MOMENTUM
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| Momentum |
The momentum of a body is defined as the product of its mass and its velocity
Momentum=Mass✕Velocity
p=m✕v
Momentum’s direction is the same as the direction of velocity of the body.
Momentum is a vector quantity.Momentum’s SI unit is kgm/s.
Concept of momentum was introduced by Newton.
In the absence of external forces, the total momentum of the system is conserved.
i.e., m1v1=m2v2 for a body. And m1u1+m2u2=m1v1+m2v2 for two bodies.
APPLICATIONS OF CONSERVATION OF MOMENTUM
When a bullet is fired from a gun, the gun recoils or gives a sharp pull in back ward direction.
While firing a bullet, the gun must be held tight to the shoulder.
When a man jump from a boat to the shore, the boat slightly moves away from the shore.
Rocket works on the principle of conservation of momentum.
If someone left on a frictionless floor desires to get out of it,he can do so by blowing air out of his mouth.
IMPULSE
An impulse is a large force acting on a body for a short time to produce a finite change in momentum.
Impulse =Force✕ Time
=Change in momentum
i.e., I= F✕ t
Impulse’s SI unit is N-s or kg-m/s.
EXAMPLES OF MOMENTUM AND IMPULSE
In catching a ball, a player by drawing his hands backwards increases the time of contact.
An athlete is advised to come to stop slowly after finishing a fast race.
The layer of bricks is broken by player.
FRICTION
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| FRICTION |
Friction is the opposing force that is set-up between the surface of contact, when one body slides or rolls or tends to do so, on the surface of another body.
The maximum value of the force of friction which comes into play before a body just begins to slide over the surface of another body is called limiting friction.
The force of friction that comes into play between the surfaces of two bodies before the body actually starts moving is called static friction.
The force of friction that opposes relative motion between two surfaces in contact is called kinetic or sliding friction.
The frictional force developed when a body rolls over a surface, is known as the rolling friction.
Static friction is a self adjusting force and it adjusts itself so that it became equal to the applied force.
LAWS OF FRICTION
Friction acts in a direction opposite to the direction of motion of the objects.Friction depends upon the nature of the two surfaces in contact.
Friction is independent of the area of contact of the two surfaces.Rolling friction is less than sliding friction.
Force of friction (F) is directly proportional to the normal reaction (R).
i.e., F∝R ⟹F=𝝁R
where, 𝝁 = coefficient of friction.
Also 𝝁 = tan ɑ, where ɑ is angle of friction.
Angle of repose = Angle of friction.
ADVANTAGES
Due to friction, we are able to move on the surface of earth.
The fibers of thread are held together due to force of friction.
The brakes applied in automobiles work only due to friction.
Sledges are used in arctic region as friction is very low on the surface of ice.
DISADVANTAGES
A lot of energy is wasted in the form of heat that causes wear and tear of the moving parts.
Due to friction, speed of automobiles cannot be increased beyond a certain limit.
METHODS FOR REDUCING FRICTION
By using lubricants e.g., grease,oil etc.
By using ball or roller bearings.
By changing sliding to rolling.
By using soap solution.
By using powder.
The tyres are threading which also increases the friction between the tyres and the road.
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