This preview shows pages 1–3. Sign up to view the full content.
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
Unformatted text preview: CHAPTER 7 Conservation of Energy 1* What are the advantages and disadvantages of using the conservation of mechanical energy rather than Newtons laws to solve problems? Generally simpler, involving only scalars; cannot obtain some details, e.g., trajectories. 2 Two objects of unequal mass are connected by a massless cord passing over a frictionless peg. After the objects are released from rest, which of the following statements are true? ( U = gravitational potential energy, K = kinetic energy of the system.) ( a ) U < 0 and K > 0 ( b ) U = 0 and K > 0 ( c ) U < 0 and K = 0 ( d ) U = 0 and K = 0 ( e ) U > 0 and K < 0 ( a ) 3 Two stones are thrown with the same initial speed at the same instant from the roof of a building. One stone is thrown at an angle of 30 o above the horizontal, the other is thrown horizontally. (Neglect air resistance.) Which statement is true? ( a ) The stones strike the ground at the same time and with equal speeds. ( b ) The stones strike the ground at the same time with different speeds. ( c )The stones strike the ground at different times with equal speeds. ( d ) The stones strike the ground at different times with different speeds. ( c ) Their kinetic energies are equal. 4 A block of mass m is pushed up against a spring, compressing it a distance x , and the block is then released. The spring projects the block along a frictionless horizontal surface, giving the block a speed v . The same spring projects a second block of mass 4 m , giving it a speed of 3 v . What distance was the spring compressed in the second case? K 1 = 1/2 mv 2 = 1/2 kx 2 ; mv 2 = kx 1 2 ; kx 2 2 = (4 m )(3 v ) 2 = 36 mv 2 = 36 kx 1 2 ; x 2 = 6 x 1 . 5* A woman on a bicycle traveling at 10 m/s on a horizontal road stops pedaling as she starts up a hill inclined at 3.0 o to the horizontal. Ignoring friction forces, how far up the hill will she travel before stopping? ( a ) 5.1 m ( b ) 30 m ( c ) 97 m ( d ) 10.2 m ( e ) The answer depends on the mass of the woman. ( c ) h = v 2 /2 g = 50/9.81 m = 5.1 m; d = (5.1/sin 3.0 o ) m = 97.4 m. 6 A pendulum of length L with a bob of mass m is pulled aside until the bob is a distance L /4 above its equilibrium position. The bob is then released. Find the speed of the bob as it passes the equilibrium position. 1/2 mv 2 = mg h ; h = L /4; v = ( gL /2) 1/2 . 7 When she hosts a garden party, Julie likes to launch bagels to her guests with a spring device that she has devised. She places one of her 200g bagels against a horizontal spring mounted on her gazebo. The force constant of the Chapter 7 Conservation of Energy spring is 300 N/m, and she compresses it 9 cm. ( a ) Find the work done by Julie and the spring when Julie launches a bagel. ( b ) If the released bagel leaves the spring at the springs equilibrium position, find the speed of the bagel at that point. ( c ) If the bagel launcher is 2.2 m above the grass, what is Julies horizontal range firing 200g bagels?) If the bagel launcher is 2....
View
Full
Document
 Spring '09
 Eduardo
 Physics, Conservation Of Energy, Energy

Click to edit the document details