Force And Pressure
Class 8th Science Term 1 Tamilnadu Board Solution
- Choose the correct answer. a) The SI unit of pressure is N/m^2 .This unit is otherwise…
- Fill in the blanks Friction is a __________ force (contact / non-contact)…
- Match the following
- Correct the given statement. The gravitational force of moon is equal to the gravitational…
- List out the following actions on the basis of contact force and non contact force a)…
- By observing the diagram, answer the following. a) How does the pressure at A differ from…
- We know Pressure = Force / Area If 50 N force is applied on a liquid and it experiences 25…
- Aswin and Anwar were playing with four bricks each. Aswin arranged his four bricks as…
- We know that pressure can also be calculated by using the formula p=hdg A rectangular…
- Swetha is wearing a sharp edged high heeled footwear. Madhu is wearing a flat footwear. If…
- Swathi went by car to Ooty last week, when the car was climbing the mountain, her ears…
- As we go higher and higher atmospheric pressure _______ (increases / decreases)…
- Kumaran went to a shop near his house on a bicycle. The bicycle made a lot of noise when…
- We know that friction depends upon mass of the body when we roll down an iron ball and a…
- When we “suck” using a straw, the liquid travels up into it. Explain why?…
- In a car, friction is essential in some parts but needs to be reduced in some parts. Give…
- Arasu went to an exhibition. There he saw a magician lying on a bed of nails. To his…
Evaluation
Question 1.Choose the correct answer.
a) The SI unit of pressure is N/m2.This unit is otherwise called _____________ (pascal, newton, joule)
b) Atmospheric pressure at sea level is approximately equal to ________ (105 N/m2, 107 N/m2, 103 N/m2)
Answer:
a) The SI unit of pressure is N/m2.This unit is otherwise called _____pascal________
The unit of pressure is named in the hour of scientist Blaise Pascal.
b) Atmospheric pressure at sea level is approximately equal to ____105N/m2____
Explanation: The atmospheric pressure at sea level is 101325 Pa / Nm-2
Question 2.
Fill in the blanks
Friction is a __________ force (contact / non-contact)
Answer:
Friction is a ____contact______ force
Explanation: Friction force is just the rubbing of two surfaces against each other, so friction is a contact force.
Question 3.
Match the following
Answer:
i) wheels and ball bearings – decrease friction
ii) grooves – increase friction
iii) earth excavators - based on Pascal’s law
iv) fall of an apple from the tree – non-contact force
Question 4.
Correct the given statement.
The gravitational force of moon is equal to the gravitational force of earth.
Answer:
The gravitational force on moon is one – sixth of the gravitational force of earth. The gravitational force of a planet depends on its mass, as the mass of Earth is greater than the mass of moon so its gravitational force is also greater.
Question 5.
List out the following actions on the basis of contact force and non contact force
a) lifting a chair
b) the falling of a coconut from the tree
c) friction between the road and the tyre of a car
d) a comb attracts bits of paper
e) attraction between two magnets
Answer:
Contact forces are those which need any point of contact to occur whereas non-contact forces are those which can show their effects without any contact.
a) lifting a chair – contact force
b) the falling of a coconut from the tree – non contact force
c) friction between the road and the tyre of a car – contact force
d) a comb attracts bits of paper – non contact force
e) attraction between two magnets – non contact force
Question 6.
By observing the diagram, answer the following.
a) How does the pressure at A differ from the pressure at B.?
b) The pressure at B is greater than the pressure at D. Is it true?. Justify your answer.
c) Compare the pressure at A and C.
d) If the water is replaced with mercury, how would this affect the pressure at A and D?
Answer:
a) The pressure at B is greater than the pressure at A.
Pressure (P) is defined as the force (F) acting per unit area(A).
As the pressure depends upon the force i.e. the weight of the water column above the given point, so the pressure at B is greater than the pressure at A.
b) No.
Pressure is the same at two points at the same level, as point B and point D are at a same level so their pressure is also the same.
c) Same.
Though the point A and point C are in different vessels, they are at the same level. So, the pressure is the same as point A and point C.
d) Pressure at point D will be greater than point A.
As the height of the mercury column above the given point is greater than at point D than at point A, so the pressure at point D will be greater than at point A.
Question 7.
We know Pressure = Force / Area
If 50 N force is applied on a liquid and it experiences 25 N/m2 pressure.
Find out the area on which the force is applied?
