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
PEi + KEi = PEf + KEf
Read section 7.1 and answer questions 1 -> 8 on page 260. Review your answers here.