Olympiad Test: Force And Pressure - 1 - Class 8 MCQ

Frictional force
Frictional force is a contact force that occurs when two surfaces are in contact and moving relative to each other. It is a force that opposes motion and is responsible for various everyday phenomena like walking, driving, and gripping objects. Frictional force can be further categorized into two types:
1. Static friction: This is the force that prevents an object from moving when a force is applied to it. It acts in the opposite direction of the applied force and can vary depending on the nature of the surfaces in contact.
2. Kinetic friction: This is the force that opposes the motion of an object that is already moving. It is generally smaller than static friction and also depends on the nature of the surfaces in contact.
Frictional force is a contact force because it requires direct contact between two surfaces for it to occur. It is different from other forces mentioned, such as magnetic force, electrostatic force, and gravitational force, which do not require direct contact between objects.
Therefore, the correct answer is B: Frictional force.

The force exerted by the earth on a body is called weight of the body.
Explanation:
When a body is in contact with the surface of the Earth, it experiences a force due to the gravitational pull of the Earth. This force is known as the weight of the body. Here is a detailed explanation to support the answer:
- Force: Force is a vector quantity that can cause an object to accelerate or deform. It is measured in newtons (N) and has both magnitude and direction.
- Weight: Weight is the force exerted by gravity on a mass. It is the product of the mass of an object and the acceleration due to gravity. Weight is also a vector quantity and is measured in newtons (N).
- Mass: Mass is a measure of the amount of matter in an object. It is a scalar quantity and is measured in kilograms (kg).
- Pressure: Pressure is defined as the force per unit area. It is a scalar quantity and is measured in pascals (Pa). Pressure can be exerted by fluids or solids, but in this context, it is not relevant to the force exerted by the Earth on a body.
- Explanation: The force exerted by the Earth on a body is the weight of the body. It is directly proportional to the mass of the body and the acceleration due to gravity. The weight of an object can vary depending on the location, as the acceleration due to gravity differs at different places on Earth. However, the mass of the object remains constant. Therefore, the correct answer is B: weight.

Explanation:
Definition of Atmospheric Pressure:
Atmospheric pressure is the force per unit area exerted on a surface by the weight of the atmosphere above it. It is caused by the gravitational attraction of the planet on the atmospheric gases.
Key Points:
- Atmospheric pressure is the pressure exerted by the Earth's atmosphere.
- It is a result of the weight of the air above a certain point.
- The pressure decreases with increasing altitude, as there is less air above to exert force.
- Atmospheric pressure is measured using a device called a barometer.
- The units commonly used to measure atmospheric pressure are millibars (mb) or hectopascals (hPa).
- Standard atmospheric pressure at sea level is around 1013.25 millibars or 1013.25 hectopascals.
Conclusion:
In conclusion, atmospheric pressure is the pressure exerted by the Earth's atmosphere. It is caused by the weight of the air above a certain point and is measured using a barometer. Atmospheric pressure decreases with increasing altitude. Therefore, option D, atmosphere, is the correct answer.

Cannot be changed by the force acting on an object
Mass:
- Mass is the amount of matter in an object and it remains constant regardless of the force acting on the object.
- The force acting on an object can change its velocity, but not its mass.
Shape:
- The shape of an object can be changed by the force acting on it.
- For example, applying force to a piece of clay can change its shape.
State of rest:
- The state of rest refers to an object being stationary or not moving.
- The force acting on an object can change its state of rest, causing it to start moving.
Direction of motion:
- The force acting on an object can change its direction of motion.
- For example, throwing a ball in a different direction will change its motion.
Conclusion:
- The mass of an object cannot be changed by the force acting on it.
- The shape, state of rest, and direction of motion can all be changed by the force acting on an object.

Pressure exerted by a liquid:
Explanation:
- When a liquid is at rest, it exerts pressure in all directions.
- The pressure exerted by a liquid depends on the depth of the liquid.
- The pressure increases with increasing depth due to the weight of the liquid above.
- The pressure exerted by a liquid at a certain depth is the same in all directions.
- This is known as Pascal's law, which states that the pressure at a point in a fluid is transmitted equally in all directions.
- The pressure at a certain depth is independent of the shape of the container or the area of the surface.
- The pressure is dependent only on the density of the liquid and the depth.
- Therefore, the statement "At the same depth, pressure is the same in all directions" is true for the pressure exerted by a liquid.
Summary:
The correct statement for the pressure exerted by a liquid is: "At the same depth, pressure is the same in all directions."

