Friction
Friction is a contact force. It arises when an object is in contact with another object or material, and an external force is applied to the object in an attempt to move it.
If the force applied is sufficient to cause the object to be in motion, the friction is called dynamic friction.
If the object does not move despite a force being applied to move it, the friction in action is static friction.
a static friction is greater than dynamic friction.
Friction and motion
Lets release a ball gently on the floor and let it roll on its own. Newton's first Law of motion suggest that if the net force on the ball is zero " Since your hand is no longer in contact with the ball", the ball will not accelerate, and therefore it will move with constant velocity. will it?
It is a common observation that a ball eventually slows down and stops. it can therefore be inferred that the net force acting on the rolling ball is not zero, but negative. The negative force opposes the motion of the ball. The rolling ball experiences negative net force because of friction.
Air resistance
When an objects move through air, the force of air pushes against the moving object to oppose its motion this is called air resistance.
Lets look at the motion of a parachutist. as he jumps off a helicopter, gravitational force pulls him downwards. Air resistance push him upwards.
If the Parachutist opens his parachute, he will experience a much greater air resistance because of greater surface area of the open parachute.
When the parachutist is no longer accelerating but travelling at a uniform velocity, air resistance and gravitational force are said to be balanced. They are equal in magnitude but act on opposite direction.
A body travelling at high speeds in contract greater air resistance. hence a speeding car consumes more fuel, as extra energy is converted to do work against the greater air resistance to maintain a car at high speed.
Worked example
At a particular instant, Mr Mohammed car is travelling at 24 m/s on a level road when it runs out of petrol. it experiencing an average restiveness force of 500 N while in motion. The total mass of the car and Mr Mohammed is 1200 kg.
(a) state and explain the motion of the car at the moment the fuel tank is empty.
(b) calculate the intial acceleration of the car.
(c) assuming that the acceleration is constant, how long will it take the car to come to rest?
(a) The engine force becomes zero when the car runs out of petrol. This means that the net force acting on the car is now negative, and it will cause the car to slow down and finally come to rest.
(b) F net = ma
0- 500 = 1200 * a
a= -0. 42 m/s2
Initial acceleration of the car is -0.42 m/s2. Note that a is negative in value. This implies that the car is slowing down.
(c) With an acceleration of -0.42 m/s2. the car slows down at a rate of 0.42 m/s every 1 s later. Hence it will take 57 s for the car come to rest.
Bird's Eye View for this chapter:
Bird's Eye View for this chapter:
ليست هناك تعليقات:
إرسال تعليق