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Daily Activity which require force • To make the toothpaste come out of the tube , you need to press it. • To make the football move , you have to push it. • You exert a force on a drawer to open or close it. You make an interaction with the drawer. • From these examples , we can infer that at least two objects must interact with each other for a force to come into play. • Solve Table 11.1 from Text book given on pg 128
FORCE – A PUSH OR A PULL • A force is the push or pull which changes or tends to change the state of rest , direction , speed and shape of a body.
Forces are due to an Interaction Suppose a man is standing behind a stationary car. Will the car move due to his presence ? Suppose man now begins to push the car , that is he applies a force on it . The car may begin to move in the direction of the applied force . Note that the man has to push the car to make it move. Now again there is three situations that may be familiar to you . Can you decide who is pulling and pushing in these cases?
In Case (a) both the girls appear to push each other while the pair of girls in fig (b) are trying to pull each other . Similarly , the cow and the man in fig (c) appear to pull each other . The girls in the two situations shown here are applying force on each other . Now what happens in the next with the man and the cow. Answer is the same only
Exploring forces • Let us learn about the two scenario which was shown in the previous slide • Lets discuss the game of tug of war • In the game of tug of war two teams pull at a rope in opposite directions . • Members of both the teams try to pull the rope in their direction . • Sometimes the rope simply does not move . • The teams that pulls harder , that is ,applies a larger force , finally wins the game . • This implies that If the two forces act in the opposite directions on an object , the net force acting on it is the difference between the two forces.
Activity 11.2 Choose a heavy object like a table or a box , which you can move only by pushing hard . Try to push it all by yourself . Can you move it ? Now ask one of your friends to help you in pushing it in the same direction . Is it easier to move it now? Can you explain why? Now Push the same object ,but ask your friend to push it from the opposite side. Does the object move? If it does , note the direction in which it moves . Can you guess which one of you is applying a larger force?
Inference of the Both Activities • Forces has both Magnitude and Direction • (i) Forces applied on an object in the same direction add to one another. • (ii) If the two forces act in the opposite direction on an object , the net forces acting on it is the difference between the two forces. • (iii) In general , more than one forces may be acting on an object at any given point. • However , force acting on an object is always the mean net force acting on it.
Why some times rope doesn’t move in a tug of war game? • We know that if the two forces act on opposite direction on an object , the net force acting on it is the difference between the two forces. • If the two teams pull the rope with equal force, the forces will be in opposite directions and will cancel each other and rope will not move.
Effect of force • When a force is applied , it may bring out different changes in the objects . Let us find out what these changes are. • 1. Forces causes movement in an object • When an object is in the state of rest , it can be brought in motion on applying a force on it • The object will move in the direction of the force applied on it.
2.Force Changes speed of a moving object • Making an object move faster or slower , or bringing it to rest means changing its speed . • If a force is applied in the direction in which the object is moving, the object will move faster . • But if a force is applied in a direction opposite to the direction of the moving object , its speed will reduce. • For example , a football can be made to move by kicking it and it can be stopped by the force applied by the goalkeeper . • Similarly , a bicycle can be stopped or slow down by applying brakes. • Hence , force is required to change the speed of a moving object.
Forces Changes direction on an object • In cricket , when a batsman hits the ball , the direction in which the ball is moving , changes. • In the game of football , the players can changes the direction of the moving ball by kicking it in different directions . When you ride a bicycle , you need to make some efforts to turn the bicycle. • Thus , force is needed to change the direction of the motion of a moving object.
Types of Forces • There are different types of forces . Some forces act when two objects are in contact with each other. • But sometimes its not necessary for objects to be in contact . • When you press a balloon , you make contact with the balloon to exert a force . But when an apple falls from a tree , the earth’s pull make it move downwards. • Here apple and the earth are not in contact with each other . • Forces , therefore , can be broadly divided into two categories: Contact forces and Non contact forces.
