৪৯তম বিসিএস ⎯ পদার্থবিদ্যা [৫১১]

When a particle moves with uniform velocity in a straight line, both the magnitude and direction of its velocity remain constant. Velocity is a vector quantity, meaning it has both magnitude and direction. "Uniform velocity" means the particle covers equal distances in equal intervals of time in the same direction. Since the particle is moving in a straight line, the direction of its velocity does not change. Also, because the magnitude of the velocity remains constant, the velocity as a whole remains unchanged. Change in direction of velocity means whether the particle turns or moves in another direction. But here, since the particle moves straight with constant speed, the direction stays the same, and the amount of change in direction is 0 degrees.

Therefore, the direction of velocity of a particle moving with uniform velocity in a straight line never changes. This is consistent with Newton’s First Law of Motion.

[Source: Higher Secondary Physics Textbook]

If there is no friction, mechanical energy is completely conserved.

Mechanical energy is the sum of kinetic energy and potential energy of an object or system. It is conserved if the energy is not transformed into other forms. In a frictionless environment, the work done remains entirely within mechanical energy, so the total mechanical energy remains constant. However, in the real world, friction is always present, which converts part of the mechanical energy into heat energy. As a result, the amount of mechanical energy decreases and mechanical energy is not conserved.

Therefore, mechanical energy is fully conserved only in a frictionless environment. This is why friction is often neglected when analyzing motion to apply the condition of mechanical energy conservation. This is consistent with Newton’s laws of motion and the law of conservation of energy.

[Source: Higher Secondary Physics Textbook]

Zero acceleration means an object’s velocity does not change; its speed and direction remain constant. It is neither positive nor negative acceleration.

• When speed increases, acceleration is present.

• When speed suddenly decreases, negative acceleration (deceleration) occurs.

• A stationary object has zero velocity but is not considered motion, so the question of acceleration in motion doesn’t apply.

• If a train moves at constant velocity, its velocity remains unchanged → acceleration = 0.

[Source: Secondary Level Physics Textbook]

Newton’s laws are primarily applicable for low-speed motion and analysis of forces on everyday large objects. However, when objects or particles move at speeds close to the speed of light — such as protons or electrons emitted from the Sun — Newton’s laws fail to provide accurate results. This is because mass, time, and distance change according to the theory of relativity.

In such cases, Einstein’s theory of relativity must be applied. The failure of Newtonian mechanics for high-speed particles emitted from the Sun is a clear example of its limitation. On the other hand, Newton’s laws work well for large rocks, trains, or rockets moving at everyday speeds.

Therefore, the emission of high-speed particles from the Sun illustrates the limitation of Newton’s laws.

[Source: Higher Secondary Physics Textbook]

When a person waves a fan, the muscles consume energy. This energy originally comes from food in the form of chemical energy. The body converts this chemical energy into mechanical energy by moving the hands and the fan.

So, the energy transformation is:
Chemical energy → Mechanical energy

[Source: Higher Secondary Physics Textbook]

Einstein’s famous equation E=mc² shows that the energy produced due to the loss of mass is equal to the mass multiplied by the square of the speed of light. Here, $m$ refers to the mass defect—the lost mass when the nucleus is formed, which is converted into energy.

When a nucleus is formed from protons and neutrons, the total mass of the nucleus is slightly less than the sum of the masses of its individual nucleons (protons and neutrons). This difference is the mass defect, which is converted into energy.

[Source: Higher Secondary Physics Textbook]

In an inelastic collision, momentum is conserved because if there are no external forces, the total momentum remains constant. However, during the collision, some portion of kinetic energy is transformed into heat, sound, or deformation energy, so total kinetic energy is not conserved.

On the other hand, in an elastic collision, both momentum and kinetic energy are conserved.

One-dimensional and two-dimensional collisions refer to the dimensionality of the collision, not the type.

Therefore, the type of collision where momentum is conserved but kinetic energy is lost is an inelastic collision.

[Source: Higher Secondary Physics Textbook]

Classical mechanics cannot explain the behavior and interactions of gas molecules at the microscopic level. To understand the kinetic theory of gases and molecular motion, quantum mechanics and statistical mechanics are required.

On the other hand, classical mechanics can somewhat explain the vibration of sound waves and the motion of falling objects. It provides limited explanations for subatomic particles, but modern physics (quantum mechanics) is more effective there.

Therefore, classical mechanics fails specifically in explaining the kinetic theory of gases.


[Source: Higher Secondary Physics Textbook]

• (a) Firing a bullet from a gun
  This involves inertia of motion and reaction forces, not inertia of rest.

• (b) Hitting a dusty cloth
  The dust particles remain at rest while the cloth moves, showing inertia of rest. This is the correct example.

• (c) Pulling a large ship’s anchor
  The ship does not move easily due to inertia, but this is not a direct example of inertia of rest.

• (d) Pulling a horse cart
  Here, inertia is overcome to set the cart in motion, so this is not an example of inertia of rest.
  
  [Source: Higher Secondary Physics Textbook]

• Option ক: Incorrect. Although the mango has more mass, gravitational acceleration remains the same, so speed is not solely determined by mass.

• Option খ: Correct. The leaf is light and has a larger surface area, causing more air resistance, which slows its fall.

• Option গ: Incorrect. Gravitational force depends on mass, not shape.

• Option ঘ: Incorrect. The leaf, being flatter and lighter, experiences more air resistance than the mango.

[Source: HSC Physics Textbook]

This law states that in a closed system, the total amount of energy always remains constant. Energy can neither be created nor destroyed but can change from one form to another. For example, mechanical energy can transform into heat energy, chemical energy into electrical energy, and so on. Although energy changes its form or location, the total energy remains unchanged. This principle is the fundamental basis for understanding energy transformations.

[Source: Higher Secondary Physics Textbook]