Chapter 6 Work and Energy

6-1 Work done by a Constant Force Work can mean many things…our everyday definition of work does not always mean the physics definition of work. Work (physics def.) is the product of displacement times the component of force parallel to the displacement. W=Fd(cosf) …this works 99% of the time, but look at your diagram to asure cosine is correct to isolate the parallel force.

Lets ‘WORK’ some examples… When an object is moved the same direction as the entire force…the angle is 0 so cosine of the angle results in 1 and the formula becomes W=Fd. A cart is moved 50 m by a 30 N force in the direction of movement... 50.0m x 30.0N = 1.50x103 Nm The SI unit of work is the Nm…a special name is given to this unit…the Joule. In the cgs system…the unit of work is the erg …it is = to 1dyne x cm… in British units work is reported as foot x lbs. 1J= 107 ergs = .7376 ft lbs

Work Without Work??? You can feel like you are exerting work without really performing work….if you exert a force without moving an object there is NO WORK DONE on the object See Example 6-1 page 147

What makes up a Joule? A joule is equal to 1Nm from Fd… The unit of F is a Newton and you know that 1 N = kg x m / s2 so a Nm = = kg x m2/s2

6-2 Work done by a varying force… The work done by a varying force can be solved graphically or by calculus. Wor done by a variable force, in a moving object, between two points is equal to the area under the Fparallel vs d curve between those two points. See page 150 figure 6-6.

HW page 172-174 q 1-4 pr 1-7