Thursday, December 8, 2016

Activity 65: Introduction to Work




Work = Fd cos(theta)

F = Applied Force

d =  the displacement of the object

theta = the angle between the Applied Force the the Direction of movement


Work is measured in Joules.

1 Joule = 1 Newton meter = 1 kg m^2/s^s

1 J is small.  ~ lifting a 100 gram apple 1 meter.

Other energy measures:  1 calorie = energy need to heat 1 g of water by 1 degree Celsius
                                        1 calorie = 4.184 J
                                       
                                         1 food calorie (kcal)  = 4185 J



Work is the means of energy transfer.



Net Work is the sum of all work done on a system:   Wnet = Fnet d cos(theta)



Net Work is the area under a Net Force vs Displacement Graph.

If the Fnet is in the same direction as the motion then Wnet = Fnet d

Since Fnet = mad

Wnet = mad


v^2 = v0^2 + 2ad

a = (v^2 -  v0^2)/(2d)

Wnet = m( (v^2 -  v0^2)/(2d))  (d)
         

Wnet = 1/2 mv^2 - 1/2 mv0^2

1/2 mv^2 = KE


Wnet = change in KE of the system


If Wnet is zero then there is no change in kinetic energy.


2 kinds of Mechanical energy

   KE = 1/2mv^2

   PE = mgh

Conservative Forces: A force is conservative depends on the starting point and ending point of an object.

NonConservative Forces: A force whose work depends on the path taken.  Friction is a good example.

Conservation of of Mechanical Energy

When on conservative forces are involved:

Wnet = 1/2 mv^2 - 1/2mv0^2  = delta KE

Wnet = delta KE

-deltaPE = delta KE

or

PEi + KEi = PEf + KEf




Read section 7.1 and answer questions 1 -> 8 on page 260.   Review your answers here.

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