Energy is what is needed and used to change or do things, measured in Joules(J). Energy is conserved, meaning it cannot be created or destroyed, thus, the amount of energy in the universe is constant, just changing forms. Energy is stored, either as potential and kinetic. Potential energy is energy stored in an object not in motion, like gravitational energy or spring energy. Kinetic energy is energy stored in a moving object, so the stored energy from the gravitational energy in an object turns into kinetic energy once it is dropped from an elevated point and is moving.
Representing Energy Transfers with LOL Charts
A system is the object(s) of interest when looking at Energy transfers. Energy can be transferred in an out of a system. For example, while energy in the entire universe is at a constant level, energy is transferred into a plant from the sun through radiation. There are three different ways energy is transferred: work(W), radiation(R), and heat(Q).
LOL charts represent energy transfers using bar graphs and the system. Two bar graphs are used (the L's) as to represent initial position (before the transfer) and the final position (after the transfer). The system is outlined (the O) in the middle, with arrows, proportionate to the bar graphs, to show energy transferred in or out to or from another object (in this example, the sun) and what kind of transfer it was (in this example, +R). LOL charts are helpful when devising energy conservation equations:
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Work
Work is a scalar, meaning it represents the magnitude of the transfer.
Work is energy transferred through forces. Thus, it occurs when an external push or pull is exerted on the system. Work can be found using this equation:
Negative Work: Energy is transferred out of the system, thus the final energy will be less than the initial energy in the system.
Work is energy transferred through forces. Thus, it occurs when an external push or pull is exerted on the system. Work can be found using this equation:
- W = F * Δx
Negative Work: Energy is transferred out of the system, thus the final energy will be less than the initial energy in the system.
Energy Problem Solving
Four essential equations are used in energy problem solving:
- W = F * Δx
- Ekinetic = 1/2 m * v^2
- Egravity = m * g * h
- Eelastic = 1/2 k * Δx^2
Power
Power is the rate at which energy is transferred:
In humans, stored energy (from food, chemical energy) turns into kinetic energy, when we move. We produce power in our movement. The quicker we are able to work, the more power we produce.
Horsepower is a unit of power, used often as a measurement for engines.
- P = W/Δt
In humans, stored energy (from food, chemical energy) turns into kinetic energy, when we move. We produce power in our movement. The quicker we are able to work, the more power we produce.
Horsepower is a unit of power, used often as a measurement for engines.
- 1 Hp = 745.7 W
- One horse's maximum power is about 15 Hp
- One human's maximum power is about 5 Hp
Energy, Force, and Motion
How they are related
In previous units, we've seen how force and motion are connected. When a force is constant, motion stays constant. Now with energy introduced, we know how energy and force, thus energy and motion are connected. Work is an energy transfer executed due to the presence of force. With a nonzero force, we know that work will accelerate an object, thus showing how energy and motion are related.
Let's look at representations of force and energy when an object is in motion.
Consider a cart being pushed along a floor. The system will be the cart. Friction is negligible.
Let's look at representations of force and energy when an object is in motion.
Consider a cart being pushed along a floor. The system will be the cart. Friction is negligible.