What is energy Ability to do work Forms
What is energy? --Ability to do work!
Forms of energy Thermal Energy the kinetic energy of atoms and molecules (remember, “heat” is the transfer of this energy between systems) Chemical energy associated with electronic structure of atoms and the electromagnetic force Nuclear energy associated with nuclear structure of atoms and the strong nuclear force
Electrical energy associated with an electric current (kinetic energy of electrons in a conductor) Radiant (light) energy associated with photons of light Mechanical energy associated with the movement of position of physical bodies (kinetic and potential energy)
Energy can be categorized into two main classes: § Kinetic energy– energy a body has because it is moving
v Kinetic energy is a scalar quantity with unit of Joules (Newton*Meter) v Since it involves velocity, it depends on the frame of reference
§ Potential Energy: energy that is stored in a body. § Gravitational potential energy: Energy associated with gravitational force. That is, energy that goes into lifting an object is stored in the object as gravitational potential energy.
v. It’s a scalar quantity with unit of Joules (Newton*Meter). v. Gravitational potential energy is relative because h is relative. A reference height has to be set first. v. Change of gravitational energy doesn’t depend on the path, only height. (conservative forces)
Practice A ball of mass m collides into a stationary ball with the same mass at a velocity of v. Suppose the collision is elastic collision. a. What are the velocities of the two balls after the collision? b. What is the total kinetic energy of the system before collision? c. What is the total kinetic energy of the system after collision? d. Is the total kinetic energy conserved?
Practice A ball of mass m collides into a stationary ball with the same mass at a velocity of v. Suppose the collision is completely inelastic collision. a. What are the velocities of the two balls after the collision? b. What is the total kinetic energy of the system before collision? c. What is the total kinetic energy of the system after collision? d. Is the total kinetic energy conserved?
Momentum Kinetic energy conserved Elastic collision Yes Inelastic collision Yes No Completely inelastic collision Yes No
Practice Drop a ball of 1 kg mass from a height of 10 meters. Compute the potential energy before it is dropped and the kinetic energy just before it hits the floor. Are they the same?
The Law of Conservation of Energy The total amount of energy of an isolated system remains the same. That is, energy can be converted from one form to another but it can’t be created or destroyed.
Practice Two pendulums with the same mass swing side-by-side. When each reaches the bottom of the swing, the speed of "A" is twice the speed of "B". Find the ratio of the starting heights of “A” and “B”.
Practice You have been asked to make a roller coaster more exciting. The owners want the speed at the bottom of the first hill doubled. How much higher must the first hill be built?
Warm Up: Two balls of the same mass roll down two slopes (one steeper than the other) from the same height. Do they have the same velocity when they reach the bottoms? (ignore friction). If not, which one has a bigger velocity at the bottom?
Newton’s cradle puzzle Two balls are released from one end. Why not one ball flies off the other end with twice the velocity? The total momentum would still be conserved in that case.
Practice A pendulum is released from a position 30 degrees from the vertical. The string is 2 meters long. What is its speed when it reaches the bottom position if its mass is 1 kg?
2 2 2
Practice A 0. 30 kg ball with a speed of 2. 0 m/s has a head-on elastic collision with a stationary 0. 70 kg ball. What are the velocities of the balls after collision?
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