Download 8.1 kinetic and potential energy

H Notes 8.1 ­ Energy Basics.notebook
Honors Notes 8.1a
Kinetic Energy Basics
Energy: the ability to produce change
Kinetic Energy (KE) is energy of a moving object.
Any object that is moving has KE.
Any object that is NOT moving has no KE
An object's Kinetic Energy depends on its:
• Velocity (more velocity = more KE)
• Mass (more mass = more KE)
Examples: Carl the cat (m = 3 kg) and Dave the dog (m = 6 kg) are going for a walk together. Isn't that nice? They both walk at 4 m/s. Find their Kinetic Energies.
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Rotational Kinetic Energy (RKE) is energy of a rotati ng object.
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At what angular velocity would a 0.60 kg basketball with a diameter of 0.24 m need to spin to have 6 J of energy?
Any object that is rotating has RKE.
Any object that is NOT rotatinging has no RKE
An object's Rotational Kinetic Energy depends on its:
• Angular Velocity (more angular velocity = more RKE)
• Rotational Inertia (more rotational inertia = more RKE)
What if the ball was somehow sliding, but not rolling? (...like on a frictionless surface)
Examples: A hoop and a solid disc each have a mass of 3 kg and a radius of 0.20 m. If they are each rolling at 4.0 m/s, find their RKE and KE.
What if it was rolling?
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H Notes 8.1 ­ Energy Basics.notebook
Honors Notes 8.1b
Potential Energy Basics
Potential Energy (PE) is energy an object has stored or saved.
Note: often Potential Energy can be converted into Kinetic Energy.
We will talk about FOUR types of Potential Energy, but only learn equations for
two:
1. Electric Potential Energy (ZPE) ­ Energy stored in a battery or other power source.
Examples:
• A battery
• A static­y sweater
2. Chemical Potential Energy (CPE) ­ Energy stored in the chemical make­up of an object.
Examples:
• Gasoline
• Food
Notice...in each case, you can burn CPE to give an object KE.
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3. Elastic Potential Energy (EPE) ­ Energy an "elastic­y" object has because it is stretched or smooshed.
Examples:
• A stretched rubberband
• A stretched slinky
• A smooshed spring
• An inflated balloon
• My loaded Nerf gun
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Example with spring in class:
Notice...in each case, if you release the object, it will move, so the EPE has been converted to KE.
A simple spring follows Hook's Law:
F = kx Here, x is the distance the spring is stretched or smooshed, and k is called a "spring constant" and measures the amount of force required to stretch or smoosh the spring 1.0 m. A higher spring constant means a stiffer spring.
To calculate EPE:
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H Notes 8.1 ­ Energy Basics.notebook
4. Gravitational Potential Energy (GPE) ­ Energy an object has stored because it is elevated or being held up above some lower point.
Examples:
• A painter on a ladder
• A sky­diver
• A skier at the top of a hill
• A rock at the top of a cliff
Examples: Carl the cat (m = 3 kg) and Dave the dog (m = 6 kg) are sitting on a balcony (4 m high) watching at the stars. Isn't that nice? Find their Gravitational Potental Energies.
Carl is jealous because Dave has twice as much GPE. So to compensate, he climbs up to the top of the roof which is twice as high...8 m tall. So now find his GPE.
Notice...in each case, if you release the object, it will move, so as it moves down, the GPE gets converted to KE.
Any object that is elevated has GPE.
Any object that is at its lowest possible point has no GPE
If both Carl and Dave are on the ground, then neither of them have any GPE.
Notice...we can work backward too! How high up would Dave the dog (m = 6 kg) need to go to have 300 J of GPE?
An object's Gravitational Potential Energy depends on its:
• Height (more height = more GPE)
• Mass (more mass = more GPE)
Also note that the GPE depends on your coordinate system...once you choose
where h = 0, stick with it!
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Notice: An object can have more than one type of energy!
Ex:
1. After you drop a ball but before it hits the ground, it has:
• GPE b/c it is still elevated above the ground
• KE b/c it is moving
2. A flying airplane has all the types of energy we've discussed.
Total Energy: The sum of all types of energy an object has at a given moment.
Mechanical Energy: The sum of an object's GPE, EPE, KE, & RKE.
in other words it is energy in an object due to its motion or position, or both
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