Explanation:
Pressure:
- Pressure is defined as the force applied per unit area.
- It is calculated by dividing the force applied by the area over which it is distributed.
Narrow straps on school bags:
- The narrow straps on school bags increase the pressure on the shoulders.
- The force is concentrated on a smaller area, resulting in higher pressure.
- This helps distribute the weight of the bag more evenly and reduces strain on the shoulders.
Pointed nails for hammering:
- Pointed nails have a smaller contact area with the wood.
- When a force is applied to hammer the nail, the pressure is concentrated on a smaller area.
- This allows the nail to penetrate the wood more easily due to the higher pressure exerted on a smaller surface area.
Blunt cutting edge on cutting tools:
- Cutting tools with a blunt cutting edge have a larger contact area with the material being cut.
- A larger contact area distributes the force over a larger surface area, reducing the pressure.
- This provides more control and stability during cutting.
Thin and flat tires on tractors:
- Tractors have thin and flat tires to increase the pressure on the ground.
- The weight of the tractor is distributed over a smaller area, resulting in higher pressure.
- This allows the tires to grip the ground better, providing more traction and reducing slippage.
Therefore, among the given options, the correct answer is option B: pointed nails are easier to hammer in wood because they have a smaller contact area, resulting in higher pressure and easier penetration into the wood.

Pressure is defined as the force per unit area. If given a constant area the force applied is directly proportional to the pressure.

If the force is held constant the pressure is inversely proportional to the area. If the area is doubled the pressure is halved.

If the pressure is held constant then the force is directly proportional to the area. If the area is halved then the force is also halved. If the area is doubled then the force is also doubled.

Explanation:

If no force acts on a body, it will either remain at rest or move with the same speed. This is based on Newton's first law of motion, also known as the law of inertia. The law states that an object at rest will remain at rest, and an object in motion will continue to move in a straight line with constant velocity, unless acted upon by an external force.

Reasoning:

• Newton's first law of motion states that if the net force acting on an object is zero, the object will maintain its state of motion.


• If no force acts on the body, the net force on the body is zero.


• If the body is at rest, it will remain at rest.


• If the body is already in motion, it will continue to move with the same speed and in the same direction.


• This is because there is no force to change the body's velocity or direction of motion.

Conclusion:

Therefore, if no force acts on a body, it will either remain at rest or move with the same speed. The body will not break, get deformed, or move with increasing speed.

To understand the direction in which the pressure is transmitted in a beaker filled with water, let's consider the following points:
1. Pressure Transmission:
When pressure is applied to a fluid, it gets transmitted in all directions equally. This is known as Pascal's principle, which states that any change in pressure applied to an enclosed fluid is transmitted undiminished to all portions of the fluid and the walls of its container.
2. Beaker filled with Water:
In the given scenario, we have a beaker filled with water. Water is a fluid and therefore follows Pascal's principle.
3. Increasing Pressure on Air:
We are increasing the pressure on the air in the beaker at its bottom. This means that we are applying a force to the air molecules, resulting in an increase in pressure.
4. Pressure Transmission Direction:
Based on Pascal's principle, the increased pressure on the air at the bottom of the beaker will be transmitted equally in all directions. This means that the pressure will propagate upwards, downwards, sideways, and in all other directions within the beaker.
Therefore, the correct answer is option C: Equally in all directions.

To determine what will happen to the pressure when the force is kept the same but the area is reduced to half, we can use the formula for pressure:
Pressure = Force / Area
Given that the force is 15N and the initial area is 1m^2, we can calculate the initial pressure:
Initial Pressure = 15N / 1m^2 = 15 Pa (Pascal)
Now, when the area is reduced to half, the new area becomes 1/2 m^2. We can calculate the new pressure:
New Pressure = 15N / (1/2 m^2) = 15N * (2/1 m^2) = 30 Pa
Therefore, the new pressure is 30 Pa.
Conclusion:
The pressure doubles when the force is kept the same but the area is reduced to half. Therefore, the correct answer is A) Pressure gets doubled.