PHY 121 Ch 6 Lecture

PHY 121 Ch 6 Lecture - Lecture 10 Tuesday February 19 1...

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1 Lecture 10 Tuesday, February 19
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2 Kinetic energy A. can be positive or negative. B. is proportional to the object’s speed. C. is a vector quantity. D. None of the above.
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3 An object has a mass of 0.5 kg and a kinetic energy of 1 J. Its speed is A. 0.02 m/s B. 2 m/s C. 200 m/s D. 20,000 m/s
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4 If a force is perpendicular to the direction of motion, the work it does is A. greater than zero. B. zero. C. less than zero. D. not enough information to tell the sign of the work.
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5 A boy stands on frictionless roller skates on a level surface, facing a rigid wall. He pushes against the wall, which makes him move. A. His kinetic energy increases because the wall normal force does work on him. B. His kinetic energy increases because the floor normal force does work on him. C. His kinetic energy increases because his weight does work on him. D. His kinetic energy increases but none of the above forces does work on him.
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6 How much force do I need to stop a moving object? ! v ! v = 0 ! F ! a x 0 d v 0 v 1 =0 0 = v 1 2 = v 0 2 + 2 a x d 0 = v 0 2 ! 2 F m " # $ % d ( Fd = 1 2 mv 0 2
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7 How much force do I need to lift an object? d 0 ! F 1 ! w ! F 2 ! w ! n ! ! F 1 = mg F 1 d = mgd F 2 = mg sin F 2 ! = mg ! sin = mgd F 1 d = F 2 !
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8 Work Work W = Fs W [ ] = N ! m = kg ! m s 2 " # $ % m = kg ! m 2 s 2 = J Units of work (Tentative definition) Displacement Force
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Improving the definition of work • For each force F acting on an object one can define the work done by this force. • If there are more than one force acting on an object, the displacement s may not be in the same direction as one of the individual forces F . • We need to extend our definition of work for
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PHY 121 Ch 6 Lecture - Lecture 10 Tuesday February 19 1...

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