INTRODUCTION OF FORCE- In our daily life we do a number of activities. These activities include push and pull of anything. Lifting, picking, throwing, pushing, striking, flicking, stopping, hoisting, shutting, opening, hitting, lowering, flying, Sliding, smashing etc. activities include push or pull. A push or pull acting on an object is known as force. Force is important to do every work. The direction of force will be the direction of push or pull applied on the object. The amount of force varies according to the type of work. The S.I. Unit of Force is Newton (N). The unit of force was given in honour of Sir Isaac Newton.
FORCE –A PUSH OR PULL- Lets understand force by a few more examples where push or pull is applied.
Activity 11.1- To Identify the actions as push or Pull Force
TYPE OF ACTION
PUSH OR PULL FORCE
Lifting a stone
Striking a coin of carom board
Flicking a ball by hockey
Opening the door of Car
Opening a drawer
So, force of pull or push is needed in every kind of work.
|Flicking the golf Ball (Push)
|Lifting the Car (Pull)
FORCES ARE DUE TO AN INTERACTION- Suppose a man is standing behind a stationary car, the car will not move due to his presence. But, he pushes that car the car moves. It is due to applying a push force. Here, interaction between the car and man occurs.
Suppose a milkman is pulling his cow by a rope toward himself then it is an example of pull force. Here, interaction between the cow and the milkman occurs.
|Pull Force is due to interaction Push
Case I-When two girls are pushing a big box in a particular direction, it becomes easier for them to move the box. It is because both are applying the force in same direction and their forces are added.
Case II-When two girls are pushing the box from opposite side, the box will move in the direction of comparatively greater magnitude. In this case lower force will be deducted from greater force. So, deduction of forces occurs in this case.
The magnitude of force denotes its strength. It is expressed as │5│N. Here, magnitude of force in two vertical lines shows that strength is 5 Newton. Direction of the force is also important.
In case I the total strength of force will be F1 + F2 = F
In case II the total strength of force will be F1 – F2 = F Where, F1 > F2
UNIT OF FORCE-The S.I. Unit of Force is Newton (N). The smaller unit of force is dyne in CGS System. 105 dyne= 1 N
So, 1 dyne = 1 N /105 = 10 _5N
We Know that, Force (F) = Mass of an Object (m) x Acceleration (a)
Or, F = m. a
If we put the units of mass and acceleration then the unit of force will be- Kg. m/s2. This in short is called as 1 N (Newton).
A FORCE CAN CHANGE STATE OF A MOTION- When a football player kicks a stopped football, a force is applied to move in a particular direction. The state of rest of football changed. When the goalkeeper stops the kicked football, a force is applied. The state of motion of football changes to zero.
The speed of a moving vehicle increases when a force is applied on it in the direction of its motion. Similarly, the speed of moving vehicle decreases when the force is applied opposite to the direction of motion. So, the state of motion either increases or decreases due to applying the force.
A FORCE CAN CHANGE DIRECTION OF A MOTION-When a goalkeeper dives to deflect the incoming ball, its direction changes. The direction of a ball changes when it strikes on a wall with an angle. The change in direction is opposite and with same angle.
A FORCE CAN CHANGE SHAPE OF AN OBJECT-Pressing down a lump of dough on a plate, pressing the spring, pulling an elastic rubber etc are examples of change in shape of objects due to force.
|Change in shape and size of elastic rubber band
A force may bring following changes-
1. make an object move from rest.
2. change the speed of a moving object.
3. change the direction of a moving object.
4. change the shape of an object.
5. change some or all of the above effects together.
The application of force to bring change in shape, size, position or state of the object is not always applicable. e.g.- Even if you apply maximum on a wall, it will not move.
TYPES OF FORCES-Some forces act on an object by other object when they are in contact with each other. Some forces act on an object by other object when they are not in contact with each other. On this basis there are two types of forces. Contact forces and non-contact forces.
