CBSE PHYSICS: 8th Force Pressure and Friction

Action of force and its effects: A force

a) can change the state or rest or motion
b) can change the shape of object
c) can change speed of a body
d) can change direction of motion

Contact and non-contact forces

Forces which act only when there is physical contact between two interacting objects are known as

Contact forces.

Forces which can act without physical contact between objects, i.e. those that can act from a distance, are called non-contact forces or field forces.

What is the definition of balanced and unbalanced force?
When two equal forces acting on an object in opposite directions called balanced force.

When something does not move the forces are balanced, e.g. Gravity pulls a table down but that table is pushed up by the floor or the ground it stands on so doesn’t’ move.

An Unbalanced force is needed for something to change movement or change direction, e.g. a seesaw moving up and down because the forces are not equal.

Net force: - The sum of all the forces acting on a body is known as net force.

Types of forces

1. Muscular force: This is the force we can exert with our bodies by using our muscles, e.g. pull, push, kick etc. These are contact forces

2. Magnetic force : Magnets exert forces of attraction or repulsion on other magnets. An important feature of magnetic force is that it can act from a distance, and is therefore of a non-contact force.

3. Electrostatic force: The force exerted by a charged body on another charged or uncharged body is known as electrostatic force. Electrostatic force is used to separate solid pollutant particles from smoke given out from factories.

4. Gravitational force : All objects in the universe exert a force on all other objects. This is called gravitational force. The gravitational force exerted by the Earth on all the bodies on its surface is called gravity.

5. Frictional force : The fact that the rolling ball comes to rest after some time shows that there must be a force acting on it which tends to slow it down. This force seems to be more on rough surfaces than on smooth surfaces. The force acting against the relative motion of surfaces in contact is called frictional force or friction.

6. Tension Force:Strings, ropes and chains can only pull on things! The force of pull supplied by strings, ropes or chains is called the tension force. The tension force is always directed along the length of the thing doing the pulling (string, rope, chain).

7. Spring Force‎: is the force exerted by a compressed or stretched spring upon any object that is attached to it.

8. Applied Force: An applied force is a force that is applied to an object by a person or another object. If a person is pushing a desk across the room, then there is an applied force acting upon the object. The applied force is the force exerted on the desk by the person.

Mass and Weight :

The mass of an object is the amount of matter that is contained by the object; the weight of an object is the force of gravity acting upon that object.

The mass of an object (measured in kg) by Pan Balance and will be the same no matter where in the universe that object is located.

On the other hand, the weight of an object (measured in Newton) by Spring balance and will vary according to where in the universe the object is.

Type of friction:

Static friction:

The block remains at rest because a force of friction, equal but opposite to the applied pull, comes into action between the surfaces. Increase the force a little. The block des not move. This means that the force of friction has increased to balance the pulling force on the block.

If the pulling force P is increased further, at a certain stage the block begins the move on the table. At this point the friction developed has reached the maximum value F for the two surfaces. This is called static friction.

Kinetic or sliding friction

Continue pulling the block with the spring balance, so that it slides at a steady speed. The reading on the spring balance is also steady and is slightly less than the static friction. This is a measure of the kinetic or sliding friction between the two surfaces.

Kinetic or sliding friction is less than static friction.

Rolling friction

Repeat the above procedure using an identical wooden block provided with wheels on either side. The reading on the spring balance when the block moves with a steady speed is much less than the sliding friction measured above.

Rolling friction is less than sliding friction.

Advantages of friction

Friction plays an important role in our daily life.

1) Without friction we would slip and fall every time we attempt to walk or run. There is very little

friction on a wet polished floor. That is why it is easy to slip on such a floor.

2) Friction causes nails and screws to hold on to walls.

3) It would not be possible to light a matchstick without friction between its head and the side of the

matchbox.

4) Cars and buses are able to run on roads because of friction between the tyres and the road.

5) Without friction writing on paper would be impossible as the tip of the pen will slip on paper.

6) It is because of friction between the brake ‘shoes’ and wheels that bicycles and automobiles stop

when brakes are applied.

