All questions of Force and Pressure for Class 8 Exam

Frictional force

Mass is the total amount of matter present in a body where as pressure is the the force applied on a particular area and force is the interaction which changes the the state of motion of a body and weight is the amount of force acting on a body which relates to its mass and acceleration due to gravitational pull of Earth so we all know that the force exerted by the earth is the gravitational force but when it acts on the body it becomes the weight of the body

D

Pressure exerted by any point in the fluid is equal in all directions. Pressure directly depends on height a particular fluid exerts equal pressure

Explanation:

When two forces act simultaneously on an object, the object may move or may remain at rest. If the object remains at rest, it means that the two forces are equal in magnitude and opposite in direction. Such forces are called balanced forces.

In the given scenario, two forces are acting on the football simultaneously. If the football is at rest, it implies that the two forces acting on it are balanced forces.

Therefore, the correct answer is option 'C' i.e., balanced force.

Summary:

- When two forces act simultaneously on an object, the object may move or may remain at rest.
- If the object remains at rest, it means that the two forces are equal in magnitude and opposite in direction.
- Such forces are called balanced forces.
- In the given scenario, two forces are acting on the football simultaneously.
- If the football is at rest, it implies that the two forces acting on it are balanced forces.
- Therefore, the correct answer is option 'C' i.e., balanced force.

Because pressure exerted by liquids is same in all direction at the same height.

The value of atmospheric pressure on the surface of the earth at sea level is approximately 1.013 bar, or 1013 millibars, or 1013 hPa (hectopascals).

Pressure is inversely proportional to area. Therefore when area is half pressure will be double.

P=F/A
p is inversely proportional to Area

Correct answer is menometre

To understand why the correct answer is option D, we need to apply Newton's second law of motion, which states that force is equal to mass multiplied by acceleration. Mathematically, it can be represented as F = m * a, where F is the force applied, m is the mass, and a is the acceleration.

In this case, we are applying a force of 1N. The acceleration due to gravity is approximately 9.8 m/s^2. Since we want to hold the mass in our hand without it falling, the acceleration can be taken as 0 (assuming no vertical movement). Therefore, the equation simplifies to F = m * 0, which means the mass is 0.

However, this is not a practical scenario as we can clearly hold objects in our hand. The reason for this is that our hand provides an opposing force (normal force) to counteract the force of gravity acting on the object. The normal force is equal in magnitude and opposite in direction to the force of gravity.

So, in order to hold a mass in our hand, the force we apply should be equal to the force of gravity acting on the object. The force of gravity can be calculated using the formula F = m * g, where F is the force of gravity, m is the mass, and g is the acceleration due to gravity (approximately 9.8 m/s^2).

Let's rearrange the formula to find the mass: m = F / g.

Substituting the values, we get m = 1N / 9.8 m/s^2 ≈ 0.102 kg or 102 grams.

Therefore, by applying a force of 1N, we can hold a mass of approximately 100 grams in our hand. Therefore, the correct answer is option D - 100g.

Constant speed do not change anything. The difference is that unbalanced forces speed up, slow down, change an objects direction or shape. Unbalanced forces change the objects motion. The question explicitly states that the motion has been changed therefore option C.

Force of Friction:

Force of friction is a force that opposes the motion of an object when it is in contact with a surface. It is a contact force, which means it only occurs when two objects are in contact. The force of friction acts in the direction opposite to the direction of motion, and it is dependent on the roughness of the surface.

Direction of Force of Friction:

The force of friction acts in the direction opposite to the direction of motion. For example, if a box is pushed to the right, the force of friction acts to the left. If the box is pulled to the left, the force of friction acts to the right. The force of friction is always in the opposite direction to the motion of the object.

Contact Force:

The force of friction is a contact force, which means it only occurs when two objects are in contact. When two surfaces are in contact, the force of friction is produced due to the irregularities on the surface. The rougher the surface, the greater the force of friction.

Smooth Surface:

If the surface is smooth, the force of friction is small. For example, if you try to push a book on a smooth table, the force of friction is small, and the book slides easily. On the other hand, if you try to push a book on a rough surface, the force of friction is greater, and the book is harder to move.

Conclusion:

In summary, the force of friction is a contact force that acts in the direction opposite to the direction of motion. It is dependent on the roughness of the surface and is small when the surface is smooth. Therefore, the correct answer is option 'D' which includes all the above-mentioned features of the force of friction.

