11/24/08
Post date: Nov 23, 2008 11:7:48 PM
Bell Activity: What is energy?
Introduction to energy
Consider a bowling ball rolling down a ramp, with the teacher's head at the bottom:
When the bowling ball hits the head, will it do work on the head? What if the ball were at rest ... does it still do work on the head?
What factors impact how much work the bowling ball can do on the head? Mass and velocity
We find, experimentally, that the amount of work a moving object can do equals 1/2*m*v^2
Why did the ball start to roll?
Gravity was pulling it down - gravity does work on the ball
How much work? W=F*d, so the work gravity can do on the ball is mass x gravity x height
When an object has the ability to do work, we say it has energy.
Energy of motion is kinetic energy, and is given by the equation KE = 1/2*m*v^2
Energy of height is gravitational potential energy, and is given by PE = m*g*h
Practice with equations:
Calculate your kinetic energy as you moonwalk across a 3 m distance. (1 lb = .453 kg)
Calculate your gravitational potential energy when you are at the top of a jump.
Conservation of energy
A fundamental law of physics says that energy cannot be created or destroyed. It can only be changed into different forms.
The equation for this is total energy at one time = total energy at another time
This equation applies for a closed system, meaning that neither matter nor energy can enter or leave the system.
Example: A 10 kg ball is dropped from rest from a height of 15 m. Find the speed of the ball when it hits the ground.
Example: Calculate the velocities of a ball doing a loop at various points on the loop.
Practice: Page 230: 9-12 due Tuesday