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VIII-EM-FORCE AND PRESSURE Q & A
LESSON - 2 - FORCE AND PRESSURE
TEXT BOOK EXERCISES
I. CHOOSE THE BEST ANSWER
1. If we apply a force against the direction of motion of a body, then the body will
a) stop moving
b) move with an increased speed
c) move with a decreased speed
d) move in a different direction
Ans : a) stop moving
2. Pressure exerted by a liquid is increased by
a) the density of the liquid
b) the height of the liquid column
c) Both (a) and (b)
d) None of the above
Ans : c) Both (a) and (b)
3. Unit of pressure is
a) pascal b) Nm-2 c) poise d) Both (a) and (b)
Ans : d) Both (a) and (b)
4. The value of the atmospheric pressure at sea level is
a) 76 cm of mercury column
b) 760 cm of mercury column
c) 176 cm of mercury column
d) 7.6 cm of mercury column
Ans : a) 76 cm of mercury column
5. Pascal's law is used in
a) hydraulic lift
b) brake system
c) pressing heavy bundles
d) All the above
Ans : d) All the above
6. Which of the following liquids has more viscosity?
a) Grease b) Water c) Coconut oil d) Ghee
Ans : d) Ghee
7. The unit of viscosity is
a) Nm2 b) poise c) kgms -1 d) no unit
Ans : b) poise
II. FILL IN THE BLANKS.
1. The pressure of a liquid column _____ with the depth of the column
Ans : increases
2. Hydraulic lift works under the principle of _____.
Ans : pascal law.
3. The property of _____ of a liquid surface enables the water droplets to move upward in plants.
Ans : capillarity action
4. A simple barometer was first constructed by _____.
Ans : Torricelli
III. STATE TRUE OR FALSE.
1. Force acting on a given area is called pressure.
Ans : True
2. A moving body comes to rest due to friction alone.
Ans : False
3. A body will sink if the weight of the body is greater than the buoyant force.
Ans : True
4. One atmosphere is equivalent to 1,00,000 newton force acting on one square metre.
Ans : True
5. Rolling friction is slightly greater than the sliding friction.
Ans : False
6. Friction is the only reason for the loss of energy.
Ans : False
7. Liquid pressure decreases with the decrease of depth.
Ans : True
8. Viscosity depends on the pressure of a liquid.
Ans : False
IV. MATCH THE FOLLOWING:
a.
Ans:
b.
Ans:
V. COMPLETE THE ANALOGY.
1. Knot in a thread : _____ friction::ball bearing : _____ friction
Ans: Static ; Rolling
2. Downward force : weight :: Upward force offered by liquid : ____
Ans : Buoyant force
VI. NUMERICAL PROBLEM
A stone weighs 500 N. Calculate the pressure exerted by it if it makes a contact with a surface of area 25 cm2.
Weight of a stone = 500 N
Area = 25 cm2 = 0.25 m2
Pressure = Force / Area
500 / 0.25 = 50000 / 25
= 2000 Nm-2
Pressure = 2000 Nm-2
VII. CONSIDER THE STATEMENTS GIVEN BELOW AND CHOOSE THE CORRECT OPTION.
1. Assertion: Sharp knives are used to cut the vegetables.
Reason: Sharp edges exert more pressure.
a) Both assertion and reason are true and reason is the correct explanation of assertion.
b) Both assertion and reason are true, but reason is not the correct explanation of assertion.
c) Assertion is true, but reason is false.
d) Both assertion and reason are false.
Ans : a) Both assertion and reason are true and reason is the correct explanation of assertion.
2. Assertion: Broad straps are used in bags.
Reason: Broad straps last for long life. Mark the correct choice as:
a) Both assertion and reason are true and reason is the correct explanation of assertion.
b) Both assertion and reason are true, but reason is not the correct explanation of assertion.
c) Assertion is true, but reason is false.
d) Both assertion and reason are false.
Ans : b) Both assertion and reason are true, but reason is not the correct explanation of assertion.
3. Assertion: Water strider slides easily on the surface of water.
Reason: Water strider experiences less buoyant force.
Mark the correct choice as:
a) Both assertion and reason are true and reason is the correct explanation of assertion.
b) Both assertion and reason are true, but reason is not the correct explanation of assertion.
c) Assertion is true, but reason is false.
d) Both assertion and reason are false.
Ans : c) Assertion is true, but reason is false.
VIII. ANSWER VERY BRIEFLY
1. Give two examples to verify that a force changes the shape of a body.
(i) Balloon changes its shape when air is blown inside.
(ii) When the sponge is squeezed, it will change its shape.
2. Give two examples to verify that a force tends to change the static condition of a body.
(i) Force applied to the football when it is at rest.
(ii) Force applied to wheat flour dough while making chapati.
3. What do you feel when you touch a nail immediately after it is hammered into a wooden plank? Why?
I feel warm because of frictional force.
4. How does the friction arise between the surfaces of two bodies in relative motion?
Frictional force is produced due to dissimilarities of the surface of the bodies, which are in relative motion.
5. Name two instruments, which help to measure the pressure of a fluid.
(i) Manometer
(ii) Barometer
6. Define one atmosphere.
One atmospheric pressure (1 atm) is defined as the pressure exerted by the mercury column of height 76cm in the barometer. It is equal to 1.01 X105Nm-2.
7. Why are heavy bags provided with broad straps?
Broader straps are provided on heavy bags for giving lower pressure by providing a larger area of contact.