Force applied to an object can produce a variety of effects. When force is applied to an object, it can change the speed or direction of motion of the object, it can change the state of rest or motion of the object, and it can also change the dimension or shape of the object. All of these effects are possible when force is applied to an object.

Interaction between objects
To understand why option 'C' is the correct answer, we need to delve into the concept of forces and how they come into play.

Definition of force
A force is a push or pull that can cause an object to accelerate, decelerate, or change its shape. Forces can be exerted by one object on another through direct contact or through fields, such as electromagnetic or gravitational fields.

Interaction between objects
For a force to come into play, the two objects must interact with each other. This interaction can occur through direct contact or through fields. Let's explore these two scenarios in more detail:

Direct contact
When two objects are in direct contact, they can exert forces on each other. These forces can be a result of one object pushing or pulling the other. For example, if you push a book on a table, the book exerts a force on the table, and the table exerts an equal and opposite force on the book. This interaction between the book and the table leads to the force coming into play.

Fields
Objects can also interact with each other through fields. Fields are regions of influence around an object that can exert forces on other objects without direct contact. For example, the Earth exerts a gravitational force on objects near its surface. This force is not a result of direct contact but rather the influence of the Earth's gravitational field. Similarly, charged objects can exert electrical forces on each other through their electric fields.

Conclusion
In conclusion, for a force to come into play, the two objects must interact with each other. This interaction can occur through direct contact or through fields. Without this interaction, there would be no force acting on the objects. Therefore, option 'C' is the correct answer, as both objects must interact with each other for a force to come into play.

C is correct option

In electrostatic force, an object which is charged will repel other charged object and attract non - charged object. Hope that clears you doubt.

The instrument used to measure the atmospheric pressure is the **barometer**. This device is specifically designed to measure the pressure exerted by the Earth's atmosphere. Here is a detailed explanation of why a barometer is used to measure atmospheric pressure:

**Barometer:**
A barometer is an instrument that measures atmospheric pressure. It consists of a long glass tube filled with mercury or another liquid, with one end sealed and the other end open to the atmosphere. The open end is submerged in a container of mercury, forming a vacuum above it. The pressure exerted by the atmosphere is balanced by the weight of the mercury in the tube, resulting in the height of the mercury column.

**Principle of Operation:**
The principle behind a barometer is based on the concept that atmospheric pressure decreases with increasing altitude. When the atmospheric pressure is high, the mercury in the tube is pushed up, resulting in a higher column height. Conversely, when the atmospheric pressure is low, the mercury column height decreases. By measuring the height of the mercury column, the atmospheric pressure can be determined.

**Types of Barometers:**
There are various types of barometers, including:

1. **Mercury Barometer:** This is the traditional barometer that uses mercury as the liquid. It is based on the principle of balancing the weight of the mercury column with the atmospheric pressure.

2. **Aneroid Barometer:** An aneroid barometer is a compact and portable instrument that uses a small, flexible metal box called an aneroid cell. The cell expands or contracts with changes in atmospheric pressure, which is then converted into a mechanical movement that can be read on a dial.

3. **Electronic Barometer:** Electronic barometers use sensors to measure atmospheric pressure and convert it into an electrical signal. These barometers provide digital readings and are often used in weather stations and meteorological instruments.

**Importance of Measuring Atmospheric Pressure:**
Measuring atmospheric pressure is important for various reasons:

1. **Weather Prediction:** Changes in atmospheric pressure indicate changes in weather conditions. High pressure is associated with fair weather, while low pressure is often associated with storms and precipitation.

2. **Altitude Determination:** By measuring atmospheric pressure, it is possible to estimate the altitude of a location. As atmospheric pressure decreases with increasing altitude, barometers can be used to determine the height above sea level.

3. **Aircraft and Weather Balloon Operations:** Atmospheric pressure measurements are crucial for aircraft operations and weather balloon launches. This information helps pilots and meteorologists understand weather patterns and make accurate predictions.

In conclusion, a barometer is the instrument used to measure atmospheric pressure. It operates on the principle of balancing the weight of a liquid column with the atmospheric pressure. By measuring the height of the liquid column, atmospheric pressure can be determined, providing valuable information for weather prediction and altitude determination.