Contact Forces • Forces that arise when the objects are in direct or indirect physical contact with each other are called the contact forces. • Examples are : • Muscular forces • Frictional forces
Muscular Forces • When you walk , run, kick a ball or lift an object , you apply a force . This force is called muscular force. • Animals exert muscular force to do heavy works such as pulling a cart, ploughing and carrying heavy loads . • Since muscular force can be applied on objects only when it is in contact with the object, it is a contact force . • Hence , the force exerted by the muscles of a human or animal body is called muscular force.
Frictional force • Frictional is a force that slows things down or prevents things from moving. • Thus , frictional force (friction) is the opposing force that arises when a body slides over another body. • Friction acts at the surfaces in contact. • For example , when a ball rolled on the ground , after certain interval of time , it comes to rest . • We generally perceive that the ball comes to rest by itself but actually there is a force applied by the ground which acts on the ball in the direction opposite to its motion. • It makes the ball stop after a certain interval of time . • In general , friction reduces the speed of the moving object . • You will study more about frictional force in the next chapter.
Non contact Forces • The force which can act even without any actual contact between the two objects is called a non-contact force. • An object can exert a force on another object only when it lies within its field of influence .. • Examples of non contacts forces are: • Magnetic forces • Electrostatic forces • Gravitational forces
Magneticforce • The force exerted by a magnet on another magnet or any magnetic substance like iron and cobalt within its magnetic field called a magnetic force. • The magnetic force acts from a distance . The effect of this force become evident when iron pins or nails are brought near any magnet . • The iron nails will get attracted towards the magnet . • But magnet has a certain field of influence within which it can exert the force. • This field of influence is called its magnetic field . • This it shows that magnetic force is an non-contact.
Activity(tb pg. 135) Take a pair of bar magnets . Place the longer side of one of the magnets over three round shaped pencils or wooden rollers as shown in the figure . Now bring one end of the other magnet near the end of the magnet placed on the rollers Make sure that two magnets do not touch each other. Observe what happens . Next , bring the other end of the magnet placed on the rollers . Note what happens to the magnet placed on the rollers every time another magnet is brought near it.
Magnetic force • Does the magnet on the rollers begin to move when the other magnet is brought near it? • Does it always move in the direction of the approaching magnet? • What does these observation suggest? • Does it mean that some force must be acting between the two magnets? • We have already learnt in previous class that like poles of the two magnet repel each other. • And unlike poles of the magnet attract each other. • Attraction or repulsion between objects can also be seen as another form of pull or push. • Do you have to bring the magnets in contact for observing the force between them. • A magnet can exert a force on another magnet without being in contact with it. • The force exerted by a magnet is an example of a non contact force.
Electrostaticforce (Activity) Take a plastic straw and cut it into nearly two equal pieces. Suspend one of the pieces from the edge of a table with the help of a piece of thread (Fig. 11.11) . Now hold the other piece of straw in your hand and rub its free end with a sheet of paper. Bring the rubbed end of the straw near the suspended straw. Make sure that the two pieces do not touch each other. What do you observe? Next, rub the free end of the suspended piece of straw with a sheet of paper. Again, bring the piece of straw that was rubbed earlier with paper near the free end of the suspended straw. What do you observe now?
A straw is said to have acquired electrostatic charge after it has been rubbed with a sheet of paper. Such straw is an example of a charged body. The force exerted by a charged body on another charged or uncharged body is known as electrostatic force. This force comes into play even when the bodies are not in contact. The electrostatic force, therefore, is another example of a non-contact force. You will learn more about electric charges in Chapter 15.
Another activity • Take a piece of paper and tear it into small bits . Rub a plastic comb 5-10 times through your dry hair . Bring the comb near these pieces . • What do you observe ? • The paper pieces are attracted towards the comb in your hair ,it gets charged. • The charge thus developed is called electrostatic charge. • Electrostatic charge exerts a force , called electrostatic force. • It is because of this force that the bits of paper move towards the comb. • As with magnetic force , the electrostatic force can be felt only when two charged objects are kept sufficiently close to each other. If they are kept too far , there will not be any attraction or repulsion. • Thus it shows that electrostatic force is a non contact force.
