Classical Mechanics - Newton's Laws & Energy Conservation Flashcards

Question 1

What is Newton's First Law of Motion?

Accepted Answer

An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced external force. This is also known as the Law of Inertia.

Question 2

State Newton's Second Law of Motion.

Accepted Answer

The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. Mathematically, it's expressed as $\vec{F}_{net} = m\vec{a}$, where $\vec{F}_{net}$ is the net external force, $m$ is mass, and $\vec{a}$ is acceleration.

Question 3

What is Newton's Third Law of Motion?

Accepted Answer

For every action, there is an equal and opposite reaction. When one object exerts a force on a second object, the second object exerts an equal and opposite force on the first. These forces act on different objects.

Question 4

Define Inertia.

Accepted Answer

Inertia is the natural tendency of an object to resist changes in its state of motion. The mass of an object is a quantitative measure of its inertia.

Question 5

What is the difference between mass and weight?

Accepted Answer

**Mass** is a measure of the amount of matter in an object and its resistance to acceleration (inertia), measured in kilograms (kg). **Weight** is the force of gravity acting on an object, measured in Newtons (N), calculated as $W = mg$, where $g$ is the acceleration due to gravity.

Question 6

Define Work in physics.

Accepted Answer

Work ($W$) is done when a force causes a displacement of an object. It is the product of the component of the force in the direction of the displacement and the magnitude of the displacement. Mathematically, $W = Fd\cos	heta$, where $F$ is the force, $d$ is the displacement, and $	heta$ is the angle between the force and displacement vectors.

Question 7

What are the SI units for Work and Energy?

Accepted Answer

The SI unit for both Work and Energy is the Joule (J). One Joule is defined as one Newton-meter ($1 	ext{ J} = 1 	ext{ N} \cdot 	ext{m}$).

Question 8

When is work considered positive, negative, or zero?

Accepted Answer

Work is:
*   **Positive**: When the force has a component in the direction of displacement ($0^\circ \le 	heta < 90^\circ$).
*   **Negative**: When the force has a component opposite to the direction of displacement ($90^\circ < 	heta \le 180^\circ$).
*   **Zero**: When the force is perpendicular to the displacement ($	heta = 90^\circ$), or when there is no displacement ($d=0$).

Question 9

Define Kinetic Energy.

Accepted Answer

Kinetic energy ($KE$) is the energy an object possesses due to its motion. It depends on the object's mass and speed. The formula is $KE = \frac{1}{2}mv^2$, where $m$ is mass and $v$ is speed.

Question 10

State the Work-Energy Theorem.

Accepted Answer

The Work-Energy Theorem states that the net work done on an object is equal to the change in its kinetic energy. Mathematically, $W_{net} = \Delta KE = KE_f - KE_i = \frac{1}{2}mv_f^2 - \frac{1}{2}mv_i^2$.

Question 11

Define Gravitational Potential Energy.

Accepted Answer

Gravitational potential energy ($PE_g$) is the energy an object possesses due to its position in a gravitational field, relative to a reference point. The formula is $PE_g = mgh$, where $m$ is mass, $g$ is the acceleration due to gravity, and $h$ is the height above the reference point.

Question 12

Define Elastic Potential Energy.

Accepted Answer

Elastic potential energy ($PE_s$) is the energy stored in an elastic material, such as a spring, when it is stretched or compressed from its equilibrium position. The formula is $PE_s = \frac{1}{2}kx^2$, where $k$ is the spring constant and $x$ is the displacement from equilibrium.

Question 13

What is Mechanical Energy?

Accepted Answer

Mechanical energy ($E$) is the sum of an object's kinetic energy and potential energy (gravitational and/or elastic). It is expressed as $E = KE + PE$.

Question 14

State the Law of Conservation of Energy.

Accepted Answer

The Law of Conservation of Energy states that energy cannot be created or destroyed, only transformed from one form to another. In an isolated system, the total amount of energy remains constant.

Question 15

When is Mechanical Energy conserved?

Accepted Answer

Mechanical energy is conserved in a system where only conservative forces (like gravity or ideal spring force) do work. If non-conservative forces (like friction or air resistance) are present and do work, mechanical energy is not conserved, but total energy still is.

Question 16

What is a conservative force? Give examples.

Accepted Answer

A conservative force is a force for which the work done in moving an object between two points is independent of the path taken. The work done by a conservative force around any closed path is zero. Examples include gravitational force and the ideal spring force.

Question 17

What is a non-conservative force? Give examples.

Accepted Answer

A non-conservative force is a force for which the work done in moving an object between two points depends on the path taken. The work done by a non-conservative force around a closed path is not zero. Examples include friction, air resistance, and tension.

Question 18

How does the presence of non-conservative forces affect mechanical energy?

Accepted Answer

When non-conservative forces do work, mechanical energy is not conserved. The work done by non-conservative forces ($W_{nc}$) equals the change in mechanical energy: $W_{nc} = \Delta E = E_f - E_i = (KE_f + PE_f) - (KE_i + PE_i)$. This work often manifests as heat or sound.

Question 19

Define Power in physics.

Accepted Answer

Power ($P$) is the rate at which work is done or energy is transferred. Mathematically, $P = \frac{W}{t}$, where $W$ is work and $t$ is time. It can also be expressed as $P = Fv\cos	heta$ for a constant force $F$ acting on an object moving with velocity $v$.

Question 20

What is the SI unit for Power?

Accepted Answer

The SI unit for Power is the Watt (W). One Watt is defined as one Joule per second ($1 	ext{ W} = 1 	ext{ J/s}$).

Question 21

Explain the concept of a "system" in energy conservation.

Accepted Answer

A system is a defined collection of objects that we are studying. An **isolated system** is one where no external forces do work and no matter or energy enters or leaves. For such a system, total energy is conserved. A **closed system** allows energy transfer but not matter.

Question 22

How does Newton's Second Law relate to the concept of net force?

Accepted Answer

Newton's Second Law, $\vec{F}_{net} = m\vec{a}$, explicitly states that it is the *net* (or resultant) external force acting on an object that causes it to accelerate. If the net force is zero, the acceleration is zero, consistent with Newton's First Law.

Classical Mechanics - Newton's Laws & Energy Conservation

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