Lecture 11: Potential Energy & Energy Conservation

1. Lecture 11: Potential Energy &Energy Conservation

2. Questions of Yesterday You slam on your brakes in a panic and skid a certain distance d down a straight and level road before coming to a stop. If you had been traveling twice as fast, what would the skidding distance be? a) 2d b) d/2 c) 4d d) d/4 2) As a pendulum swings back and forth, the forces acting on the pendulum are the force of gravity and tension in the supporting cord. Which of these forces does no work on the pendulum? a) Gravity b) Tension c) neither one does work on the pendulum d) they both do work on the pendulum

3. Gravitational Potential Energy There is a 10 m high and a 2 m high diving board. You know that the faster you are going when you hit the water the bigger splash you will make. Which board will you jump off of to make the biggest splash? Which board gives you the greatest kinetic energy when you hit the water? Is work done on you as you fall? How do you know? What is doing work on you? The higher up you are the greater the potential you have to gain kinetic energy by gravity doing work on you

4. Gravitational Potential Energy Instead of looking at the work done by gravity on an object during any vertical displacement… we can look at GRAVITATIONAL POTENTIAL ENERGY (PE) or the energy associated with the potential for gravity to do work on an object Potential energy is the property of a system with a set “zero point” or “ground level” Potential Energy always refers to some object at some certain point in space relative to the “ground” Choosing your reference frame is important!

5. Potential Energy Work done by Gravity Energy (J) distance fallen (m) Gravitational Potential Energy Does the work done on an object by gravity increase or decrease as it falls? What about the potential energy?

6. Potential Energy Work done by Gravity Energy (J) distance fallen (m) Gravitational Potential Energy Does the work done on an object by gravity increase or decrease as it falls? What about the potential energy? The change in Potential Energy between two points = The negative of the work done by gravity through that displacement

7. DPE = mg(yf - yi) Gravitational Potential Energy What is the work done by gravity on the book from height yi to yf? Wg = (Fgcosq)Dy Wg = mgcos0o(yi - yf) yi DPE = -Wg yf

8. Units kg*m2/s2 = Joule (J) Scalar Quantity Always Positive! PE = mgy Gravitational Potential Energy y = vertical distance (height) of an object from the Earth’s surface or some other “zero” point g = magnitude of acceleration due to gravity

9. Wg = mg(yi - yf) Wnc = DKE + DPE Gravitational Potential Energy PE = mgy Recall the work energy theorem… Wnc + Wc = DKE

10. Reference Levels, y = 0 PE = mgy yi When calculating PE you can choose “y = 0” to be at any vertical height in your system yf y = 0 Wnc = DKE + DPE Only the CHANGE in PE matters! y = 0

11. KEi + PEi = KEf + PEf when Wnc = 0 Energy Conservation Wnc = DKE + DPE If only conservative forces are acting on a system… Energy is Conserved! The sum of the kinetic energy and potential energy of an isolated system remains CONSTANT at all times!

12. E = KE + PE Ei = Ef Mechanical Energy KEi + PEi = KEf + PEf Kinetic Energy (KE) + Potential Energy (PE) = Mechanical Energy (E) In any isolated system of objects interacting only through conservative forces, the total mechanical energy E = KE + PE of the system remains the same at all times!

13. KE = (1/2)mv2 PE = mgy (1/2)mvi2+ mgyi = (1/2)mvf2 + mgyf Energy Conservation KEi + PEi = KEf + PEf Energy conservation when gravity is the only conservative force acting on the system

14. Practice Problem Assuming the ramps are frictionless and the balls of equal mass start from rest…. Which ball has a greater speed when it reaches the bottom? Which ball reaches the bottom first? h h 30o 60o If the ramps have friction (equal coefficients of friction)… would your answers change?

15. Practice Problem A 70-kg diver steps off a 10-m tower and drops from rest straight down into the water. If he comes to rest 5.0 m beneath the surface, determine the average resistive force exerted on him by the water.

16. Practice Problem Starting from rest, a 10.0 kg block slides 4.00 m down to the bottom of the frictionless ramp inclined 30.0o. The block then slides an additional 5.00 m along the floor before coming to a stop. Determine: -The speed of the block at the bottom of the ramp -The coefficient of kinetic friction between the block and floor -The mechanical energy lost due to friction What if the coefficient of friction between the ramp and the block was 0.5?

17. Questions of the Day A 50-kg student starting from rest slides down a frictionless waterslide of height 10 m while a 100-kg student slides down a similar slide that is only 5 m high. Which student is going faster when they reach the bottom? a) the 50-kg student b) the 100-kg student c) they are going the same speed 2) A women pulls a crate up a rough (with friction) inclined plane at a constant speed. Which statement is NOT true? a) The work done on the crate by the normal force of the inclined plane on the crate is ZERO b) The work done on the crate by gravity is ZERO c) The work done by the net force on the crate is ZERO d) The gravitational PE is increasing