Download - 1 work
![Page 1: 1 work](https://reader034.vdocuments.net/reader034/viewer/2022051413/554f630db4c905bb178b48d5/html5/thumbnails/1.jpg)
WORK AND WORK AND CONSERVATION OF CONSERVATION OF
ENERGYENERGYSTYMVERLY GAWAT
JENERUS JUAN
AND ALWEN AGYAM
![Page 2: 1 work](https://reader034.vdocuments.net/reader034/viewer/2022051413/554f630db4c905bb178b48d5/html5/thumbnails/2.jpg)
Work is the transfer of energy through motion. In order for work to take place, a force must be exerted through a distance. The amount of work done depends on two things: the amount of force exerted and the distance over which the force is applied. There are two factors to keep in mind when deciding when work is being done: something has to move and the motion must be in the direction of the applied force. Work can be calculated by using the following formula: Work=force x distance
WorkWork
![Page 3: 1 work](https://reader034.vdocuments.net/reader034/viewer/2022051413/554f630db4c905bb178b48d5/html5/thumbnails/3.jpg)
Work is done on the books when they are being lifted, but no work is done on them when they are being held or carried horizontally.
WorkWork
![Page 4: 1 work](https://reader034.vdocuments.net/reader034/viewer/2022051413/554f630db4c905bb178b48d5/html5/thumbnails/4.jpg)
Work can be positive Work can be positive or negativeor negative
• Man does positive work lifting box
Man does negative work
lowering box
Gravity does positive work when box lowers
Gravity does negative work when box is raised
![Page 5: 1 work](https://reader034.vdocuments.net/reader034/viewer/2022051413/554f630db4c905bb178b48d5/html5/thumbnails/5.jpg)
Work done by a constant ForceWork done by a constant Force
Ekin = Wnet
• W = F s = |F| |s| cos = Fs s
|F| : magnitude of force |s| = s : magnitude of displacement Fs = magnitude of force in direction of displacement :
Fs = |F| cos
: angle between displacement and force vectors• Kinetic energy : Ekin= 1/2 m v2
• Work-Kinetic Energy Theorem:
F
s
![Page 6: 1 work](https://reader034.vdocuments.net/reader034/viewer/2022051413/554f630db4c905bb178b48d5/html5/thumbnails/6.jpg)
Conservation of Mechanical EnergyConservation of Mechanical Energy
Total mechanical energy of an object remains constant
provided the net work done by non-conservative forces
is zero: Etot = Ekin + Epot = constantor
Ekin,f+Epot,f = Ekin,0+Epot,0
Otherwise, in the presence of net work done bynon-conservative forces (e.g. friction):
Wnc = Ekin,f – Ekin,0 + Epot,f-Epot,i
![Page 7: 1 work](https://reader034.vdocuments.net/reader034/viewer/2022051413/554f630db4c905bb178b48d5/html5/thumbnails/7.jpg)
Example ProblemExample Problem
Suppose the initial kinetic and potential energies of a system are 75J and 250J respectively, and that the final kinetic and potential energies of the same system are 300J and -25J respectively. How much work was done on the system by non-conservative forces? 1. 0J 2. 50J 3. -50J 4. 225J 5. -225J
correct
Work done by non-conservative forces equals the difference between final and initial kinetic energies plus the difference between the final and initial gravitational potential energies.
W = (300-75) + ((-25) - 250) = 225 - 275 = -50J.
Samar HathoutSamar Hathout
![Page 8: 1 work](https://reader034.vdocuments.net/reader034/viewer/2022051413/554f630db4c905bb178b48d5/html5/thumbnails/8.jpg)
ExampleExample
Samar Hathout
![Page 9: 1 work](https://reader034.vdocuments.net/reader034/viewer/2022051413/554f630db4c905bb178b48d5/html5/thumbnails/9.jpg)
Conservation of EnergyConservation of Energy
Conservative forces:• Gravity, electrical, QCD…Non-conservative forces:• Friction, air resistance…Non-conservative forces still conserve energy!Energy just transfers to thermal energy
PE f KE f PEi KEiKE PE
Samar Hathout
![Page 10: 1 work](https://reader034.vdocuments.net/reader034/viewer/2022051413/554f630db4c905bb178b48d5/html5/thumbnails/10.jpg)
ExampleExample
A diver of mass m drops from a board 10.0 m above the water surface, as in the Figure. Find his speed 5.00 m above the water surface. Neglect air resistance.
9.9 m/s
![Page 11: 1 work](https://reader034.vdocuments.net/reader034/viewer/2022051413/554f630db4c905bb178b48d5/html5/thumbnails/11.jpg)
ExampleExample
A skier slides down the frictionless slope as shown. What is the skier’s speed at the bottom?
H=40 m
L=250 m
start
finish
28.0 m/s
![Page 12: 1 work](https://reader034.vdocuments.net/reader034/viewer/2022051413/554f630db4c905bb178b48d5/html5/thumbnails/12.jpg)
Example Example
Three identical balls are thrown from the top of a building with the same initial speed. Initially, Ball 1 moves horizontally. Ball 2 moves upward. Ball 3 moves downward.
Neglecting air resistance, which ball has the fastest speed when it hits the ground?A) Ball 1
B) Ball 2C) Ball 3D) All have the same speed.
![Page 13: 1 work](https://reader034.vdocuments.net/reader034/viewer/2022051413/554f630db4c905bb178b48d5/html5/thumbnails/13.jpg)
Springs (Hooke’s Law)Springs (Hooke’s Law)
Proportional to displacement from equilibrium
F kx
![Page 14: 1 work](https://reader034.vdocuments.net/reader034/viewer/2022051413/554f630db4c905bb178b48d5/html5/thumbnails/14.jpg)
Potential Energy of Potential Energy of SpringSpring
PE=-Fx
x
F
PE 1
2(kx)x
PE 1
2kx2
![Page 15: 1 work](https://reader034.vdocuments.net/reader034/viewer/2022051413/554f630db4c905bb178b48d5/html5/thumbnails/15.jpg)
x
Example Example
b) To what height h does the block rise when moving up the incline?
A 0.50-kg block rests on a horizontal, frictionless surface as in the figure; it is pressed against a light spring having a spring constant of k = 800 N/m, with an initial compression of 2.0 cm.
3.2 cm
![Page 16: 1 work](https://reader034.vdocuments.net/reader034/viewer/2022051413/554f630db4c905bb178b48d5/html5/thumbnails/16.jpg)
Power Power
Average power is the average rate at which a net force
does work:
Pav = Wnet / tSI unit: [P] = J/s = watt (W)
Or Pav = Fnet s /t = Fnet vav
![Page 17: 1 work](https://reader034.vdocuments.net/reader034/viewer/2022051413/554f630db4c905bb178b48d5/html5/thumbnails/17.jpg)
Example Example
A 1967 Corvette has a weight of 3020 lbs. The 427 cu-in engine was rated at 435 hp at 5400 rpm.a) If the engine used all 435 hp at 100% efficiency during acceleration, what speed would the car attain after 6 seconds?b) What is the average acceleration? (in “g”s)
a) 120 mph b) 0.91g
![Page 18: 1 work](https://reader034.vdocuments.net/reader034/viewer/2022051413/554f630db4c905bb178b48d5/html5/thumbnails/18.jpg)
Example Example
Consider the Corvette (w=3020 lbs) having constantacceleration of a=0.91ga) What is the power when v=10 mph?b) What is the power output when v=100 mph?
a) 73.1 hp b) 732 hp (in real world a is larger at low v)