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Lecture17

# Lecture17 - PHYSICS 149 Lecture 17 Chapter 6 Conservation...

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PHYSICS 149: Lecture 17 • Chapter 6: Conservation of Energy 7 Elastic Potential Energy – 6.7 Elastic Potential Energy – 6.8 Power • Chapter 7: Linear Momentum – 7.1 A Vector Conservation Law – 7.2 Momentum Lecture 17 Purdue University, Physics 149 1

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ILQ 1 A force of 5 N is applied to the end of a spring, and stretches 10 cm ow much "farther" will it stretch it stretches 10 cm. How much farther will it stretch if an "additional" 2.5 N of force are applied? A) 2.5 cm ) 5 cm B) 5 cm C) 10 cm ) 15 cm D) 15 cm Lecture 17 2 Purdue University, Physics 149
ILQ 2 A mass is attached to the bottom of a vertical spring. This causes the spring to stretch and the pg mass to move downward. Does the potential energy of the spring increase or decrease? Does the ravitational potential energy of the mass increase gravitational potential energy of the mass increase of decrease? A) PE of spring decreases; PE of mass increases B) both decrease C) both increase D) PE of spring increases; PE of mass decreases Lecture 17 3 Purdue University, Physics 149

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ILQ 3 A simple catapult, consisting of a leather pouch attached to rubber bands tied to two forks of a wooden Y, has a spring constant k and is used to shoot a pebble horizontally. When the catapult is stretched by a distance d , it gives the pebble a speed v . What speed does it give the same pebble when it is tretched to a distance 4 stretched to a distance 4 d ? A) 16 v B) 4 v C) 4 sqrt(4) v ) 64 Lecture 17 Purdue University, Physics 149 4 D) 64 v E) sqrt(4) v
Energy and Work • Work: Transfer of Energy by Force θ W F = |F| |s| cos θ • Kinetic Energy (Energy of Motion) K = 1/2 mv 2 • Work-Energy Theorem: + Σ W NC = Δ K + Δ U Gravitational Potential Energy: Lecture 17 Purdue University, Physics 149 5 gy U grav = mgy = -W grav 12 mm UG r =−

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Hooke’s Law • The deformation–change in size or shape–of the object is proportional to the magnitude of the force that causes the deformation. – Magnitude: k|x | – Direction: Whether the spring is compressed or stretched, the Hooke’s force always points toward its relaxation position ( x =0). – Spring constant k • A characteristic of a spring • Unit: N/m x =0 |F x |= k|x | to left Lecture 17 6 Purdue University, Physics 149
Work by Variable Force •W = F x Δ x ork is area under F vs x plot Force Work is area under F vs x plot – Spring: F = k x Work • Area = ½ k x 2 =W spring Distance Work is the area under the F vs x plot Force p Work Lecture 17 Purdue University, Physics 149 Distance 7

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Work and Potential Energy of a Spring • The force of a spring F = -kx he work The work 2 2 1 1 x x 0 x x x f i = = 2 2 i f kx kx W + = 2 2 1 kx W = • The potential energy 2 2 2 1 2 1 i f spring kx kx W U = = Δ 2 1 0 kx U x x x f i = = = Purdue University, Physics 149 Lecture 15 page 8 2 Lecture 17
ILQ You compress a spring by a distance x and tore 10 J of energy How much energy is store 10 J of energy. How much energy is stored if you compress the spring a distance 2 x ?

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Lecture17 - PHYSICS 149 Lecture 17 Chapter 6 Conservation...

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