GRAVITATIONAL FORCE • Gravitational force is the force of attraction that always exists between any two objects having mass. • The earth exerts a force of attraction on all objects , trying to pull them towards its center all the time. • When you drop an object from a height , it always comes down towards the earth. • If you throw an object upwards , it may go up to a certain height and then fall down. It is due to the gravitational force exerted by the earth. • Earth is a huge body . It has a large mass. It attracts every object towards it .It is because an object with larger mass exerts more gravitational pull than any object with smaller mass. • That is why a ball thrown upwards ultimately comes down; a ripened fruits falls from a tree down to the earth.
It is the gravitational force of the earth that enables us to walk conveniently on the ground . Otherwise we would have floated away. Gravitational force makes the earth move around the sun and also makes the moon go around the earth. Objects or things fall towards the earth because it pulls them. This force is called the force of gravity, or just gravity. This is an attractive force. The force of gravity acts on all objects. The force of gravity acts on all of us all the time without our being aware of it. Water begins to flow towards the ground as soon as we open a tap. Water in rivers flows downward due to the force of gravity.
Weight of an object • Weight is a measure of the earth’s gravitational pull on an object. When you weigh an object using a spring balance ,you measure the earths gravitational pull on the object . • The weight of a body can be measured by hanging it on a spring balance and letting the pull of gravity stretch the spring in the balance . • The greater the pull , the more the spring stretches and more is the weight of the body . • This is the reason a big stone has more weight than a small stone
Weight and mass are two different physical quantities . Mass is the material content of an object whereas weight is a force that changes with the change in gravity. Force of gravity on the moon is 1/6th of the force of gravity on the earth . If your weight on the earth is 48 kg , you will weigh 8kg on the moon.
Unit of force • The S.I Unit of force is newton . • In the name of famous scientist Sir Isaac Newton.
PRESSURE • You have learnt in Class VII that strong winds during a storm or a cyclone can blow away even the roof-tops. You also learnt that winds and cyclones are caused by the differences in air pressure. • Is there any relation between pressure and force? Let us find out. Try to push a nail into a wooden plank by its head. • Did you succeed? Try now to push the nail by the pointed end Could you do it this time? Try cutting vegetables with a blunt knife and then with a sharp knife. • Which is easier? Do you get the feeling that the area over which the force is applied (for example, the pointed end of the nail) plays a role in making these tasks easier? The force acting on a unit area of a surface is called pressure. pressure = force / area on which it acts
Pressure • Why is it easier to cut an apple with a sharp edged object like a knife rather than a blunt edged object like a ruler. • It is because the sharp edge has a very small area of contact with the surface of the apple whereas a blunt edge has a larger area of contact . • The impact of force is distributed all over the surface of contact but in case of a knife , the impact of force is at fixed area. • Thus we see that the impact of force depends upon the area over which the force is applied as well as the magnitude of the force . The impact of force is called the pressure.
The more the force , the more is the pressure if the area of contact is constant. • However , bigger the area , lesser is the pressure if the force applied is contact. • Pressure is directly proportional to the force and inversely proportional to the area of contact.
Definition of pressure • Pressure is defined as the force per unit area . The SI unit of pressure is pascal (Pa) • Which is newton/square meter(N/m2) • Mathematically pressure can be represented as, • Pressure (in Pa) = Force (in newton)/Area (in m2) • At this stage we consider only those forces which act perpendicular to the surface on which the pressure is to be computed.
NUMERICAL • A solid weighs 200 N. Calculate the pressure exerted by the solid when it is kept on a glass surface , if the area of contact is 500cm2 • Area = 500cm2 = 500/10000 = 1/20 • = 1/20m2 • Pressure = Force / Area = 200/1/20 • = 200x20 = 4000 Pa
