Energy in Motion Study Guide

Main Concepts:

According to the Law of Conservation of Energy, the total energy of a system never changes. Thus, as Kinetic Energy increases, potential energy will decrease and vice versa. A common misconception is that, when there is maximum kinetic energy, there is no potential energy. This is not necessarily true since potential energy is energy due to an object’s position in space and maximum kinetic energy means that it is in the lowest point in its path and it has the fastest speed(Note: Kinetic energy is always positive or zero since it has no direction). When the object is at its lowest position, it can still have potential energy but it would have minimum potential energy. Potential energy is only zero when the object is on the ground or has a height of zero according to the potential energy formula, PE = mgh.

Similarly, when an object has maximum potential energy, it can still have kinetic energy but the kinetic energy would be at its minimum. An object can still have speed even when it is at its highest location in space.

For Newton’s laws, the most common misconception is that mass and weight are the same. However, mass is the amount of matter in an object and always remains the same regardless of location(i.e. it would have the same mass and gravity does not have an impact on it). Weight is the effect of gravity on mass and it depends on location.

Another main concept in this unit is the relationship between energy and force. If a force results in motion, it has performed work, meaning that energy was expended. Thus, energy and forces are related through the transfer and transformation of one type of energy to another through forces and work.

Vocabulary:

Potential Energy(PE)- stored energy in an object due to position(For objects lifted off the ground, its formula is PE = mgh, where m is mass of the object, g is the gravity, h is the height of the object from the ground); Ex. a stretched bow has PE due to change in its shape by work.

Maximum Potential Energy- stored energy; an object can still be moving while having maximum potential energy. The maximum potential energy is always at the highest position of an object’s trajectory and the slowest point of an object’s travel.

Kinetic Energy(KE)- energy due to motion; formula: KE = 1/2 * mv² where m is the mass of the object and v is the velocity/ speed of the object. A change in an object’s motion depends on the forces acting on the objects and its mass.

Maximum Kinetic Energy- energy of motion; Always at the spot of initial force(throw/arc); lowest point in an object’s trajectory, fastest point in an object’s travel

Total Energy- the sum of all the types of energies in an object; Total Energy does not transfer and always remains the same

Mechanical Energy- the sum of the kinetic energy and potential energy; Mechanical energy of an object remains the same unless it transfers some of it to another object; Even if mechanical energy is constant, KE and PE can still increase and decrease

Magnitude- size

Energy transformation- change from one form of energy to another; any form of energy can change into any other form of energy.

System-Open- an open system where matter and energy can go in and out

System-Closed- a closed system where matter and energy cannot go in or out

Model-representation of something

Friction- a contact force; a type of thermal energy created from the sliding motion of two surfaces; amount of friction depends on the materials that are touching; friction can also act on objects moving through air/water; coefficient of friction shows amount of friction between two surfaces. Larger coefficient of friction means greater friction; friction ALWAYS creates heat

Law of Conservation of Energy: Energy is never created nor destroyed, but is transferred and transformed in a system. The total amount of energy in a closed system always adds up to the same amount of total energy. Energy can change from one form to another but always adds up to the same amount of total energy.

Force- a push or pull exerted by an object.Force is a vector quantity(has magnitude and direction) measured in Newtons(N) and is needed to overcome inertia; Balanced forces acting on an object do not cause a change in motion because they cancel out. The motion of an object will only change when unbalanced forces act on an object. Contact force is when two objects are touching when a force is exerted. Field force is a force exerted over a distance without objects touching.

Acceleration- a change in velocity(speed and direction); Unit: m/s²

Types of Variables:

Independent Variable:the one thing that is changed in an experiment and the variable controlled to test the dependent variable

Dependent Variable: the one thing in an experiment that is being measured and the variable that changes due to the independent variable

Control: the variables that stay the same in order to observe the relationship between the independent and dependent variables

Energy

Energy is the ability to do work. Work is done when a force causes an object to move in the direction of the force. When an object does work on another, energy is transferred from the first object to the second object. Energy’s SI unit is Joule(J) which is equal to Newtons(N) * meters(m).

Types of Energy

Thermal energy is the total kinetic energy of the random motion of the particles in an object and is dependent on the number of the particles as well. For example. Steam has greater temperature than lake water but the lake water has greater thermal energy because it has more water particles.

Chemical energy is the energy of a compound and changes when the atoms are rearranged. It is a type of potential energy because it is dependent on the position and arrangement of atoms.

Electrical energy is the energy of moving electrons. Generators turn magnets inside loops of wire and its changing position makes electricity run through the wires. Electrical energy is stored as potential energy before use.

Sound energy is caused by electrical vibrations. For example, when a guitar string is stretched, it has potential energy in it because it experiences a change in shape due to work. When the string is released, it vibrates with kinetic energy. Some of the kinetic energy is transmitted into the air, creating air vibrations, thus creating sound.

Light energy is produced by the vibrations of electrically charged particles. Both light energy and sound energy are caused by vibrations. Unlike sound energy however, light energy does not require a medium to travel through, meaning that light can travel through a vacuum(area of no matter).

How are Energies and Forces related?

If a force results in motion, it preformed work, meaning that the energy is expended. Therefore, energy and forces are related through the transfer and transformation of one type of energy to another through forces and work. However, energy is the ability to do work and a force is a push or pull exerted on an object. A force may not necessarily result in work done, thus, it may not always be related with energy. Energy can also be transferred between two interacting objects by force. For example, lifting objects results in more potential energy.

Newton’s Laws of Motion

Newton’s first law of motion(Law of inertia)- Objects that are at rest or moving tend to resist any change in motion. An object at rest stays at rest and an object in motion will continue to be in motion with the same speed and direction unless acted upon by an external force.

Newton’s Cradle(representation of Newton’s first law of motion) Image Source

Inertia- an object’s tendency to resist any change in motion

Newton’s Second Law of Motion- F = ma, Force equals mass times acceleration

Newton’s Third Law of Motion- For every force, there is an equal and opposite reaction.

Gravitation

Gravitation is a force that causes objects with mass to be attracted towards each other without touching. Thus, it is a field force. Gravitational attraction by Earth on an object is gravity.

Mass and Weight

Mass is the amount of matter in an object and always remains the same regardless of location. Mass is measured in grams(g). Weight is the effect of gravity on mass and depends on location. W=mg. W is weight, m is mass, g is gravity. Thus, weight equals mass times gravity.