Answer:
Given,
Force (F) =50N
Pressure (P) = 25 N/m2
Area (A) =?
Now,
Hence, the area is 2 m2.
Question 8.
Aswin and Anwar were playing with four bricks each. Aswin arranged his four bricks as shown in figure A. Anwar arranged his bricks as shown in figure B, in order to be a taller one.
Now let us complete the following sentences by choosing the right option below
(equal to, less than, more than)
a) The force of A on the ground is _________ the force of B on the ground.
b) The area that A occupies is ________ B on the ground.
c) The pressure exerted by A is _______ the pressure exerted by B.
Answer:
a) The force of A on the ground is ___ less than ______ the force of B on the ground.
b) The area that A occupies is ___ equal to _____ B on the ground.
c) The pressure exerted by A is __ less than ____ the pressure exerted by B.
Question 9.
We know that pressure can also be calculated by using the formula p=hdg
A rectangular storage tank is filled with paraffin. The height of the tank is 2m. Density of paraffin is 800 kg/m3, the value of g is 10 m/s2 Calculate
a) the pressure at the bottom of the tank
b) the pressure at a depth of 1 m.
Answer:
Given,
Height of the tank (h) = 2m
Density of paraffin (d) = 800 kg/m3
g = 10 m/s2
a) The pressure is given by
b) The pressure at depth of 1m
Hence, the pressure at depth of 1m is 8000 Pa.
Question 10.
Swetha is wearing a sharp edged high heeled footwear. Madhu is wearing a flat footwear. If both the girls are having same weight and both are stepping over your foot, whose footwear will cause more pain to your foot? Why?
Answer:
The high heeled footwear of Swetha is having a sharp or small area of the heel in comparison to that of Madhu who is wearing a flat heel footwear. As both the girls have the same weight, so the force acting will be same and only the area will determine the amount of pressure ( pain to my foot). Hence, Swetha will cause more pain than Madhu if they step on my foot.
Question 11.
Swathi went by car to Ooty last week, when the car was climbing the mountain, her ears popped. She felt uneasy but after some time she felt better. Why did her ears pop when she climbed the mountain?
Answer:
Swathi’s ear pops because of the changing air pressure while scaling the mountain and the difference between the air pressure outside and inside of the ear. With elevation, the air pressure decreases but the air pressure inside the ear remains constant which forces the eardrums to bulge outwards causing discomfort. If some of the air from the ear can escape through the air canal, the pressure equalizes and the eardrum comes back to its initial position with a pop.
Question 12.
As we go higher and higher atmospheric pressure _______ (increases / decreases)
Answer:
As we go higher and higher atmospheric pressure ___decreases____.
Question 13.
Kumaran went to a shop near his house on a bicycle. The bicycle made a lot of noise when he pedalled it. After coming home, he applied some oil on some parts of the bicycle. Then there was no noise, why?
Answer:
The cycle was making a lot of noise due to friction between its different parts. As oil is a lubricator, it lubricates the surfaces and reduces friction which in turn reduces the noise.
Question 14.
We know that friction depends upon mass of the body when we roll down an iron ball and a football on the ground, which ball will travel more distance? Why?
Answer:
Mass of an iron ball is much greater than that of a football. As friction depends upon mass of the body, the frictional force on the iron ball must be greater than that of football and as friction force is an opposing force, the distance travelled by the football must be greater than that of the iron ball.
Question 15.
When we “suck” using a straw, the liquid travels up into it. Explain why?
Answer:
When we suck a straw just dipped in water the air inside is removed which creates a region of low pressure. As the atmospheric pressure acting on the surface of the water is also acting on the submerged tip of the straw (by Pascal’s Law), the water flows upward along the straw.
Question 16.
In a car, friction is essential in some parts but needs to be reduced in some parts. Give two examples of where friction is a) Essential and b) Needs to be reduced in a car.
Answer:
The parts in which friction is essential:
•Between the wheels and the brake shoe.
•Between the engine and rear wheel
The parts in which friction needs to be reduced:
•Inside the engine
•Between the clutch plates
Question 17.
Arasu went to an exhibition. There he saw a magician lying on a bed of nails.
To his wonder, the magician was not hurt at all. Help Arasu to understand the phenomenon.
Answer:
A person will get hurt if he experiences greater pressure. The magician does not get hurt while lying on the bed of nails because the huge number of nails increases the surface area, decreasing the amount of pressure. If the number of nails were lesser then the magician would have got hurt due to increased pressure.