1. Work F d 

2. Work is a Bridge to Energy Land of Forces & Motion Land of Energy WORK Conceptual Bridge PE = mgh KE = ½mv 2 F resistance F forward F ground-on-car F gravity

3. Conceptual Definition of Work Work is a transfer of energy from one object to another object by a force –In other words, one object loses energy and another gains it –The interaction between the objects is a force –Example: A bat hitting a baseball –The bat loses energy by transferring it to the ball

4. Work Work is done on an object by a force –The force comes from some other object: so we say that one object does work on another W = F  x W = Fd (book’s notation) –W is work (measured in joules) –F is magnitude of the force (newtons) –d =  x is the displacement (meters) xx F

5. Work (cont.) Work is done only by the component of force parallel to the displacement W = (Fcos  )·d = Fd·cos  F  d

6. Work (cont.) If there are multiple forces acting on an object, we can find the net work W net = F net dcos  net work = net force  displacement  cosine of the angle between them

7. Work (cont.) Work can be positive or negative –If force is in opposite direction from displacement, then work is negative –Friction does negative work on sliding things: W = F f d·cos(180  ) =  F f d FfFf d

8. Work (cont.) We can calculate the work done by each and every force acting on an object –Each one of these forces may be doing: Positive work Negative work Zero work –Positive, negative, or zero, depends on direction of the force relative to the object’s motion FAFA FgFg FNFN FfFf

9. Work (cont.) The SI unit for work is the joule (J): 1 J = 1 N  m The joule is also the unit for energy –We will see that all forms of energy are equivalent in some sense and can use the same unit of measure: the joule –Since work is a transfer of energy, it uses the same unit as energy itself

10. Example Work Problem: A 7.0-kg box of books is pulled 5.0 m along the floor by a rope at a 25  angle from the horizontal. Assume  = 0.20. –If the tension in the rope is 20 N, what is the work done by the rope on the box? F  m  = 0.20 d

11. Work-Energy Theorem Assume a net force, F net, acts on an object over a parallel displacement, d W net = F net d·cos  = F net  x·cos(0  ) = F net  x = ma  x From Chapter 2 we have v f 2 = v i 2 + 2a  x  a  x = ½(v f 2  v i 2 ) F net d

12. Work-Energy Thm (cont.) So we have: W net = ma  x = m·½(v f 2  v i 2 ) = ½ mv f 2  ½ mv i 2 Define kinetic energy: KE = ½ mv 2 Finally this gives us W net = KE f  KE i =  KE net work = change in kinetic energy

13. Work-Energy Thm Example A baseball player throws a ball by exerting a 113 N force over a distance of 1.03 m. What is the most likely field position of this player? (Hint: Find the speed of the ball as it leaves his hand.) The mass of a baseball is 144 g.