Chapter 1 Physical Environment Short Answer Questions
Question 1. What is meant by 1 Newton force?
Answer:
1 Newton Force:-
The force which produces an acceleration of 1 m/s² when it acts on a mass of 1 kilogram is 1 Newton.
Question 2. What do you mean by force? What is its SI unit?
Answer:
Force:
Force is an agent acting on a body that actually changes the state of rest or uniform motion of the body.
The SI unit of force is Newton.
Question 3. A body kept on the surface of the table, just tends to move when it is pushed with a force of 5N. Based on this, complete the following table:
Answer:
| Force | The magnitude of frictional force |
| A. Zero | |
| B. 2N |
What is the limiting value of static friction in this case?
Magnitudes of frictional force:
- zero
- 2N
Since the body just tends to move when 5N force is applied, hence limiting (or maximum) value of static friction is 5N.
Question 4. What do you mean by frictional force?
Answer:
Frictional Force:-
Whenever the surface of one body is sliding over the surface of another body, each body exerts a force on one another.
This force on each body is always in the direction opposite to its motion relative to the other body and it acts in parallel to the surfaces in contact. This force is called frictional force.
Question 5. What do you mean by the force of static friction?
Answer:
Force Of Static Friction:
The frictional forces operating between the two surfaces at rest with respect to each other are called forces of static friction.
Question 6. What do you mean by force of kinetic friction?
Answer:
Force Of Kinetic Friction:-
The frictional forces operating between surfaces in relative motion are called forces of kinetic friction or forces of sliding friction.
Question 7. Mustard oil floats on water. Which one has greater density among mustard oil or water? Compute the pressure exerted by a 500g mass when it is placed on a table on its side having an area of 0.4 sqm. (g = 9.8m/s2).
Answer:
Water has a greater density than mustard oil.
Pressure = Force / Area = mxg= kg x 9.8 m/s²
Force= mx g=500/1000kgx 9.8m/s²
Pressure = 4.9/0.4 = 12.25 N= 12.25 N/m² (or Pa).
Read And Learn More WBBSE Solutions For Class 8 School Science Short Answer Type Questions
Question 8. State two advantages of frictional force.
Answer:
Two advantages of frictional forces are:
We can stand and walk or run easily on a surface due to the presence of frictional forces. An automobile can smoothly run on the road due to this force.
If the roads were very smooth, the frictional force would have reduced considerably, and then controlling the movement of the automobile would have been difficult.
We can hold various things in our hands due to the existence of frictional forces which exists between the object and our hand. In absence of frictional forces, we could not have gripped any object properly.
Question 9. State two disadvantages of frictional force. Two disadvantages of frictional forces are:
Answer:
Disadvantages Of frictional force:-
When an object moves over a surface, for example, a car is moving on the road, it feels frictional forces that always try to reduce its velocity.
So to maintain a steady velocity, additional expenditure of energy is required. Some machine parts experience frictional forces during their operation.
As a result machine parts suffer wear and tear, and they have to be replaced from time to time. Thus maintenance cost of such machines increases.
Question 10. What do you mean by the density of a substance? What is its CGS and SI unit?
Answer:
Density of a substance:-
The quantity of matter (mass) contained in a unit volume of a substance is called the density of that substance.
Density = mass/volume
The unit of density in CGS unit is gram per cubic centimeter (g/cc), and in SI unit is kilogram per cubic meter (kg/m³).
Question 11. The density of a liquid is 10 kg/liter. Express it in the g/cc unit.
Answer:
1 litre of water = 1000 cc of water
and, 10 kg of water = 10000 g of water
So, the density of water = 10 kg/liter = 10000g/1000cc=10g/cc.
Question 12. What do you mean by the pressure of a liquid? Write its CGS and SI unit.
Answer:
Pressure of a liquid:-
The pressure of a liquid is the force exerted by the liquid per unit area of the surface, acting perpendicularly on it.
The CGS unit of pressure is dyne/cm² and the SI unit of pressure is N/m² (or Pascal).
Question 13. What are the characteristics of pressure exerted by a liquid?
Answer:
Characteristics of pressure exerted by a liquid:-
The characteristics of pressure exerted by a liquid are the following:
Pressure at a point inside a liquid depends on the density of the liquid. Pressure at a point within a liquid increases with depth.
So long as the depth remains the same, the pressure exerted by a liquid is the same in any direction. In other words, liquid exerts pressure evenly in all directions at a given depth.
Question 14. A man is standing on the ground. What are the action-reaction forces in this case?
Answer:
When a person is standing on the floor, the person is exerting a force on the floor by virtue of his weight acting vertically downward.
According to Newton’s third law, the floor is also exerting an equal amount of force on the person. If the former is called action, the latter is the reaction force.
Question 15. What do you mean by buoyant forces?
Answer:
Buoyant Forces:-
When an object is immersed in a liquid, partially or completely, it experiences an upward force called “buoyant force”. As the object is immersed in a liquid, it displaces some volume of liquid and occupies that position.
The weight of the displaced liquid is equal to the magnitude of this upward force or buoyant force. This buoyant force is responsible for a decrease in the weight of the object when it is immersed in a liquid.
