1. Physics 218: Mechanics Instructor: Dr. Tatiana Erukhimova Lecture 18

2. James Prescott Joule b. Dec. 24, 1818, Salford, Lancashire, England d. Oct. 11, 1889, Sale, Cheshire Discovered some basic laws of electricity and thermodynamics (Joule’s Law and Joule- Thomson Law); established the basis of the Law of Conservation of Energy and The First Law of Thermodynamics Main occupation and source of funding: Brewery

3. Was home-schooled by some of the finest scientists of his time (including John Dalton) Since childhood, he was a fearless and meticulous experimenter As a boy, he hiked in the mountains with a pistol, studying echo and the speed of sound. He sounded the depth of Lake Windermere to be 198 ft. He tortured servants with crazy experiments. Spent his honeymoon climbing in French Alps and measuring the temperature at the top and the bottom of waterfalls

4. 1843 (Age 24): Paddle-wheel experiment: mechanical work can be converted into heat! Therefore, heat is one of the forms of energy. Met with hostility and disbelief. It took decades before Joule’s discovery was accepted. It led to the First Law of Thermodynamics It also showed that energy is conserved. Energy Conservation Law! Only one person believed Joule. It was William Thomson (later Lord Kelvin). They started working together.

5. As a teenager, Joule was trying to replace steam engines with electric engines in his brewery. He failed, but became interested in the connections between mechanical work, heat, and electricity. In 1840, at the age of 21, he discovered “Joule’s Law”: Heat generated in a wire=Resistancex Current ^2 It showed that electricity can be converted into heat!

6. Work Energy Theorem

7. x

8. Problems from handout

9. Problem 3 A 5.00 kg block is moving at v 0 =6.00 m/s along a frictionless, horizontal surface toward a spring with constant k=500 N/m that is attached to a wall. a)Find the maximum distance the spring will be compressed. b)If the spring is to compress by no more than 0.150 m, what should be the maximum value of v 0 ?

10. Problem 1 How many joules of energy does 100 watt light bulb use per hour? How fast would a 70-kg person have to run to have that amount of energy?

11. Problem 2 A car is stopped by a constant friction force that is independent of the car’s speed. By what factor is the stopping distance changed if the car’s initial speed is doubled?

12. Quiz Ch 7 (a) A block of mass m is attached to a spring, spring constant k, and is on a table with coefficient of friction . The spring is unstretched at x=0. 1.Find the work done by the spring and the work done by the table if the mass moves from A to B. 2. Find the work done by the spring and work done by the table if the mass moves from B to A. 3.Find the total work done by the spring for the round trip. 4.Find the total work done by the table for the round trip. m x=0 x=A x=B

13. A block of mass M is on a horizontal surface and is attached to a spring, spring constant k. If the spring is compressed an amount A and the block released from rest, how far will it go before stopping if the coefficient of friction between the block and the surface is  ? How will this answer change is the block is not attached to the spring?? Quiz Ch 7 (b) Consider the spring force given by F x = –kx+nx 2 Consider  =  0 (1 + x/d).

14. Quiz Ch7 (c) y A block of mass m slides down on the larger block of mass M (see the figure below). There is non-zero coefficient of friction between the two blocks. Neglect the friction between the block of mass M and the table. 1) Plot the free body diagram for each of the blocks; 2) Find the work done by gravity on the block of mass m (the smaller one) in two coordinate systems shown above, if the height of the inclined plane is H. x x y

15. Have a great day! Reading: Chapter 8 Hw: Chapter 7 problems and exercises