Disadvantages of friction

Friction is a nuisance in some circumstances.

1) The heat produced in the moving parts of machinery due to friction results in wear and tear of the parts.

2) Forest fires are caused due to friction between branches of threes rubbing against each other.

3) Tyres of vehicles and soles of footwear wear out because of friction.

4) Energy is wasted in overcoming the force of friction.

Minimizing friction

1) By using a suitable lubricant, like oil (for light machinery) or grease (for heavy machinery). This

helps because fluid friction is less than solid friction

2) By using wheels and ball bearings.

Use of wheels between surface moving over each other reduces friction. Ball bearings have small balls of steel between steel surfaces. Because of the balls the steel surfaces can easily moves over each other.

3) By making the rubbing surfaces smooth by polishing them.

4) Friction due to air (air resistance) or water is reduced by using streamlined shapes in aeroplanes or ships.

A streamlined shape is narrow in front and broader at the back. Birds and aquatic animals have streamlined shapes which held them in flying or swimming.

Increasing friction

1) By making the moving surfaces rough, e.g. tyres have designs and patterns with grooves on the surface to increase resistance with the road. This prevents slipping of the tyres on a wet road.

2) Sand and gravel is strewn on slippery ground during the rainy season to increase friction. It is them easier to walk on the ground.

3) To increase friction, spikes are provided in the soles of shoes used by players and athletes.

Pressure :

Pressure is defined as the force acting on a unit area. Pressure = Force/ Area

If the force increases the pressure increases.

If the area over which the force acts decreases, the pressure increases

Unit of pressure

The SI unit of pressure is the pascal (Pa). It is the pressure exerted by a force of 1 N acting over an area of 1 m².

Thus 1Pa = 1N/m²

Solved Problem

Q.1: A liquid gives force of 100N over an area of 2m2. What is the pressure?

Solution: Force = 100N Area = 2m2 Pressure = 100N / 2m2 = 50 N/m2 Pressure = 50 N/m2

Q. A liquid’s force is acting over an area of 4m2. If the pressure is 25 N/m2, what is the force?

Solution : F = P x A = 25 N/m2 x 4m2 = 100N

Everyday examples

@ When you cut an apple with a knife, the shaper the knife, the smaller is the area of contact of the knife with the apple. Thus a sharper knife exerts a greater pressure making it easier to cut the apple.

@ It is easier to pierce a piece of cardboard with a pin if the pin has a sharp point, as the area of contact is then smaller.

@ School bags and shopping bags have broad belts or straps as handles. Narrow string-like handles cause severe pain in the hand because the weight of the bag acts on a small area and so the pressure is

considerably higher.

Q Porters wear turbans when they have to carry heavy loads on their heads, to increase the area of

contact. This reduces the pressure on the head.

Q Lorries and trucks carrying heavy loads have 8 tyres instead of four, and the tyres are broader. This

increases the area of contact with the ground, this reducing the pressure exerted on the ground.

Q War tanks move on caterpillar tracks which are broad and chain-like and cover the wheel. This
causes large increase in the area of contact with the ground. The pressure on the round reduces so much that tanks can even move on soft wet ground without sinking.

Atmospheric Pressure

The Earth is surrounded by air and this covering of air is known as the atmosphere.

The atmospheric air exists to height of about 1000 km. But beyond 100 km the air is very thin. The force exerted by the atmosphere on unit area is called atmospheric pressure.

Liquid Pressure

a) Water exerts pressure downwards

b) Water exerts pressure in the upward direction

c) Water exerts pressure in all directions on the sides. This is called lateral pressure.

d) Liquid pressure increases with depth.

Pressure at the bottom of the ocean is very high. Deep sea divers wear steel diving suits to prevent their bodies from begin crushed by the tremendous pressure of water exerted on all sides.

Submarines are built of hard thick sheets of metal to withstand the high pressure under water. Fro the

same reason dams are broader at the base than at the top.

Measuring liquid pressure – the manometer

An instrument called a monometer is used to measure liquid pressure.

@ A liquid exerts pressure in all directions (downward, upward and sideways).

