WorkEnergy Theorem PHYS116 A01 92214 Lecture 13 Momchil

Work-Energy Theorem PHYS-116 A-01, 9/22/14, Lecture 13 Momchil Velkovsky

How fast will be our sled moving? l l Fx=800 N m=2000 kg 3. 0

A nonzero net force acts on an object. Which of the following quantities could be constant? ¡ ¡ A. the object’s kinetic energy B. the object’s velocity C. both of the above D. none of the above

Two iceboats (one of mass m, one of mass 2 m) hold a race on a frictionless, horizontal, frozen lake. Both iceboats start at rest, and the wind exerts the same constant force on both iceboats. Which iceboat crosses the finish line with more kinetic energy (KE)? 1. 2. 3. 4. 5. the iceboat of mass m: it has twice as much KE as the other the iceboat of mass m: it has 4 times as much KE as the other the iceboat of mass 2 m: it has twice as much KE as the other the iceboat of mass 2 m: it has 4 times as much KE as the other they both cross the finish line with the same kinetic energy

Work for composite systems x 1 F s/b x 0 Fw/s x 1 moves, x 0 does not! Fs/b does work, Fw/s does not, although they may have the same magnitude.

Work by forces acting on composite objects ¡ ¡ ¡ The static friction force F is the net force. It creates the acceleration. It is not doing work! Still the kinetic energy of the motorcycle changes! a F

Work and energy with varying force

The stretch of a spring and the force that caused it

Motion with a varying force ¡ ¡ ¡ k=2. 25 N/m m=1 kg At the point where the spring is un-stretched v 1 =2 m/s mk= 0. 408 What is the distance traveled to the point where v=0 ?

Motion on a curved path h

Watt about power? 1 hp=746 W P=d. W/dt=F. dr/dt P=F. v

Force, Power, Work ¡ ¡ Kilowatts, kilowatt hours and Joules Climbing up a mountain y a=0 Fg x L N F x H Fg Steeper slope –larger force, but the total work is the same: W=mg sin(q)L, L=H/sin(q), W=mg. H , Wg=-mg. H=-mg(y 2 -y 1)=-(U 2 -U 1) U(y)=mgy ¡ Potential Energy U: W=-DU, Recall Wtot=DK, Wrest -DU =DK, Wrest =DU+DK=DE, E=K+U

Athletes and the conservation of energy l Notice how potential and kinetic energy interchange as the athlete jumps.

What’s the speed in a vertical circle?

Next time ¡ The rest of Chapter 8