A. CONTACT FORCES– Those forces which develop between two objects in contact are called contact forces. e.g.-
1. Muscular Force-The force resulting due to the action of muscles with an object is known as the muscular force.
e. g.-The drought animals like oxen, horses, donkeys, camel, Yak etc. are used to perform a number of tasks like carrying, ploughing etc by applying muscular force.
|Muscular Force of Animals and Humans
e. g.- The inhalation and exhalation of air from our lungs is due to movement of muscles. etc.
2. Frictional Force– The force which arises between two surfaces in contact is known as frictional force. Out of two at least one object should be in motion. If both the surfaces in contact are in motion then also friction will be there. For rough surface frictional force is greater than smooth surface.
The force of friction always develops in between moving surfaces in contact. The direction of force is opposite to the direction of motion.
|Rough surface produces more friction than the smooth surface
e. g. Stopping of a moving ball on the ground by its own without any external force.
e.g. Stopping of a boat after some time without sailing in still water.
e.g.- Burning of matchsticks on rough surface of match box. etc.
The frictional force produces heat. It is useful as well as a harmful force. When we walk, our energy is consumed more due to friction. But, friction helps us to not skid. A football, cricket, tennis, sprinter like players wear spiked shoes to increase his/her friction so that skidding during running can be prevented.
B. NON-CONTACT FORCES- Those forces which develop between two objects without any contact are called non-contact forces. e.g.-
1. Gravitation Force- The attractive force which develops between the earth and any other object with or without contact on it is called gravitational force or force of gravity or simply gravity. Isaac Newton has discovered this force. All the objects have their own gravitational force. The gravity depends on the mass of objects and distance between them. The weight of an object varies from the Earth to the Moon. It is because of change in gravity.
|Earth Attracts all objects towards it.
2. Magnetic Force– The fore exerted by a magnet on another magnet and magnetic substance is called magnetic force. Similar poles repel each other while dissimilar poles attract each other.
e. g. A magnetic force of attraction is found between dissimilar poles of two magnets.
N – N = Repulsion
S – S = Repulsion
N-S = Attraction
e.g. A magnetic force of attraction is found between a magnet and iron.
Loadstone is a natural mineral that behaves like a magnet. Iron, cobalt and Nickel and their alloys are ferromagnetic. They are attracted towards the magnets.
3. Electrostatic Force- The force exerted by a charged body on another charged or uncharged body is called electrostatic force.
|Electrostatic force of attraction
Electrostatic forces may be attractive or repulsive. Similar charges attract each other and dissimilar charges repel each other. Electroscope device is used to know the nature of charge present on anybody. Electrostatic force is also called as Coulomb force or Coulomb interaction.
PRESSURE- The force acting on a unit area of a surface is called pressure.
Pressure= Force/Area on which it acts
Unit of Pressure- Newton (N)/m2or N.m-2 Newton per square meter) = 1 Pascal
Pascal is S.I. derived unit. One Pascal is defined as one newton per square meter. Pascal was named after Scientist Blaise Pascal.
e.g. When an iron nail is hammered with its pointed end into a wooden plank, it gets inserted easily. But, it is very difficult to insert that iron nail into wooden plank with its head. It is because the surface area of head is more than the pointed end. More is the force, more will be the pressure.
e.g. The porters place a round piece of cloth on their heads to increase the surface area so that the pressure of luggage on a particular point on head can be reduced and distributed to comparatively bigger area. It helps the porters to carry luggage with better comfort due to reduced pressure.
PRESSURE EXERTED BY LIQUIDS AND GASES- When any liquid is filled in a container, the liquid puts pressure on its walls.
e.g. A rubber sheet is fixed at one of the end of a transparent glass tube or plastic pipe. The pipe is kept in vertical position. The rubber sheet bulges out on pouring water. The bulging is more on pouring more water. So, it confirms that liquids exert pressure on the walls of their containers.
e.g. The fountains of leaked water in supply water pipes is because water is supplied with force. If any leakage or hole is there, water comes out with pressure.
ATMOSPHERIC PRESSURE- The pressure exerted by air is known as atmospheric pressure. Atmospheric pressure is very large which is counter balanced by our internal body pressure.
