List of Contents

• 1. Force
• 2. Work
• 3. Energy

Force is a push or pull exerted on an object. Force is a vector quantity because it has both magnitude and direction. If unopposed, a force will change the motion of the object. The SI unit of force is Newtons or kg.m/s². Note that 1 N = 1 kg.m/s² and represented by the symbol F. The study of force is called Dynamics.

Inertia refers to the tendency of an object at rest to stay at rest and an object in motion to stay in motion, unless acted on by a force. An object at rest will only move when an unbalanced force is applied to it through a distance- that is the force acting in one direction is greater than the force acting in the opposite direction.

Force = Mass (kg) * Acceleration (m/s²) = kg m/s² = N

Mechanical force is said to occur when the objects are in physical contact. Friction is an example of a mechanical force. Sliding friction always acts in the opposite direction as the motion of the object.

Non-mechanical force can act on an object at a distance. These include gravitational force, electric force, magnetic force.

Discovery

Sir Isaac Newton described the motion of all objects using the concepts of inertia and force, and in doing so he found they obey certain conservation laws. In 1687, Newton published his thesis Philosophia Naturalis Principia Mathematica.

In this work Newton set out three laws of motion that to this day are the way forces are described in physics.

Newton's First Law

An object will remain at constant velocity until an unbalanced force act upon it.

Free-body diagrams are used to illustrate the forces acting on an object. will be discussed in more details.

Newton's Second Law

The second law states that the rate of change of momentum of a body is directly proportional to the force applied, and this change in momentum takes place in the direction of the applied force.

The acceleration is inversely proportional to the mass (called the LAW OF INERTIA) which implies that more massive objects require more force to accelerate.

This second law can be summarised as

F = ma

The second law also implies the conservation of momentum: When the net force on the body is zero, the momentum of the body is constant. Any net force is equal to the rate of change of the momentum. Any mass that is gained or lost by the system will cause a change in momentum that is not the result of an external force. A different equation is necessary for variable-mass systems.

Newton's laws and Gravity

The force of gravity acting on an object is often called weight
The formula F_g = mg explains why al objects fall at the same rate of acceleration if air resistance is negligible. Air resistance, such as that experienced by a falling feather, results in varying rates of acceleration.

Newton's Third Law

Whenever one body exerts a force on another, the latter simultaneously exerts an equal and opposite force on the first. This law is sometimes referred to as the action-reaction law. This law has had several applications in innovations such as space rockets, hot air balloons etc.

Friction

Friction is an example of a mechanical force and it always acts in a direction opposite to the motion of the object. Friction can be measured by seeing how much force is needed to slide an object along at a constant speed. The force of friction depends on two variables: Normal force - force perpendicular to the surface (that is the support force F_N or F_s), and Coefficient of friction between the two surfaces (μ or "mu")

Where F_f is the Force of friction (N), μ is the coefficient of friction (has no units) and F_N is Normal force (N).

For objects on flat surfaces-

Vertical Forces

Objects that are accelerating upwards will feel heavier than those that are not. This force can be registered on a weight scale. When the upward acceleration is at a constant velocity, the object starts to feel normal. If the acceleration reduces, the object will feel lighter. If the downward acceleration is equal to 9.81m/s², the object will feel weightless.

Whenever a force moves an object through a distance that is in the direction of the force, then work is done on the object. If the object does not move, then there is no work being done. The area under a force - distance graph represents how much work is done.

Work = Force (N) * Distance (m) = N.m = kg.m²/s² = J

Energy is defined as the ability to do work, therefore W = ΔE, or work is equal to the change in energy. When work is done an object gains energy. In the absence of any outside forces, such as friction, the total work input is equal to the total work output/energy output.

Forms of Energy

1. 1. Chemical Energy: Is the potential energy stored in the chemical bonds of compounds such as fossil fuels.
2. 2. Electrical energy is the work done by moving charges.
3. 3. Nuclear energy is the potential energy stored in the nucleus of an atom.
4. 4. Solar energy results from a hydrogen-hydrogen nuclear fusion reaction with the release of radiant energy.
5. 5. Kinetic energy is the energy an object has due to its motion.
6. 6. Gravitational potential energy is the energy an object has due to its position relative to the Earth’s surface.
7. 7. Mechanical energy is the sum of an object’s kinetic energy and gravitational potential energy.
8. 8. The amount of thermal energy in a substance is determined by the average kinetic energy of the individual atoms.
9. 9. Potential Energy
   • Elastic Potential Energy: The energy stored in an object that is stretched or compressed and will return to its original form if released.
   • Chemical Potential Energy: The energy stored in the bonds of chemical compounds. Any substance that can be used to do work through a chemical reaction has chemical potential energy.

Kinetic energy

Kinetic energy is the energy associated with the motion of an object. Kinetic energy is calculated as:

E_k = 0.5*mv²

As shown in the formula, an object with a mass and a speed will have a certain amount of kinetic energy. Another object with twice the mass and the same speed will have twice the kinetic energy. A third object with the same mass but twice the speed will have four times as much kinetic energy.

Mechanical Energy

Mechanical energy is defined as the energy due to the motion and position of an object. Mechanical energy can be calculated as:

E_m = E_p + E_k

Energy Conversions

• Photosynthesis converts light energy, carbon dioxide, and water into chemical energy in the form of glucose.
• Cellular respiration converts chemical energy in the form of glucose into chemical energy in ATP and heat energy.
• Combustion reactions convert chemical energy in fossil fuels into thermal energy.
• Hydroelectric dams convert the potential energy of water stored behind the dam into electrical energy - Hydroelectric power.
• A coal-burning power station converts the chemical energy of fossil fuels into electrical energy - Thermal power station.
• Solar cells convert solar energy directly into electricity
• In a hydrogen fuel cell, hydrogen reacts with oxygen to form water and release energy.

Energy Flow in Systems

A system is a set of interconnected parts, everything else is considered the environment.

An open system is one that exchanges both matter and energy with its surroundings (such as a tree).

A closed system is one that cannot exchange matter but can exchange energy with its surroundings (such as the Earth).

An isolated system is one that cannot exchange matter or energy with the environment (such as the Universe).

Laws of Thermodynamics

The first law of thermodynamics states that energy cannot be created or destroyed. It can only be transformed from one form to another, and the total amount of energy never changes.

The second law of thermodynamics states that thermal energy always flows naturally from a hot object to a cold object.

Machines

Machines make work easier by converting an initial energy input to the type of desired (useful) energy output. All energy output that is not being used to do useful work is considered waste energy (such as energy lost due to friction).

Efficiency (of a machine) is a measurement of how effectively a machine converts energy input into useful energy output. It is calculated as:

Efficiency = (Useful work output) / (Total work input).

Percent efficiency is Efficiency *100.

Renewable and Non-Renewable Energy

Renewable energy sources are continually and infinitely available (such as solar, wind, water, geothermal, tidal, and biomass).

Non-renewable energy sources are limited and irreplaceable (such as nuclear and fossil fuels).

The Effects of Energy Use

The extraction and combustion of fossil fuels may negatively affect environment in ways such as:

• Ecosystem disruptions.
• Oil spills
• Production of greenhouse gases (GHGs)
• Release of chemicals and other toxins, such as those that contribute to acid rain