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Work and Energy

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**Work is done when an external force is used to change the energy of the system.**

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**Energy is the ability to create change or do work.**

Energy and work are both measured in Joules (J =Nm). Energy and work are scalar quantities. They only have magnitude, no direction

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**There are many different forms of Energy:**

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Kinetic Energy The energy of motion. Is the object moving?

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**Gravitational Potential Energy**

The energy due to the height of an object. Does the object have a height?

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**Elastic Potential Energy**

The energy stored in a stretched or compressed spring. Is there a loaded spring? k = The spring constant (N/m) x = distance stretched or compressed (m)

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**Is there a force of friction acting?**

Internal Energy The energy transferred to the molecules of the objects in the due to friction. HEAT Is there a force of friction acting? f = The force of friction. ∆x = The distance traveled.

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**Chemical Potential Energy**

The energy released due to a chemical reaction. Is there a chemical reaction occurring? ASK A CHEMISTRY TEACHER FOR THE FORMULA

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**Conservation of Energy**

For a closed system the sum of the original energy (Eo) and the work (W) done is equal to the final energy (Ef).

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**Using Pie Charts to understand Energy transfers**

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**A ball is dropped from rest. (Include air friction)**

Example 1: A ball is dropped from rest. (Include air friction) v = 0m/s A C B D Eint = B A D C Ug K Eint Eint K Ug Ug K h = 0

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**A pendulum swings from A to E (Neglect air resistance)**

Example 2: A pendulum swings from A to E (Neglect air resistance) D B A V=0m/s V=0m/s E h =0 C = B A D C E Ug Ug Ug K K K Ug

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**A spring launches a block across a horizontal table.**

Example 3: A spring launches a block across a horizontal table. v=0m/s v A B C D = B A D C Eint Eint Us K Eint K

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**A biker rides up a hill with at a constant speed.**

Example 4: A biker rides up a hill with at a constant speed. v 8m D C B h = 0 A = B A D C K Ug Ug Ug UC

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**Let’s do some quantitative problems:**

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Example 1: A ball is dropped from a height of 15 meters. What is its velocity just before it hits the ground? v = 0m/s 15m v h = 0

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Example 2: A pendulum is released from rest at point A and has a velocity of 6 m/s at point C. Find the initial height (h) from which the pendulum was released. (Neglect air resistance) C A V=0m/s v = 6m/s h

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Example 3: A spring is compressed 20cm and launches a 400 gram block across a horizontal table. The block comes to rest after traveling 5 meters. The coefficient of friction is What is the spring constant (k)? v=0m/s 5m

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Example 4: A 70kg biker has a velocity of 10m/s at the bottom of a 8 meter hill. The biker does 6000J of work in climbing the hill and 2000J is transferred to internal energy as he climbs the hill. What is the final velocity of the biker? v 10m/s 8m

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