Laboratory 6: Conservation Energy - General Physics | PHYS 111, Lab Reports of Physics

Download Laboratory 6: Conservation Energy - General Physics | PHYS 111 and more Lab Reports Physics in PDF only on Docsity! Phys 111L Fall 2008 Laboratory 6: Conservation of Energy The mechanical energy of a physical system obeys ∆E = Wnc (1) where ∆E = Ef − Ei is the change in mechanical energy from an initial instant to a final instant and Wnc is the work done by all the non-conservative forces between these instants. Whenever Wnc = 0 the total mechanical energy is conserved: ∆E = 0. (2) mcart msusp Figure 1: Experimental setup In this experiment, you will consider energy conservation in the context of a cart and suspended mass system as illustrated in Fig. 1. It can be shown that Wnc = 0 and thus mechanical energy is conserved. In this situation, there are two types of mechanical energy to consider: kinetic energy, KE, and gravitational potential energy, PEgrav. The mechanical energy of the system is: Etotal = KEtotal + PEgrav total (3) where KEtotal is the kinetic energy of all parts of the system and PEgrav total is the gravitational potential energy of all parts of the system at any single moment. In this series of experiments the cart and suspended masspiece will initially be held at rest and then released. The task will be to compare the total energy at the moment of release to the total energy just before the suspended masspiece hits the floor. Calling these the initial and final moments respectively, the conservation of total energy is equivalent to ∆KEtotal = −∆PEgrav total (4) where ∆KEtotal = KEtotal f − KEtotal i (5) ∆PEtotal = PEtotal f − PEtotal i. (6) A variety of straightforward measurements enable calculation of the various energies involved, and ultimately the conservation of mechanical energy can be checked. 1 Qualitative Considerations The cart and suspended masspieces will be released from rest. Consider the initial in- stant to be that at which the cart is released and the final instant to be that immediately before the suspended masspiece hits the floor. a) Is the change in gravitational potential energy for the cart positive, negative or zero? b) Is the change in gravitational potential energy for the masspiece positive, negative or zero? c) Is the change in kinetic energy for the cart positive, negative or zero? d) Is the change in kinetic energy for the masspiece positive, negative or zero? e) Is the change in total gravitational potential energy positive, negative or zero? f) Is the change in total kinetic energy positive, negative or zero? 2 Theory and Experimental Design a) Describe how to determine a numerical value for ∆PEtotal. Your description should include a list of the quantities that you need to measure and a procedure for cal- culating ∆PEtotal from these measured numbers. b) Describe how to determine a numerical value for ∆KEtotal. Your description should include a list of the quantities that you need to measure and a procedure for cal- culating ∆KEtotal from these measured numbers. c) You will need to measure the velocity of the cart at the instant before the suspended masspiece hits the floor. Explain, using physical reasons, why it is sufficient to measure the velocity of the cart after the masspieces have hit the floor. d) You will be supplied with a cart, suspended masspieces and an electronic timer which can measure the speed of the cart. Briefly describe how you could use these to build an experiment to verify that the mechanical energy is conserved in the process described above. 3 Experiment a) Level the track and check that the pulley is adjusted so that the string runs hori- zontally and free. 2