Let us imagine a unit area over our head which is filled with air in a very long cylindrical air column. If the area over our head is 15 c.m. X 15 c.m then an atmospheric pressure of 2250 N or 225 kg will be there.
e.g.- The rubber sucker pressed on a plane glass is very difficult to remove because the air inside it is almost removed after pressing. The outer air puts atmospheric pressure.
|Partial vacuum and low air -pressure inside and high atmospheric pressure outside
SOLVED EXERCISE QUESTIONS
Q.1 Give two examples each of the situations in which you push or pull to change the state of motion of objects.
Ans- Examples of push to change state of motion-
1. Kicking a football
2. Opening of doors
Examples of push to change state of motion-
1. Opening a drawer
2. Drawing a bucket of water from a well
Q.2 Give two examples each of the situations in which applied force causes a change in the shape of an object.
Ans- 1.The shape of seat changes on sitting.
2. The shape of lump of dough changes on pressing
Q.3 Fill in the blanks in the following statements.
(a) To draw water from a well we have to ……………………… at the rope.
(b) A charged body ………………………an uncharged body towards it.
(c) To move a loaded trolley we have to ………………….. it.
(d) The north pole of a magnet ……………………….the north pole of another magnet.
Ans- (a) pull (b) attracts (c) push or pull (d) repels
Q.4 An archer stretches her bow while taking aim at the target. She then releases the arrow, which begins to move towards the target. Based on these information fill up the gaps in the following statements using the following terms.
muscular, contact, non-contact, gravity, friction, shape, attraction
(a) To stretch the bow, the archer applies a force that causes a change in its ………………
(b) The force applied by the archer to stretch the bow is an example of ……….. force.
(c) The type of force responsible for a change in the state of motion of the arrow is an example of a …………….. force.
(d) While the arrow moves towards its target, the forces acting on it are due to …………. and that due to ………………of air.
Ans- (a) shape (b) muscular (c) contact (d) gravitational force (gravity) , friction
Q. 5 In the following situations identify the agent exerting the force and the object on which it acts. State the effect of the force in each case.
(a) Squeezing a piece of lemon between the fingers to extract its juice.
(b) Taking out paste from a toothpaste tube.
(c) A load suspended from a spring while its other end is on a hook fixed to a wall.
(d) An athlete making a high jump to clear the bar at a certain height.
Agent exerting the force
Object on which it acts
Effects of the force
Change in shape of lemon and
juice comes out
Change in shape of toothpaste tube
and paste comes out
Stretching of Spring
Change in athlete’s state of motion
Q.6 A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect the piece of iron?
Ans- The muscular force of the blacksmith changes the shape of the piece of iron.
Q.7 An inflated balloon was pressed against a wall after it has been rubbed with a piece of synthetic cloth. It was found that the balloon sticks to the wall. What force might be responsible for the attraction between the balloon and the wall?
Ans- An electrostatic charge develops on inflated balloon due to rubbing by synthetic cloth. The force between charged inflated balloon and the wall is due to electrostatic force.
Q.8 Name the forces acting on a plastic bucket containing water held above ground level in your hand. Discuss why the forces acting on the bucket do not bring a change in its state of motion?
Ans- In this case, two forces namely ‘muscular force’ and ‘gravitational force’ acting on the plastic bucket containing water and held above ground by the hand. Muscular force is acting upward and gravitational force is acting downward. So, due to equal and opposite forces there is no change in the state of motion of the bucket.
Q.9 A rocket has been fired upwards to launch a satellite in its orbit. Name the two forces acting on the rocket immediately after leaving the launching pad.
|Gravity and Drag on Flying Rocket
Ans- Two Forces acting on the rocket are-
1. Gravitational force
2. Atmospheric friction (drag)
Q.10 When we press the bulb of a dropper with its nozzle kept in water, air in the dropper is seen to escape in the form of bubbles. Once we release the pressure on the bulb, water gets filled in the dropper. The rise of water in the dropper is due to
(a) Pressure of water
(b) Gravity of the earth
(c) Shape of rubber bulb
(d) Atmospheric pressure
Ans- (d) Atmospheric pressure