PHYSICS 570 Master's of Science Teaching. Electricity Lecture 8 Work, Potential Energy and Kinetic Energy

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Gladys Reed
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1 1 PHYSICS 570 Master's o Science Teaching Electricity Lecture 8 Work, Potential Energy and Kinetic Energy Instructor Richard Sonneneld mpsonneneld@gmail.com

2 Course Goals Math 2 You will learn scientiic notation, and how to calculate with it. CHECK! You will learn about vectors and how to add them with pictures and with math ( by components ) CHECK! 2 You will get GREAT applications or Trig., algebra, exponents and ractions CHECK!

3 Course Goals Physics 3 I hope at the end o the irst part o the course you will know the dierence between charge, current, electric ield, potential, voltage, potential energy, power, resistivity and resistance and be able to teach these concepts. 3

4 QUANTITY SYMBOL UNITS 4 Charge q, Q Coulomb (C) Force F Newtons (N) Current I Amperes (A) C/sec Resistance Ohms Electric ield E Newt/Coul Potential Volts (V) Joule/Coul Potential Energy U Joules ( ) Work W Joules ( ) 4 N m N m

5 Today's theme 5 Electric potential, relation to work and potential energy. 5

6 Building up to potential 6 We will try to understand electric potential energy and voltage by comparing it to gravity. A positive charge in a constant electric ield acts just like a mass in a gravitational ield. 6

7 Gravitational Potential Energy Total Energy=m g h 1 i 2 m v 2 =m g h 1 i 2 m v 2 7 I the box begins at rest, and the ground is rictionless, how high does it go? A? B? C? 7

8 Gravitational Potential Energy 8 Demo the loop the loop track. 8

9 Gravitational Potential Energy and Work. 9 Work in physics is deined as Force times change in position i the orce is constant. W=F x So or gravity, W = mgh assuming you were going straight up. Potential energy is always Work with a minus sign. So U = W 9 For constant gravity, U = mgh

10 How do you deine work i Force and change 10 in position aren't in same direction? Only the part o motion that is in the SAME DIRECTION AS THE FORCE counts or work. In vector terms, only the component along the orce matters. Mathwise that is Where the black dot is a special kind o multiplication called a dot product that means only the component along the orce matters 10 W= F r

11 The brick and the pea 11 The dierence between physics work and what most people think is work. 11

12 Calculus Bonus (Optional!) 12 Work in physics is deined as Force times change in position i the orce is constant. W=F x I the orce is not constant, then and For gravity, F(x)= constant = mg so work by gravity is W= F x dx x dx W= mg dx= mg x= mg h 12 For a spring, F= kx W= kx dx= 1 2 kx2

13 What i the orce isn't constant? 13 For certain orces (called conservative orces), Work is the same no matter what path you take. Gravity and electrical orce are both conservative. So i you CAN ind a path with constant gravity, you can be sure that the Work on other paths is the same. 13

14 Gravitational Potential Energy 14 Total Energy=U i K i =U K Total Energy=m g h 1 i 2 m v 2 =m g h 1 i 2 m v 2 A sledder is going 2 m/s at the top o a hill. What is her speed at the bottom? 14 Homework: Lecture 8, problem 1

15 Total Energy=U K =U K i i Total Energy=m g h 1 i 2 m v 2 =m g h 1 i 2 m v

16 Gravitational Potential Energy 16 Total Energy=U i K i =U K Total Energy=m g h 1 i 2 m v 2 =m g h 1 i 2 m v 2 A 1000 kg car approaches a hill at shown at 10 m/second. What is its speed at the bottom o the hill? What is its kinetic energy at bottom o hill? 16 HW: Lecture 8, Problem 2

17 Total Energy=U K =U K i i Total Energy=m g h 1 i 2 m v 2 =m g h 1 i 2 m v 2 v :? K :? 17 17

18 Gravitational Potential Energy 18 Total Energy=U i K i =U K Total Energy=m g h 1 i 2 m v 2 =m g h 1 i 2 m v 2 A skier is at rest at top o a ski run. At what speed does he launch o the end o the ramp? Homework, Lecture 8, Problem 3 18

19 Gravitational Potential Energy 19 Total Energy=U i K i =U K Total Energy=m g h 1 i 2 m v 2 =m g h 1 i 2 m v 2 A skier is at rest at top o a ski run. At what speed does he launch o the end o the ramp? This works even though skiers path is curved! 19

Fs (30.0 N)(50.0 m) The magnitude of the force that the shopper exerts is f 48.0 N cos 29.0 cos 29.0 b. The work done by the pushing force F is

Fs (30.0 N)(50.0 m) The magnitude of the force that the shopper exerts is f 48.0 N cos 29.0 cos 29.0 b. The work done by the pushing force F is Chapter 6: Problems 5, 6, 8, 38, 43, 49 & 53 5. ssm Suppose in Figure 6. that +1.1 1 3 J o work is done by the orce F (magnitude 3. N) in moving the suitcase a distance o 5. m. At what angle θ is the orce