# Science Notes - 3: Physics - Friction - Important Points

Image Credits: wikimedia Vishakha.malhan

Friction

① Whenever a body moves or tends to move over the surface of another body, a force comes into play that acts parallel to the surface of contact and opposes the relative motion. This opposing force is called friction.

② Friction depends upon the nature of the surfaces in contact with each other and at the state of motion of the object on the surface. The weight of the object, if the weight of the object is high then friction is also high.

③ Static Friction is the frictional force acting on a body when the body has not started moving yet and is at rest, also this friction is equal to the amount of force applied on the body and is variable to a certain limit.

④ Sliding Friction is the friction acting on the body when it is in motion and this friction is constant and is always less than the maximum value of static friction.

⑤ When a roller rolls over a surface, the frictional force that comes into action is rolling friction.

⑥ Rolling friction is less than sliding friction while sliding friction is less than static friction.

⑦ Laws of Limiting Friction

ⅰ Friction depends on the nature of the surfaces in contact and their state of polish.

ⅱ It acts tangential to the two surfaces in contact and in a direction opposite to the direction of motion of the body.

ⅲ The value of limiting friction is independent of the area of the surface in contact so long as the normal reaction remains the same.

ⅳ The limiting friction (fsmax ) is directly proportional to the normal reaction R between the two surfaces.

⑧ Ball bearings are used in machinery to reduce friction i.e. ball bearings replace the sliding friction by rolling friction (lesser than sliding friction)

⑨ Friction due to gases and liquids is called fluid friction and is also called drag.

⑩ Air exerts the force of friction on all the bodies which move through it and opposes their motion. Shooting stars (Meteors showers) are due to atmospheric friction.

⑪ Friction is a necessary evil.

⑫ The Important methods of reducing friction are:

ⅰ By polishing.

ⅱ By applying lubricants or greasing surfaces.

ⅲ By using ball-bearing.  Ball  bearing reduces sliding friction into rolling friction.

ⅳ By using rollers and heels.

ⅴ By streamlining the shape of bodies (e.g. ships, cars, aeroplanes)

Q1(MCQ): It is easier to roll a barrel than to pull it because

(a) the full weight of the barrel comes into play when it is pulled

(b) rolling friction is much less than sliding friction

(c) the surface area of the barrel in contact with the road is more in the case of pulling

(d) of a reason other than those mentioned

Answer: (b) rolling friction is much less than sliding friction

Q2(MCQ): One finds it more difficult to walk on ice than on a concrete road because

(a) ice is soft and spongy whereas concrete is hard

(b) the friction between the ice and the feet is less than that between the concrete and the feet

(c) there is more friction on ice than on concrete

(d) none of these

Answer: (b) the friction between the ice and the feet is less than that between the concrete and the feet