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Force and Pressure · Maharashtra Board STD 8 (MSBSHSE - Semi English Medium). 20 questions.

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Question 13 Marks
A glass of water has an ice cube floating in water. The water level must touches the rim of the glass. Will the water overflow when the ice melts? Give the reason.
Answer
The water will not overflow when the ice melts. The water level will remain the same. Ice floats on water because its density is less than that of water. When ice melts, the volume of the water formed is less than the volume of the ice which has melted.

When ice in water melts, this difference equals the volume of the water formed when the part of ice above the surface of water melts. Therefore, the water level remains the same. Hence, there is no overflow of water when the ice melts.

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Question 23 Marks
State whether the body will float or sink in a liquid if the density of the body is 1. greater than that of the liquid 2. less than that of the liquid 3. equal to that of the liquid.
Answer
  1. The body will sink in the liquid if the density of the body is greater than that of the liquid.
  2. The body will float in the liquid if the density of the body is less than that of the liquid.
  3. The body will float inside the liquid if the density of the body is equal to that of the liquid.
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Question 33 Marks
Question 57.
Using the formula for the magnitude of the buoyant force, state under what conditions the body will
1. sink in the fluid
2. float in the fluid
3. remain in equilibrium anywhere within the fluid.
Answer
Magnitude of the buoyant force on the body = mg (ρ1ρ)
= magnitude of the weight of the body × (ρ1ρ)
1. If the density of the fluid (ρ1) is less than the density of the body (ρ), the magnitude of the buoyant force on the body will be less than the magnitude of the weight of the body. Therefore, the body will sink in the fluid.|

2. If the density of the fluid is greater than that of the body, the magnitude of the buoyant force on the body will be greater than that of the weight of the body. Therefore, the body will float in the fluid.

3. If the density of the fluid is equal to that of the body, the magnitude of the buoyant force on the body will be equal to that of the weight of the body. Therefore, the body will remain in equilibrium anywhere within the fluid.

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Question 43 Marks
Using Archimedes’ principle, determine the magnitude of the buoyant force.
Answer
Let m = mass of the body (object) immersed in the fluid, V = volume of the body, ρ = density of the body, ρ1 = density of the fluid, g = magnitude of the acceleration due to gravity. Suppose that the body is completely immersed in the fluid. Then the volume of the fluid displaced by the body = V. According to Archimedes’ principle, magnitude of the buoyant force = magnitude of the weight of the fluid displaced by the body = mass of the displaced fluid × g = volume of the displaced fluid × density of the fluid × g
(as density = mass/volume)

$=V \rho_1 g=\left(\frac{m}{\rho}\right) \rho_1 g=m g\left(\frac{\rho_1}{\rho}\right)\left(\because \rho=\left(\frac{m}{V}\right)\right)$

If the body is partially immersed in the fluid, the volume of the fluid displaced by the immersed part of the body = xV ; here 0 < x < 1.
In this case, the magnitude of the buoyant force

$=x V \rho_1 g=x\left(\frac{m}{\rho}\right) \rho_1 g=x m g\left(\frac{\rho_1}{\rho}\right)$

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Question 53 Marks
Explain the origin of pressure due to a gas enclosed in a container.
Answer
Molecules of a gas are in a state of continuous random motion. These molecules possess energy due to motion. When the molecules collide with a wall of the container, they rebound. A large number of molecules collide with the wall every second. Hence, a significant force is exerted on the wall. Pressure is the force per unit surface area. Thus, a pressure is exerted by the gas on a wall of the container.
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Question 63 Marks
Why is the opening of a dropper very narrow?
Answer
The pressure produced by a given force is inversely proportional to the area of the surface on which the force acts. The opening of a dropper is very narrow. Hence, its area of cross-section is very small. As a result, even if the dropper has a small amount of ink it, its pressure can equal the atmospheric pressure. As the opening is narrow, it is easier to transfer the ink to the pen. The possibility of ink spilling is very low.V
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Question 73 Marks
Describe a simple experiment to demonstrate atmospheric pressure.
Answer
Fill a glass completely with water (to its brim) and cover it with a flat and stiff card paper (or a piece of glass). Holding your palm on the card, turn the glass upside down and take the palm away from the card.

Image

You will find that the water does not spill. The atmospheric pressure on the card (acting upward) is greater than the pressure of the water in the glass (acting downward). Hence, the water in the glass does not spill.