8. How does surface tension help a plant?
Xylem tissues present in plants are very narrow and the water molecules absorbed by the roots and these narrow vessels help the water to rise upward due to capillary action which is caused due to surface tension.
9. Which has greater viscosity, oil or honey? Why?
i)Each and every liquid moves with a different speed.The friction between the layers of honey is more than oil.
ii)Honey has greater viscosity than oil. Because it has strong intermolecular attractive forces between the molecules.
IX. ANSWER BRIEFLY.
1. Define friction. Give two examples of the utility of friction in day to day life.
Friction is the force that resists motion when the surface of one object comes in contact with the surface of another
(i) Lighting a candle using a matchstick.
(ii) Writing on the paper with a pen.
2. Mention any three ways of minimising friction.
(i) Decreasing the area of contact. (ii) Using lubricants.
(iii) Using ball bearing.
3. State Pascal's law and mention its applications.
Pascal's law.
The pressure applied at any point of a liquid at rest, in a closed system, will be distributed equally through all directions of the liquid.
Applications:
a.The hydraulic press is used to make the compressed bundles of cotton or cloth so as to occupy less space.
b.The automobile brake system works according to Pascal’s law.
4. Why is a ball bearing used in a cycle hub?
The purpose of using ball bearing is to reduce rotational friction and support radial and axial load.
X. Answer in detail:
1. "Friction is a necessary evil"- explain.
Knowingly or unknowingly friction is used in most of the situations of our day to day activities. In Spite of all the advantages and disadvantages one can not run their life without using frictional force. Because of its negative effects, friction is called ‘necessary evil’.
Advantages:
(i) Friction allows us to grip and catch different objects
(ii) It helps us to walk comfortably on the surface.
(iii) It helps us to write on paper or blackboard.
Disadvantages:
(i) Friction causes wear and tear in objects.
(ii) It causes damage to the parts of machines.
(iii) The machines or tools require regular maintenance due to which a lot of money is wasted.
2. Give the different types of friction and explain each with an example.
Types of friction :
Friction can be classified into two basic types;
1.Static friction and
2.Kinetic friction.
1. Static friction: The friction experienced by the bodies, which are at rest is called static friction. (E.g. All the objects rigidly placed to be at rest on the Earth, a knot in a thread.)
2. Kinetic friction: Friction existing during the motion of bodies is called kinetic friction.
Kinetic friction can be further classified into
i)sliding friction and
ii)rolling friction.
(a) Sliding friction: When a body slides over the surface of another body, the friction acting between the surfaces in contact is called sliding friction.
(b) Rolling friction: When a body rolls over another surface, the friction acting between the surfaces in contact is called rolling friction.
Rolling friction is less than sliding friction. That is why wheels are provided in vehicles, trolleys, suitcases etc.
3. Describe an experiment to prove that friction depends on the nature of a surface.
The below activity will illustrate how the nature of the surface affects the friction.
-i)Take a marble or pencil and put it on a clean table
-ii) Push the marble or pencil slightly, see and mark the point where the marble or pencil stops.
-iii)Now put some oil on the table and ensure it is evenly applied on the table
-iv) Repeat the steps i) and (ii) again
-v) observe the difference of distance between the first and second activity
-vi) You may observe that the distance covered in the second activity (after applying oil on the table) is more when compared to the first one.
As there is less friction exerted in the later case, which eases the movement
The above activity reveals the increase or decrease of the 'effect of the force of friction', depending on the nature of the surface.
4. Explain how friction can be minimised.
Using lubricants:
A substance, which reduces the frictional force is called a lubricant. E.g.: grease, coconut oil, graphite, castor oil, etc.
The lubricants fill up the gaps in the irregular surfaces between the bodies in contact. This provides a smooth layer thus preventing a direct contact between their rough surfaces.
Using ball bearing:
Since, rolling friction is smaller than sliding friction, sliding is replaced by rolling with the usage of ball bearings.
Example:Lead shots in the bearing of a cycle hub.
5. Describe an experiment to prove that the pressure in a liquid increases with depth.
The pressure in a liquid increases with depth.
Get a can or a large plastic bottle with a series of holes drilled in the side one above the other. Cover the holes with your fingers or a piece of sticky tape and then fill the container with water.
Now remove the tape. The water gushes out but comes out faster from the holes nearer the bottom where the water is deeper and so the pressure of the water is greater.
XII. HIGHER ORDER THINKING QUESTIONS.
1. Why is it not advisable to take a fountain pen while travelling in an aeroplane?
Generally fountain pens are manufactured in such a way to balance the pressure with atmospheric pressure at sea level. As the atmospheric pressure decreases with an increase of height, fountain pen kept on aeroplanes starts leaking due to the increase of pressure inside the pen than the air pressure in the aeroplane.
2. Is there any possibility of making a special device to measure the magnitude of friction directly?
Yes, it is possible to measure the magnitude of friction directly. The name of the device used to measure the magnitude of friction is 'Tribometer'.
3. Vidhya feels that mercury is costly. So, instead of mercury she wants to use water as a barometric liquid. Explain the difficulty of constructing a water barometer.
We can use water as a barometric liquid but the density difference between water and mercury is 13.6 times high. So,the height of the column to be designed accordingly to get the same pressure as measured in mercury barometer.
Mercury is used in barometers only because of its high density and the size of the column can also be constructed reasonably.
