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Work and energy

Work-Energy Theorem: W = ?KPre-Lab1. Suppose a hockey puck of mass m is at rest on the ice. A surly Canadian hits the puck and sends it sailingacross the ice at velocity v. According to the Work-Energy theorem, how much work did the player's stickdo on the puck?2. Suppose the referee lifts the

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Work and energy

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    1. Work and energy

    3. Half Atwood machine A cart of mass M is initially at rest on a frictionless track. A mass m is attached to it with a massless string hung over a massless pulley as shown. After released it falls a distance d.

    4. Q: How can you eliminate friction in the experiment?

    5. Ponderable: Work Draw a FBD for the cart, and write an expression for the work energy theorem for the cart. Draw a FBD for the hanging mass and apply the work energy theorem for the mass. Combine the two energy equations in order to find the total work done on the system (cart and mass) and the change of kinetic energy of the total system.

    6. Discussion: Defining the system as cart and hanging mass ?Ksystem = Wdone on system

    7. Ponderable: Graphs Sketch a force versus position (distance) graph for the system. What is the area underneath the force-position graph? How do you obtain the change of kinetic energy from this graph?

    8. Tangible: Determine the speed experimentally and theoretically (use table next slide). Measure the time and the distance the cart travelled and calculate the experimental speed of the system Apply the work-energy theorem to find the kinetic energy and from this the theoretical speed and compare with the experimental speed.

    9. Table1:

    11. Table 2:

    13. Table 3:

    14. Non-conservative Forces: If the work done does not depend on the path taken, the force is said to be conservative. If the work done does depend on the path taken, the force is said to be non-conservative. An example of a non-conservative force is friction. When pushing a box across the floor, the amount of work that is done by friction depends on the path taken. Work done is proportional to the length of the path!

    15. Energy dissipation: e.g. sliding friction

    16. Non-conservative Forces: Friction Suppose you are pushing a box across a flat floor. The mass of the box is m and the coefficient of kinetic friction is ?k. The work done in pushing it a distance D is given by: Wf = Ff • D = -?kmgD.

    17. Non-conservative Forces: Friction Since the force is constant in magnitude and opposite in direction to the displacement, the work done in pushing the box through an arbitrary path of length L is just Wf = -?mgL. Clearly, the work done depends on the path taken. Wpath 2 > Wpath 1

    18. Problem: Block Sliding with Friction A block slides down a frictionless ramp. Suppose the horizontal (bottom) portion of the track is rough, such that the coefficient of kinetic friction between the block and the track is ?k. How far, x, does the block go along the bottom portion of the track before stopping?

    19. Problem: Block Sliding with Friction... Using WC+WNC = ?K Wc = mgd WNC = work done by friction = -?kmgx. ?K = 0 since the block starts out and ends up at rest. WC+WNC =0 -?kmgx +mgd=0 x = d / ?k

    20. Tangible: Half Atwood with friction Level the track such that it is perfectly horizontal and use a wood block rather than the cart. State the generalized work-energy theorem (Yes right, you need to look at the theory section)

    21. Table4:

    22. Lab report Names and roles (1point) 2211L-Course number Title (max 2 points) Abstract (max 15 points) Introduction (max 5 points) Materials and methods (max 10 points) Reporting the results (max 10 points) Tables 1-4 and all the calculations below these tables.

    23. Lab report Discussion (max 10 points) Conclusion (max 10 points) Data (max 10 points) Presentation (max 10 points) Overall impact (max 10 points)

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