Question 16. A solid object always weighs less in water than in air. Explain.
Answer:
Given:
A solid object always weighs less in water than in air
When a solid object is immersed in a liquid, partially or completely, it experiences an upward force called “buoyant force”.
When the object is immersed in a liquid, it displaces some volume of liquid and occupies that position.
The weight of the displaced liquid is equal to the magnitude of this upward force or buoyant force. This buoyant force is responsible for making the weight of the object less than its weight in the air.
Question 17. State Archimedes’ principle.
Answer:
Archimedes’ Principle:-
Archimedes’ Principle states that, when a solid object is immersed in a liquid (partially or completely), it experiences an upward buoyant force, which is equal to the weight of the liquid displaced by the immersed part of the object.
Question 18. Submarines are not allowed to sink beyond a certain depth in the sea. Why?
Answer:
Submarines are not allowed to sink beyond a certain depth in the sea.
A Submarine is a small warship capable of going down water, when necessary. We know that pressure exerted by a liquid at a point within it depends on the depth of that point.
The more the depth of that point more will be the more pressure exerted by the liquid. The body of a submarine is made to withstand a certain amount of pressure exerted by the liquid.
This is equivalent to a certain depth within the water. Beyond this depth, the pressure exerted by water is so large that the submarine cannot withstand it.
So sinking beyond a certain depth is not permissible.
Question 19. On what factors does the buoyancy of a liquid depend?
Answer:
Factors affecting buoyancy are the following:
The buoyant force increases as the volume of the object immersed in the liquid increases. The buoyant force is maximum when the object is completely immersed in the liquid.
As the density of the liquid increases, the buoyant force exerted by the liquid on the object also increases.
Question 20. A body has a volume of 10 cm³ and the density of its material is 8g/cm³. Find out whether the body would float or sink if it is completely immersed in water.
Answer:
Mass of the body = volume x density
M=10×8 = 80g
When completely immersed in water, the body displaces 10 cm³ (equal to its own volume) of water.
Mass of displaced water = volume of displaced water x density
= 10 cm³x 1g/cm³ = 10g.
Since the mass of the body is more than the mass of the displaced water, the weight of the body is greater than the buoyancy of the displaced water. Thus the body sinks in water.
Question 21. You are holding a 2kg mass. How much force do you experience in this condition and in which direction?
Answer:
The force experienced is the weight of the mass.
Weight = mg = 2 x 9.8 = 19.6 N (g = 9.8 m/s²)
The force is directed vertically downward towards the center of the earth.
Question 22. An object is kept on a table and it is pulled. The magnitude of the pulling force is increased until the object moves. Why does the object remain stationary till the pulling force attains a certain value? When does the object start to move?
Answer:
What kind of friction comes into play when a block of wood kept on cylindrical iron rods moves?
The object does not move because its motion is opposed by the force of static friction which acts in the opposite direction of the pulling force.
As the pulling force is increased, the friction also goes on increasing. Ultimately when the applied pulling force becomes equal to or a little more than the maximum frictional force or the limiting value of static frictional force, the object tends to move on the surface of the table.
The frictional force acting between two surfaces has a maximum value beyond which it cannot increase. This maximum value is called the limiting value of static friction.
The pulling force has to be greater than this limiting value of static friction in order to make the object move.
Rolling friction comes into play when a block of wood kept on cylindrical iron rods moves.
Question 23. A large bus and a car, both moving with the same velocity, have a head-on collision and both of them come to a halt after that.
- Which vehicle experiences the greater force of impact?
- During motion, which vehicle has greater momentum?
- Which vehicle experiences greater acceleration?
Answer:
From Newton’s third law, action and reaction are the same in magnitude. Therefore, both vehicles experience the same force of impact.
Since they were moving with the same velocity, the bus having greater mass had bigger momentum.
Acceleration, a= F/ m
For the same force of impact (F), if the mass of the car is small, the car would experience greater acceleration.
Question 24. Two metal plates A and B having the same breadth but different lengths I, and 12 respectively are placed at the same depth inside the water such that their breadth is held exactly in vertical positions.
Answer:
Find the ratio of pressure acting on A and B by water in this situation. As air bubbles rise from the bottom to the top of a water tank, the size of the bubbles increases. Why does it happen?
Since the pressure inside a liquid varies as the depth of the point from the free surface, hence both plates A and B would experience the same pressure as they are placed at the same depth inside water.
Hence the ratio of pressure acting on A and B by water is 1:1.
The pressure exerted by water on the bubbles decreases as the height of the water column above them decreases. Due to the decrease in pressure, the volume of bubbles tends to increase.
Question 25. When a body is immersed in water, it displaces 5 Kg of water. How much is the buoyant force acting on the body? (g = 9.8 m/s2).
Answer:
A given solid is weighed in the air using a spring balance. It is then weighed by immersing fully in each of the following three vessels containing water as shown. In which vessel the loss of weight will be maximum and why?
The buoyant force is the weight of displaced water.
Buoyant force = mxg=5×9.8=49 N Loss in weight does not depend on the shape of the vessel. Therefore, loss in weight will be equal in all three vessels marked 1,2, and 3.
