Work and Kinetic Energy PHYS116 A02 92211 Lecture

Work and Kinetic Energy PHYS-116 A-02, 9/22/11, Lecture 8 Momchil Velkovsky

Consider the vectors shown. What is the dot product C. D ? ¡ ¡ ¡ A. (120 m 2 ) cos 78. 0° B. (120 m 2 ) sin 78. 0° C. (120 m 2 ) cos 62. 0° D. (120 m 2 ) sin 62. 0° E. none of these

Consider the two vectors What is the dot product ¡ ¡ ¡ A. zero B. 14 C. 48 D. 50 E. none of these

Work, a force through a distance l As in the illustration, pushing in the same direction that the object moves

Use the parallel component if the force acts at an angle

How can it be such a great “workout” with no work? • When positive and negative work cancel, the net work is zero even though muscles are exercising.

Stepwise solution of work done by several forces Fg

A 6. 00 -kg block and an 8. 00 -kg block are connected as shown. When released, the 6. 00 -kg block accelerates downward and the 8. 00 -kg block accelerates to the right. After each block has moved 2. 00 cm, gravity has done ¡ ¡ A. more work on the 8. 00 -kg block than on the 6. 00 -kg block. B. the same amount of work on both blocks. C. less work on the 8. 00 -kg block than on the 6. 00 -kg block. D. not enough information given to decide

A 6. 00 -kg block and an 8. 00 -kg block are connected as shown. When released, the 6. 00 -kg block accelerates downward and the 8. 00 -kg block accelerates to the right. After each block has moved 2. 00 cm, the total work done on the 8. 00 -kg block ¡ ¡ A. is greater than the total work done on the 6. 00 -kg block. B. is the same as the total work done on the 6. 00 -kg block. C. is less than the total work done on the 6. 00 -kg block. D. not enough information given to decide

The work-energy theorem • Work done on an object will change its velocity.

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

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

Daily physics joke One of the pioneers of Quantum Mechanics – Paul Dirack married the sister of another quantum physicist –Eugene Wigner. Soon after that, a friend of Dirack, who knew nothing about his wedding, came to visit him at his home. During the visit he noticed a young woman, who called Dirack by his first name, poured tea and behaved as the hostess of the house. After some time, Dirack noticed his guests surprise and said: “Oh, I am sorry, I forgot to introduce you to … Wigner’s sister. ”

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

Next Time Read Ch. 8 ¡ I’ll post the new homework and the Reading question tonight ¡