@The pressure is the same in all directions at the same depth.

@ Pressure increases with depth.

The pressure exerted by water in the oceans increases with depth. Very deep down the pressure is enough to crush the human body. That is why deep-sea divers wear special suits, which can withstand such high pressures.

Short answer questions

1. What is meant by force?

2. What changes can a force produce to the motion of an object?

3. What is elastic force? Give two examples.

4. How is pressure related to the area over which a force acts?

5. What is the relation between pascal and N/m2?

6. If you wear pointed heels in one leg and slippers on the other leg, which leg exerts greater pressure

on the ground?

7. What is atmospheric pressure?

8. Explain how it is possible to drink a liquid by using a straw.

9. Why are dams over rivers made broader at the bottom than at the top?

10. Differentiate between mass and weight. What are the SI units?

Long answer questions

1. Explain contact and non-contact forces. Give two examples for each.

2. What are the advantages and disadvantages of friction?

3. Illustrate static, kinetic and rolling friction with an example. Which is the highest?

4. (a) How can friction be reduced? (b) How can it be increased?

Related search:
1. Force , Frictions ad Pressure
2. Assignment By Physics adda	Download
3. Notes and Key concept	Download
4. Calculate Net force Worksheet	Download

15 September 2012

Worksheets for Class-VIII SCIENCE TOPIC: Force and Pressure

Worksheets for Class-VIII SCIENCE TOPIC: Force and Pressure

1. Name the force used in removing iron scrap from a heap of mixed scrap.

2. When is the pressure on the ground more – when a man is lying or when a man is standing? Explain.

3. You can fill a bucket from a downstairs tap quicker than from an upstairs tap. Why?

4. Why do mountaineers suffer from nose bleeding at high altitudes?

5. Trucks and buses have double tyres in the rear end. Why?

6. Why do deep-sea divers have to wear special suits before going for diving at the depths?

7. Why is it easier to hammer a sharp nail into the wood than a blunt one?

8. Why does a balloon stick to the wall after it is rubbed on polythene?

9. State the type of force acting in the following conditions-

(a) Lifting a school bag.

(b) Bullocks ploughing the field

(d) Attraction of iron nails towards a magnet.

(e) Stopping a goal by a goalkeeper.

(f) Coming downwards of the object even when it is thrown upwards.

10.An elephant weighing 50,000 Newton stands on one foot of area 1000cm². What is the

Pressure exerted on the ground?

11.A drawing pin is pushed against a wooden table with a force of 10 N. Calculate pressure exerted on the ground by the person, when he is standing.

12.Give reasons-

(a) Buildings have wide foundations.

(b) It is easier to hold a school bag with wide straps than the one with thin strings.

13. (a) State the relation between pressure exerted and area of the surface.

(b) State and define the unit of pressure.

14. How is gravity different from gravitational force?

15. How much force should be applied on a100cm² area using pressure of 25 Pascal?

16. Fill in the blanks

(a) S.I unit of pressure is ____________.

(b) The amount or the strength of force is called its_______________.

(d) Pressure in liquids depends upon _____________ and ______________.

(e) Force generated by moving of non living materials is called ______________.

Class VIII – SCIENCE WORKSHEET

21 April 2012

8th Frictions Concepts

Frictions Formative assessments - Oral Questions

Friction is a force that resists the motion of an object that is in contact with another object or material.

Causes of friction force are :

(a) Roughness of two surface (b) due to force of adhesion ( molecular attraction)(c) Weight of body

If the objects are not moving relative to each other, this is called static friction force is called static.

If force F is greater than friction F_r (written as F > F_r), then the object will slide or move. The friction is considered kinetic friction, which means moving friction.

There are mainly three types of frictions: Static friction, Sliding friction and rolling friction

What is static friction?

Friction is self adjusting force. Friction that holds an object in place is greater than the kinetic friction that slows down a moving object is called static friction

What is Sliding friction?

When two solid objects are in contact and a force is applied to slide one object against the other it is called sliding friction. Sliding friction force resists the motion.

