## 21 April 2012

### 8th Force and frictions study key concepts

Frictions   Formative assessments - Oral Questions
Friction is a force that resists the motion of an object that is in contact with another object or material.
Causes of friction force are :
(a) Roughness of two surface (b) due to force of adhesion ( molecular attraction)(c) Weight of body
If the objects are not moving relative to each other, this is called static friction force is called static.
If force F is greater than friction Fr (written as F > Fr), then the object will slide or move. The friction is considered kinetic friction, which means moving friction.
There are mainly three types of frictions: Static friction, Sliding friction and rolling friction
What is static friction?
Friction is self adjusting force. Friction that holds an object in place is greater than the kinetic friction that slows down a moving object is called static friction
What is Sliding friction?
When two solid objects are in contact and a force is applied to slide one object against the other it is called sliding friction. Sliding friction force resists the motion.
What are causes of sliding(kinetic) friction?
The causes of sliding friction are molecular attraction or adhesion between the materials, surface roughness of the materials, and deformation resistance in the case of soft materials.
What rolling friction force?
The resistive force that slows the wheel's motion on the other solid surface. Much of rolling friction is caused by adhesion between the surfaces.
What is Fluid Friction?
When a solid object is in contact with a fluid and a force is applied to either the object or to the fluid, there is a friction force that resists the motion it is called fluid friction . Examples where fluid friction occurs are water flowing through a hose, an airplane flying through the atmosphere and oil lubricating moving parts.
What are important uses of friction?
Friction is a resistance force that slows down or prevents motion,
How is friction a nuisance?
Too much friction can be a nuisance, because it can hinder motion and cause the need for expending extra energy.
In some situations, friction is very helpful.
(a) Friction holds our shoe to the ground, allowing us to walk
(b) A pencil eraser uses friction to rub off mistakes
(c) Friction between paper and pencil help in on paper.
(d) Friction of the brakes and the tires help to drive car and to stop
(a) Friction between various parts of machines led loss of energy and reduces performance
(b) Friction wears and machine parts.
(c) Friction makes the movement of object difficult
(d) The friction of parts rubbing together creates heat that cause damage to a machine
Ways to reduce friction are:
(a)By lubrication or applying oils                   (b) By polishing surface
(c) By using ball bearing                             (d) By streamlining the body
(e) Applying fine powder
Force of friction applied by liquid or gas is called Drag.it is minimized by making body of object streamline.  It depends on
(a) Speed of object with respect to fluids (b) Shape of object  (c) Nature of fluid or density
Q. Explain why it is so painful to be stepped accidentally by high-heel shoes.
Ans: Heels have very small surface area in contact with the foot.  Hence, the pressure on that small area is very high and this high pressure causes pain.
Q. Explain why the bodies of airplanes and birds are streamlined.
Ans: Streamlined body reduces the air resistance and enables them to move easily through the air.
Q  Why is it difficult to hold a school bag having a strap made of a thin and strong string?
It is difficult to hold a school bag having a strap made of a thin and strong string because it apply the large pressure on the shoulders due to very small contact surface area . we know that the pressure is inversely proportional to the surface area on which the force acts
Q. Why the cutting edge of a knife should be as sharp as possible?
The cutting edge of a knife should as sharp to create greater pressure even for a very small applied force for quick action.
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