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Chapter 7 Conservation of Energy (cont’d) Mechanical energy conservation Examples Work by non-conservative forces March 3, 2011 4/5/2017 Phys 201, Spring 2011 1 Mechanical energy is of two forms: kinetic (by motion) potential (by relative positions) kinetic potential transformable. Both of the ability to do work (work-energy theorem). Energies in Nature come with several (known) forms: Mechanical; Electromagnetic & Chemical (heat); Atomic or nuclear … 4/5/2017 Phys. 201, Spring 2011 2 Recollect: Conservation of Energy • If only conservative forces are present, the total kinetic plus potential energy of a system is conserved. Mechanical Energy = Potential Energy (U) + Kinetic Energy (K) Conservative forces interchange U K (work done), but E = K + U is a constant. ΔE = ΔK + ΔU = 0 Work-kinetic energy theorem: ΔK = W thus ΔU = -W, for conservative forces only. • But, if non-conservative forces act, then energy can be dissipated in other forms (heat, for example) 4/5/2017 Phys 201, Spring 2011 3 Example: The simple pendulum • Suppose we release a mass m from rest a height h1 above its lowest possible elevation. Assuming no friction (air drag): – What is the maximum speed of the mass and where does this happen? – To what elevation h2 does it rise on the other side? m h1 4/5/2017 h2 v Phys 201, Spring 2011 4 Example: The simple pendulum • Total mechanical energy: E = ½mv2 + mgy – Initially, y = h1 and v = 0, so E = mgh1. – Since E = mgh1 initially, and energy is conserved, E = mgh1 at all times. y m h1 y=0 4/5/2017 Phys 201, Spring 2011 5 Example: The simple pendulum E = ½mv2 + mgy Velocity is maximum where potential energy is lowest, at bottom of the swing. So, at y = 0, ½mv2 = mgh1 v2 = 2gh1 v = (2gh1)1/2 y m h1 y=0 4/5/2017 v Phys 201, Spring 2011 6 Example: The simple pendulum • To find maximum elevation on other side, note that maximum is reached when v=0. Since E = mgh1, and maximum potential energy on right is mgh2, h2=h1. The ball returns to its original height. y m h1 h2 y=0 4/5/2017 Phys 201, Spring 2011 7 Example: Airtrack and Glider • A glider of mass M is initially at rest on a horizontal frictionless track. A mass m is attached to it with a massless string hung over a massless pulley as shown. What is the speed v of M after m has fallen a distance d? v M m d 4/5/2017 Phys 201, Spring 2011 v 8 Example: Airtrack and Glider • Kinetic + potential energy is conserved since all forces are conservative. • Choose initial configuration to have U=0. ΔK = -ΔU 2mgd 1 2 m M v mgd v 2 mM v M m d 4/5/2017 Phys 201, Spring 2011 v 9 Problem: Toy car • A toy car slides on a frictionless track shown below. It starts at rest, drops a height d, moves horizontally at speed v1, rises a height h, and ends up moving horizontally with speed v2. – Find v1 and v2. v2 d h v1 4/5/2017 Phys 201, Spring 2011 10 Problem: Toy car • K+U is conserved, so ΔK = -ΔU • When the elevation decreases a distance D, ΔU = -mgd, ΔK = ½mv12. • Solving for the speed: v1 2gd d v2 2g(d h) v2 h v1 4/5/2017 Phys 201, Spring 2011 11 A projectile of mass m is propelled from ground level with an initial kinetic energy of 450 J. At the exact top of its trajectory, its kinetic energy is 250 J. To what height, in meters, above the starting point does the projectile rise? Assume air resistance is negligible. A) 50/(mg) B) 250/(mg) C) 700/(mg) K1 U1 K 2 U 2 , with U i =mgh i So mg(h 2 -h1 )=K1 -K 2 h 2 -h1 =(K1 -K 2 )/(mg) E) 350/(mg) 4/5/2017 Phys 201, Spring 2011 12 Question • A box sliding on a horizontal frictionless surface runs into a fixed spring, compressing it to a distance x1 from its relaxed position while momentarily coming to rest. – If the initial speed of the box were doubled and its mass were halved, what would be the distance x2 that the spring would compress? A) x2=x1 B) x2=x1√2 C) x2=2x1 ½ kx2 = ½ mv2, Thus v = x(k/m)1/2 x = v(m/k)1/2 So, 2v and m/2 increases x by √2. x 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. Work done is proportional to the length of the path! – The mechanical energy is converted to heat. 4/5/2017 Phys 201, Spring 2011 14 Spring pulls on mass: with friction • Suppose spring pulls on block, but now there is a nonzero coefficient of friction μ between the block and the floor. • The total work done on the block is now the sum of the work done by the spring, Ws (same as before), and the work done by the friction Wf, Wf f r mgd (not related to either kinetic energy or potential energy) Work-energy theorem now reads: Wnet = Wcons + Wf = ΔK r m stretched position (at rest) d m 4/5/2017 f = mg relaxed position vr 201, Spring 2011 Phys i 15 Spring pulls on mass: with friction • Knowing Wnet = Ws + Wf = ΔK 1 2 1 2 Wf mgd Ws kd K mv r 2 2 1 2 1 2 kd mgd mv r 2 2 k 2 vr d 2gd m r m stretched position (at rest) d m 4/5/2017 f = mg relaxed position vr 201, Spring 2011 Phys i 16 Many forms of energy: 4/5/2017 Phys 201, Spring 2011 17 Question Which statement is true? A. Mechanical energy (U+K) is always conserved B. Total energy is always conserved C. Potential energy is always conserved Energy of one form can be converted to another form, but the total energy remains the same. 4/5/2017 Phys 201, Spring 2011 18 Newton’s laws Conservation of energy F = m dv/dt = m dv/dx v F dx = m v dv When Fnc=0, then –ΔU = ΔEk But Conservation of energy is more broadly applicable than Newton’s laws • Newton’s laws do not apply to systems that are fast-moving close to the speed of light (where Einstein’s theory of relativity applies) and to very small systems (where quantum mechanics applies), but conservation of energy is always valid. Conservation laws are the consequence of symmetries: Energy conservation Time translation invariance. 4/5/2017 Phys 201, Spring 2011 19