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Question 83 Marks
When a rubber sucker is pressed onto a flat glass surface, it sticks tightly on the surface. Why? You need a large force to separate it from the surface. Why?
Answer
When a rubber sucker is pressed onto a flat glass surface, practically all the air between the surfaces of the sucker and the glass is pushed out. The air pressure there becomes much less than the atmospheric pressure. Hence, the sucker sticks to the glass due to the external atmospheric pressure. The atmospheric pressure is about 105 Pa. It is very large. Hence, to work against it to separate the sucker from the glass, a large force is needed.
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Question 93 Marks
Why do school bags have broad shoulder straps?
Answer
The pressure produced by a given force depends on the area of the surface on which the force acts. The greater the surface area, the less is the pressure produced. School bags have broad shoulder straps so that the weight of the bag is distributed over a large surface area thereby decreasing the pressure on the shoulders of the student carrying the bag.
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Question 103 Marks
Why do the blades of a pair of scissors have sharp edges?
Answer
The pressure produced by a given force depends on the area of the surface on which the force acts. The less the surface! area, the greater is the pressure produced, The blades of a pair of scissors have sharp edges so that the area is decreased and hence the pressure is increased. This makes cutting • an object such as cloth easier.
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Question 113 Marks
Why do we fall sideways when we are sitting in a bus and it takes a sharp turn?
Answer
When we sit in a bus and the bus is in motion, we are in a state of motion in the same direction. When the bus takes a sharp turn, our body tends to maintain the state of motion in the straight line due to inertia. The portion of our body in firm contact with the seat acquires the motion along the curved path, but the upper portion of our body, tends to move in the initial direction of motion. Hence, we fall sideways.
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Question 123 Marks
What is inertia of direction? Give two examples of inertia of direction.
Answer
The inherent property of a body by virtue of which it cannot change its direction of motion is called the inertia of direction.
Examples:
  1. While sharpening a knife, sparks fly off tangentially in the sense of motion from the grinding stone.
  2. When a vehicle moves, the mud particles sticking to its wheels fly off tangentially in the sense of motion. Hence, mudguards are fitted to vehicles.
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Question 133 Marks
What will happen if the force is removed completely when an object acquires a certain speed?
Answer
If the force is removed completely when an object acquires a certain speed, the object will move with the velocity it has at the instant the force is removed.

For example, a body moving with constant speed along a circular path in a horizontal plane will fly tangentially in the sense of motion if the centripetal force (the force directed towards the centre of the circle) is removed completely.

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Question 143 Marks
Explain the term unbalanced force.
Answer
A single force acting on a body is an unbalanced force. It produces acceleration in the body. If two or more forces act on a body such that their resultant is not zero, the resultant is an unbalanced force responsible for accelerating the body.
Example: When a ball lying on the ground is hit with a bat, the ball is set in motion by the applied force.
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Question 153 Marks
Explain the term balanced forces.
Answer
Consider a rigid body acted upon by two forces, equal in magnitude, opposite in . direction and having the same line of action. These forces are called balanced forces as their net effect on the body is zero.
Example: A glass slab kept on a table is acted upon by two balanced forces: (i) the weight of the slab acting downward and (ii) the upward force on the slab due to the table. Their net effect on the slab being zero, the slab remains at rest.
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Question 163 Marks
What is a contact force? Give one example.
Answer
A force that acts through a direct contact of two objects or via one more object, is called a contact force.
Example: If a ball at rest on the ground is kicked, it starts moving.
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Question 173 Marks
The volume of a plastic-covered sealed box is $350 cm^3$ and the box has a mass 500 g . Will the box float on water or sink in water? What will be the mass of water displaced by the box?
Answer

Image
It is greater than that of water.
Hence, the box will sink in water.
The volume of water displaced by the box $( V )=$ the volume of the box $=350 cm^3$
Image
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Question 183 Marks
The volume of an object is $20 cm^3$ and the mass is 50 g . The density of water is $1 gm ^{-3}$. Will the object float on water or sink in water?
Answer

ImageIt is greater than the density of water.
Hence, the object will sink in water.
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Question 193 Marks
Why do the load carrying heavy vehicles have large number of wheels?
Answer
The pressure produced by a given force depends on the area of the surface on which the force acts. The greater the surface area, the less is the pressure produced. Load carrying heavy vehicles have large number of wheels so that the load (weight, force) is distributed over large surface area of the wheels in contact with the road. Hence, the pressure decreases and the tyres do not burst.
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Question 203 Marks
A plastic cube is released in water. Will it sink or come to the surface of water?
Answer
It will come to the surface of water.
[Note: This is because its density is less than that of water. When it floats, the unbalanced force acting on it is zero.]
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