chapter - force by Yuvraj singh 8 A
Force and pressure
. A force is a push
. A force is a pull
The force is required to make a body at rest start moving
You apply force to change a speed of a swing
You apply force to change a direction of a shuttlecock
The force is needed to change a shape and size of an object
Advantage and disadvantage of friction during cycle driving.
Aeroplane are streamlined to reduce air resistance
electrostatic force can act from an distance
What do we need to open a door or close a door ? We push or pull it
A force is a push or a pull
What things a force can do ?
1.Force causes movement
2.Force changes speed
3.Force changes direction of motion
4.Force changes shape and size
1.Force causes movement
The force causes movement i.e when we kick a ball lying on ground it will start moving we can move anything which is in the state of rest So the force is required to move a body at rest
2.Force changes speed
Force can change the speed of a body i.e when you friend is on a swing and you push it and the speed of a swing increase and if we pull a swing it will either become slow or come at rest so a force is required to change a speed of a moving body .
3.The force changes direction of motion
The force can change a direction of a motion i.e when a goalkeeper punches a ball away from a goal he or she use force to change a direction of a ball so a fore is required to change a direction of a motion.
4.Force changes shape and sizes
The force can change a shape and size of an object i.e when we fold clothes and stretch and rubberband so we use force to change a shape and size of an object
Type of forces
1.muscular force
2.contact force
3.non contact force
1 . Muscular force
The force we apply by execution of our muscles is called muscular force
2.contact force
Muscular force is example of this force
Tge force applied when a thing is in the contact of our body is called conatct force
i.e frictional force
2.1 frictional force
Thsi is a most important contact force
You have thought a question in your mind that when you kick a ball why it cant roll endlessly? Thus is due to friction force
Friction force always act in a direction opposite to that of a motion.Frictional force always opposes motion , it always try to stop a movement of one surface over another .
And it also depend on the nature of two surfaces. the tiny bumps on both surface rub with each other and it causes friction
The smoother the surface the lesser is friction.
Static and kinetic friction - friction dont depend on the nature of surface .It also depend on the things it is moving or stationary
When a force is applied to move a body ,frictional force resist the movement. As the force increases ,the frictional force also increases , when the force become greater then the frictional force the body start moving .As the body start moving ,the frictional force start decreasing .when the force applied manage to get past the limit
Frictional force looses heart and become weaker .two points to be noted are
1. A body start moving when a force applied manage to become greater than frictional force .
2.static force ,which act when a body is stationary ,and it is greater than kinetic force which act when body is moving.
When a frictional force is helpful
This force is very helpful to us .life would be impossible without it.we would not be able to walk without friction because it is friction between floor and the soles of your feet which allow us to walk properly . Without friction we will slip as on new polished floor
,a banana skin ,or when anyone spill water on a floor
Similarly,without friction between tyre and road the vehicle would skid like on wet roads the brakes would also fail because they also work on friction .
Similarly , when we write with pencil tge carbon of pencil rub with page and leave the mark .It would be impossible without it .
We require friction to light a match .if we strike a match on smooth surface , it will not light.
When friction causes problem
Friction makes surface wear down.The soles of our shoes get worn out due to friction. You can see for yourself how friction causes wearing down by rubbing and eraser first over a paper then over Sandpaper . The sand paper will wear down the eraser much faster because it is rougher .
Friction wastes energy too .when you ride a bicycle , for example, you use energy to overcome the friction between the moving part of cycle . As you pass air also opposes your movement its called air resistance.
Anotger disadvantage is that is generate heat . When you rub your palm against each other for sometime you fell heat generated by friction . When you strike a match the heat of friction help ignite the match . The heat generated by friction can damage machine .
Reducing friction
Friction wastes energy and wear down machinery and cause heating .engineer always try to reduce it. One way is to make the surface smooth by polishing. Another way is oiling Oil reduce the friction by filling up the dents on the surface that move against each other is machine .
Ball bearing are used in cycle to reduce friction between wheel and axle.
Another way is ball bearing .Bearing come in many shape and sizes amd are used to reduce friction when the rod rotates inside a hole like in bicycle wheel .
Ball bearing are free to roll are placed between moving part of a machine .In fact ,the invention of wheel also invemted that rolling friction is lesser than slidding friction .peaple must have noticed placing logs under sleds make it easier to push a sleds along .
Streamlining
Air and water offer resistence to movement . Birds and fish are designed to reduce that resistence .that is called streamlined body .
This body has smooth outlines . When such a body move it disturb a nature flow of water and air very little This reduce the resistence offer by air or water .car amd aeroplane are designed like this .
Increasing friction
In some situation it is advantage to increase friction .the soles of the shoe and tyre of vehicle. if they were smooth there would be a chance of slipping or skidding .The shoes worn by athletes and mountaineer have spikes underneath to improve grip .the peddle of bicycle .a steering wheel of car ,
The handle of a knife you could think of many ecample where surfave are made rough to increase friction
Now it is a time to know about non contact force.
The first non contact force is magnetic force.
Magnetic force
Things made of iron are attracted by magnet.T he force of magnet to attract these things is called magnetic force ,and this phenomenon of attraction is called magnetism .Magnetic force can act from a distance that is why it is called non contact force
Magnetic force can act from distance.
Electrostatic force
If you run a comb from your dry hair a few times and then hold a comb over small piece of paper ,they will get drawn to comb .this force is called electrostatic force .This is also a non contact force.
Gravitational force
We know that everything on or near the earth is attracted towards it by the force of gravity .This Make the ball come down when you click it upward. Moon to pull things near it with its own force of gravity. And so do all the other planet the sun and all the stars . in fact, any two body in the universe attract each other by the force called gravitational force
This law is called Newton's law of gravitation in honour of Sir Isaac Newton the English physicist and mathematician who discovered it.
You may wonder why you do not feel your school bus or your classmates attractive with the force of gravitation the problem is that gravitational force depend on the masses of the two body concerned and unless one of the body is very massive it cannot be felt this also explain why the earth force of gravitational is much greater than that of the moon it is because the earth's mass is much greater than that of the Moon
Gravitational field
When we drop a ball from a height it is pulled down by the earth's gravitational force allthough ball is not in the touch with the Earth this mean that are gravitational pull can act at a distance just like magnetic and for electrostatic force is preferred that the Earth applied this force indirectly through something that is in the touch with the ball they said that the mass of an object producer gravitational field around the object it is a field that exert a gravitational force on object near and far
Weight
We know that the weight of a body depends on its mass the weight of a body is actually the force with which the earth attracts it if the earth didn't attract as we would be weightless even though we would still have a same mass that we have now a spring balance is a device you can use to measure the weight of the body you cannot use a beam balance to measure the weight of a body because it's merrely compare the mass of a body with the known mass of the standard weight.
Unit of force
There is a very close relation between the force of gravity acting on the mass of hundred gram and as a unit of force called a Newton the weight of hundred gram mass of the force of gravity acting on it is 0.98 Newton therefore
The weight of 1 kg mass =0.98 N
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