What are causes of sliding(kinetic) friction?

The causes of sliding friction are molecular attraction or adhesion between the materials, surface roughness of the materials, and deformation resistance in the case of soft materials.

What rolling friction force?

The resistive force that slows the wheel's motion on the other solid surface. Much of rolling friction is caused by adhesion between the surfaces.

What is Fluid Friction?

When a solid object is in contact with a fluid and a force is applied to either the object or to the fluid, there is a friction force that resists the motion it is called fluid friction . Examples where fluid friction occurs are water flowing through a hose, an airplane flying through the atmosphere and oil lubricating moving parts.

What are important uses of friction?

Friction is a resistance force that slows down or prevents motion.

How is friction a nuisance?

Too much friction can be a nuisance, because it can hinder motion and cause the need for expending extra energy.

How friction is helpful?

In some situations, friction is very helpful.

(a) Friction holds our shoe to the ground, allowing us to walk

(b) A pencil eraser uses friction to rub off mistakes

(d) Friction of the brakes and the tires help to drive car and to stop

Disadvantage of friction

(a) Friction between various parts of machines led loss of energy and reduces performance

(b) Friction wears and machine parts.

(d) The friction of parts rubbing together creates heat that cause damage to a machine

Ways to reduce friction are:

(a)By lubrication or applying oils (b) By polishing surface

(e) Applying fine powder

What is drag?

Force of friction applied by liquid or gas is called Drag.it is minimized by making body of object streamline.

It depends on

(a) Speed of object with respect to fluids (b) Shape of object (c) Nature of fluid or density

Q. Explain why it is so painful to be stepped accidentally by high-heel shoes.

Ans: Heels have very small surface area in contact with the foot. Hence, the pressure on that small area is very high and this high pressure causes pain.

Q. Explain why the bodies of airplanes and birds are streamlined.

Ans: Streamlined body reduces the air resistance and enables them to move easily through the air.

Q Why is it difficult to hold a school bag having a strap made of a thin and strong string?

Ans: It is difficult to hold a school bag having a strap made of a thin and strong string because it apply the large pressure on the shoulders due to very small contact surface area . we know that the pressure is inversely proportional to the surface area on which the force acts

Q. Why the cutting edge of a knife should be as sharp as possible?

The cutting edge of a knife should as sharp to create greater pressure even for a very small applied force for quick action.

VIII force and Pressure test paper and workseet

Force and pressure Formative assessments

Oral Questions

1. Name one contact and one non-contact force?

2. What is SI unit of Force?

3. Which force act from distance and pull iron object?

4. A plastic comb when rubbed with hair can attract piece of paper. Name force and its nature?

5. Can balance force bring about any change in the state of motion?

6. What happen to the speed of moving object when it is pushed in the direction of motion?

7. Two forces 200N and 300N acting on a body in same direction. What is resultant force?

8. If several forces act on a body in different directions, In which direction body move?

9. What is the SI unit of pressure?

10. Name two quantities that define force?

11. Where the pressure due to water higher in sea – near base or at top?

12. How does pressure at a point with in a liquid change with its depth?

Increases/Decreases

13. Pressure of liquid is ------ in all directions at same depth (same/vary)

14. Pressure doesn’t depends upon the size , shape of the container (T/F)

15. When pressure is applied on a liquid, it get transmitted throughout equally and in all directions .This law is given by is

(Pascal’s/Newton’s Law)

16. On what principal, Hydraulic Disk brakes based on?

Note: Pascal's law. According to this law whenever pressure is applied on fluid it travels uniformly in all the directions

17. Gasses and liquids are collectively known as -----------------?

18. Name the single force equal to the net force of all forces acting on body?

19. Force is ----- and change in motion is ----------. ( effect, cause)

20. Force is applied in form of ---- and --------?

21. Force required to lift a body of mass 1kg vertically is called……………….( 1000gf or 1kgf).

22. Force that acting perpendicular to the object is -------------(Thrust or pressure)

23. Pressure is ratio of area and thrust ( T/F)

24. Pressure exerted by air at any point on the earth is called ---------.

25. Air exert pressure on the earth due to its ---------------( Mass or Weight)

26. -------------- is amount of matter present in an object?

