pushes the accelerator pedal too hard, you would expect the rear wheels to spin in a clockwise direction. The point on the rear wheels in contact with the ground would move to the left relative to the ground, and the friction reaction would oppose this motion. The friction reaction will act in this direction even if the wheels do not slip relative to the ground.

There are no friction reactions between the front wheels and the road because these wheels are not being used to power the car forwards.

4 car, slowing down

Consider the same car, which is now trying to slow down. The driver has applied their foot to the brake pedal, which activates brakes at all four of the wheels.

You would expect the wheels to slip relative to the road if the person applies the brakes too quickly (it would skid, with all four wheels locked). Therefore, at each wheel the point in contact with the ground will slip to the right relative to the ground. Friction reactions will act in the opposite direction to this motion.

There are friction reactions at the rear and front wheels as all four wheels have brakes applied to them. These reactions (FA and FB) will in be general be different, depending on the position of the center of mass, rate of deceleration, etc.


Please feel free to post as many doubts on our discussion forum as you can. If you find any question Difficult to understand - post it here and we will get you the answer and detailed solution very quickly.
 We are all IITians and here to help you in your IIT JEE preparation.

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7 years ago

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