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# Work energy power 2 reading assignment -revision 1

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### Transcript of "Work energy power 2 reading assignment -revision 1"

1. 1. In physics we say work is done when a force acts on something and makes it move. No motion, no work. Simple as that.
2. 2. The amount of work done by a force on an object depends on 1. How big that force is, and 2. How far the force moves the object. In physics, we combine two unlike quantities by making a product. The work done W by a force F when the distance moved in the direction of the force is s is defined as W = F . ∆x
3. 3. Determine the equation to find the work done on the following object : A) B) C) D) Question 1
4. 4. Basic Energy Model SystemEnvironment Energy Transfer by using force (pulling/expanding) Energy Transfer by using force (pushing/Compressing) work is done ON the system SystemEnvironment work is done BY the system
5. 5. We can calculate the amount of energy removed/added to the system by using: So… How to determine the amount of Energy removed/ Added to the system? W = F . ∆x Remember : The force should be in line with the displacement . So, we may need cosine or sine to find the component first, before we can calculate the amount of energy removed/added)
6. 6. Mechanical Energy • Mechanical Energy is energy that is possessed by an object due to its motion or its position. Types of Mechanical Energy: Kinetic Energy Potential Energy
7. 7. Kinetic Energy ‘Kinetic energy of an object is the energy it possesses by virtue of its motion.’ An object has kinetic energy because it has motion. If an object of mass m has a speed v then it must have been accelerated from rest to that speed by some agent, doing work on KE = (½ )mv2 The Kinetic Energy is defined as :
8. 8. Question 2
9. 9. Question 3 • A 5.00 g bullet moving at 600 m/s penetrates a tree trunk to a depth of4.00 cm. (a) Use the work-energy theorem, to determine the average frictional force that stops the bullet. (b) Assuming that the frictional force is constant, determine how much time elapses between the moment the bullet enters the tree and the moment it stops moving?
10. 10. GPE = 0 h m GPE = mgh Potential Energy what is Gravitational Potential Energy? We define GPE of an object in a the body-Earth as the energy it has by virtue of its position relative to the earth. The chart illustrate that the GPE of the object – earth system is defined as : G(PE) =mgh F mg
11. 11. Let’s Brainstorm…. • We Already know that : • Now imagine, Suppose you throw a ball upward : 1. What does work while it is flying through the air? GRAVITY 2. Is the CHANGE in kinetic energy POSITIVE or NEGATIVE? NEGATIVE 3. Is the CHANGE in potential energy POSITIVE or NEGATIVE? POSITIVE Thus, we can conclude that : ENERGY IS CONSERVED -∆KE = ∆PE - (KE-KEo) = PE- PEo - KE + KEo = PE- PEo PEo + KEo = PE +KE Energy Before = Energy After
12. 12. The law of conservation of mechanical energy states: Energy Cannot Be Created Or Destroyed, Only Transformed AND TRANSFERED! KEo KE PEo PE
13. 13. PE KE PE +KE
14. 14. Now, Let’s investigate how does the energy transform Challenge yourself : try to fill in the blank before you move to the other slide
15. 15. PE KE
16. 16. KE PE PE
17. 17. continue • The Energy Bar Chart below ilustrates how the transformation energy occur in the previous cases Position 1 Position 2 Position 3 KE KE KEPE PE PE
18. 18. Question 4 • Use the figure in slide 10 as your reference. if the mass of the boy is 56 kg and the initial velocity of the skateboard is 5 m/s. calculate the energy transformation (KE,PE,and TME) in each position. Ilustrate the transformation in Energy Bar chart
19. 19. Power Power is the amount of transfer/ transformation of energy in certain time taken to realise the transformation. Thus, the amount of Power can be defined as: P = ∆E System/ ∆ t. In the special case that there is no heat involved (i.e. no transfer of energy by using differences of temperature), this reduces to: P = W/ ∆ t
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