27. Atmospheric pressure is measured by using ----------- that is made by E. Torricelli.

28. ------------ Barometer does not use any liquid -----------( Mercury barometer or Aneroid barometer)

29. Atmospheric pressure -------------- with altitude (Increase Or, Decrease)

30. Pressure exerted by 76 cm high column of mercury is equal to ----- atmospheric pressure ( 1 Or, 2)

31. ------------ and ----------- are use to measure weight and mass of body respectively?

32. -------------- is the change in velocity in a unit time interval.

33. If the amount of time the force is applied is very short, that force is called an ---------- ( impulse)

35. One ------------ is the force required to give a mass of 1 kilogram (1 kg) an acceleration of 1 meter

per second per second (1 m/s²).

Note: It is abbreviated as N and equal to 100,000 dynes.

36. A torque is a special form of force that turns an axle in a given direction. It is sometimes called a rotational force(T/F)

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16 April 2012

CBSE PHYSICS: Fore Fiction and Pressure for VIII Physics-Notes a...

FORCE AND PRESSURE

A force is a push or pulls acting on an object which changes or tends to change the state of the object.

In the international system of units (SI System), the unit of force is Newton (N) that is named after Sir Isaac Newton.

There are also other units like dyne, kilogram weight and pound.

Action of force and its effects: A force

CBSE PHYSICS: Fore Fiction and Pressure for VIII Physics-Notes Click Here

03 December 2011

MCQ:Thrust , Presure,Buyoancy and Density (9th physics)

MCQ’s Based On Thrust , Presure,Buyoancy and Density

1. Units of pressure are :-

(a) N/m2 (b) m2/N

2. Units of Relative Density are :-

(a) Kg/m3 (b) Unit less

(d) Depend on the density of water

3. Pressure at a point in the liquid is

(a) Same in all directions (b) Greater in the upward direction

4. If the area of an object is less then the pressure acting on that object will be

(a) Less (b) More

5. If cross – sectional area of an object is more than the pressure applied by the external force is :-

(a) Less (b) More

6. Mass per unit volume of a substance is called

(a)Density (b)Relative density

(c)specific gravity (d)None

7. A buoyant force of 200 g wt. acts on a body dipped completely in water. If the apparent weight of body is 100 g wt., then its actual weight is

(a)200 g wt. (b)100 g wt.

8. If the weight of the floating body is equal to the buoyant force then body

(a)Sinks (b)Rises

(c)Floats (d) First floats and then sinks

9. When a body is placed in a liquid, the buoyant force experienced by it is equal to the

(a)weight of liquid displaced (b)Density of the body

(c)volume of body (d)Density of liquid

10. A body immersed in a liquid will rise to the surface if the buoyant force acting on it is

(a)greater than its actual weight (b)less than its actual weight

(c)equal to its actual weight (d)equal to Zero

11. When a body is submerged in a liquid, its weight

(a)Remains same (b)Increases

(c)Decreases (d)Reduces to zero

12. A body weighing 200 g wt. is dipped in water. Its weight in water as indicated by the spring balance is 150 g wt. The buoyant force acting on the body is

(a)200 g wt (b)1000 g wt

13. A body is floating in upright position in water. Then force of gravity and buoyant force acting on the body are

(a) In the same direction long the same line

(b) In opposite direction but along the same line

(d) In opposite to direction and from couple which turns and only anti-clockwise.

14. A body floats in vertical position, when its centre of buoyancy is

(a) Below C.G. of the body (b) Above the C.G. of the body

15. In case of a body placed in liquid if the buoyant force is equal to its acutal weight, then its apparent weight

(a) Is equal to the actual weight (b) Is equal to buoyant force

16. A block of ice is floating on water contained in a beaker. When all the ice melts, the level of water

(a)rises (b)falls

(c)remains unchanged (d)None

17. A boat floating in a tank is carrying passengers. If the passengers drink water, the water level of the tank

(a) rises (b) falls

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