Download presentation

1
**Chapter 7 Conservation of Energy**

2
**Recap – Work & Energy The total work done on a particle**

is equal to the change in its kinetic energy

3
Potential Energy The total work done on an object equals the change in its kinetic energy But the total work done on a system of objects may or may not change its total kinetic energy. The energy may be stored as potential energy.

4
**Potential Energy – A Spring**

Both forces do work on the spring. But the kinetic energy of the spring is unchanged. The energy is stored as potential energy

5
**Conservative Forces If the ski lift takes you up a displacement h, the**

work done on you, by gravity, is –mgh. But when you ski downhill the work done by gravity is +mgh, independent of the path you take

6
**Conservative Forces The work done on a particle by**

a conservative force is independent of the path taken between any two points

7
**Potential-Energy Function**

If a force is conservative, then we can define a potential-energy function as the negative of the work done on the particle

8
**Potential-Energy Function**

potential-energy function associated with gravity (taking +y to be up) The value of U0 = U(y0) can be set to any convenient value

9
**Potential-Energy Function of a Spring**

By convention, one chooses U0 =U(0) = 0

10
**Force & Potential-Energy Function**

In 1-D, given the potential energy function associated with a force one can compute the latter using: Example:

11
**7-1 Conservation of Energy**

12
**Conservation of Energy**

Energy can be neither created nor destroyed Closed System Open System

13
**Conservation of Mechanical Energy**

If the forces acting are conservative then the mechanical energy is conserved

14
**Example 7-3 (1) How high does the block go? Initial mechanical energy**

of system Final mechanical energy of system

15
**Example 7-3 (2) Forces are conservative, therefore,**

mechanical energy is conserved Height reached

16
**Example 7-4 (1) How far does the mass drop? Initial mech. energy**

Final mech. energy

17
Example 7-4 (2) Final mech. energy = Initial mech. energy

18
Example 7-4 (3) Solve for d Since d ≠ 0

19
**Example 7-4 (4) Note is equal to loss in gravitational potential**

energy

20
**Conservation of Energy & Kinetic Friction**

Non-conservative forces, such as kinetic friction, cause mechanical energy to be transformed into other forms of energy, such as thermal energy.

21
**Work-Energy Theorem Work done, on a system, by external**

forces is equal to the change in energy of the system The energy in a system can be distributed in many different ways

22
**Example 7-11 (1) Find speed of blocks after spring is**

released. Consider spring & blocks as system. Write down initial energy. Write down final energy. Subtract initial from final

23
**Example 7-11 (2) Initial Energy Take potential energy of system**

to be zero initially Kinetic energy of system is zero initially

24
**Example 7-11 (3) Final Energy Kinetic and potential energies of**

system have changed

25
**Example 7-11 (4) Subtract initial energy from final energy**

But since no external forces act, Wext = 0, so Ef = Ei

26
Example (5) And the answer is… Try to derive this.

27
**E = mc2 E = mc2 In a brief paper in 1905 Albert Einstein wrote**

down the most famous equation in science E = mc2

28
**Sun’s Power Output Power 1 Watt = 1 Joule/second**

100 Watt light bulb = 100 Joules/second Sun’s power output 3.826 x 1026 Watts

29
**Sun’s Power Output Mass to Energy**

Kg/s = x 1026 Watts / (3 x 108 m/s)2 The Sun destroys mass at ~ 4 billion kg / s

30
Problems To go…

31
Ch. 7, Problem 19

32
Ch. 7, Problem 29

33
Ch. 7, Problem 74

Similar presentations

OK

Potential Energy (PE or U) Definition: The energy that an object has by virtue of its position relative to the surface of the earth. PE = mgh Compare the.

Potential Energy (PE or U) Definition: The energy that an object has by virtue of its position relative to the surface of the earth. PE = mgh Compare the.

© 2018 SlidePlayer.com Inc.

All rights reserved.

To ensure the functioning of the site, we use **cookies**. We share information about your activities on the site with our partners and Google partners: social networks and companies engaged in advertising and web analytics. For more information, see the Privacy Policy and Google Privacy & Terms.
Your consent to our cookies if you continue to use this website.

Ads by Google

Download ppt on pollution Ppt on steps in curriculum development Ppt on acid-base titration calculations Ppt on central limit theorem explained Download ppt on fundamental rights and duties citizens Ppt on product advertising Ppt on vehicle chassis Ppt on endangered species tiger Ppt on high voltage engineering jobs Ppt on energy giving food meaning