AP Physics C: Mechanics Unit 3 Work Power and Energy

Power

• The Rate at which work gets done Or how fast energy is changed

• Slope on Work or Energy vs. time graph

Energy

• Energy is the “ability to do work”

• Work leads to transfers and transformations of Energy

  

Benefits of Energy

• Energy is a totally different way of modeling the world

• No VECTORS!!!  Energy is a scalar

• Energy will allow you to solve problems you could not solve with just Forces, Kinematics, and Momentum

• It can also make problems much easier to solve

Law of Conservation Of Energy

• Energy Cannot Be Created or Destroyed!!

• It can be Transformed into different forms

• It can be Transferred from one object to another.

Linear Kinetic Energy (K)

energy of moving objects

Gravitational Potential Energy(Ug)

• Energy stored in an object due to its position in a gravitational field

• Ug= Weight x Height

Conservative Forces

• U_g and U_s can convert with K interchangeably without significant loss to other forms

• F_g and F_s are Conservative Forces

• path independence: it does not matter what path you take, the same energy must be transferred/transformed.

Non-Conservative Forces

• Converts energy to other forms such as heat that cannot easily become K again

• Kinetic F_f and F_air are Non-conservative Forces

• The path matters: the further you go, the more energy goes to heat

Spring Potential Energy (Us)

• Energy stored in a Spring due to its stretch

• Remember, it can be stretched OR compressed from the equilibrium position

Thermal Energy(Q)

• The Kinetic Energy of tiny particles, their “temperature”

• Can act as potential energy if there is a lot of Q in a small volume, like a car’s engine

• Usually thermal energy can’t be converted to Mechanical Energy, it dissipates

• Some Sources

  • Friction

  • Air Resistance

  • Inelastic Collisions

  • And many others like fires, heaters, etc.

• Most of the energy we use will eventually be thermal

Gravitational Potential Energy

• U_g=F_gh as long as g is a constant

• BUT. . .we just learned F_g changes

• Work increases r and in turn increases U_g

Gravitational Potential Well

• As you remember, U_g=0 height is free choice

• In Universal Gravitation, U_g is always negative

• Highest U_g is 0, planets and moons are trapped in “wells”

area under this graph

What concept is the slope of this graph?

What number is the slope of this graph?

W = ∆E

How can you find the Work done here?

How can you find the Force exerted here?