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**Unformatted text preview: **4/25/08 10:29 AM Ch5 HW3 S2008 Page 1 of 6 http:[email protected][email protected] Ch5 HW3 S2008 (Homework) KRISTIN SEILOFF PHYS 2211 M & N Spring 08, section N05, Spring 2008 Instructor: Jennifer Curtis Description The Energy Principle: Change of identity; Work done by non-constant forces Instructions Reading: Sections 5.5-5.7 Web Assign Current Score: 25 out of 35 Due: Monday, February 25, 200812:00 PM EST 1. 2/2 points | 2/4 submissions You push a box out of a carpeted room and along a hallway with a waxed linoleum floor. While pushing the crate 2 m out of the room you exert a force of 35 N; while pushing it 5 m along the hallway you exert a force of 13 N. To bring it to a stop you exert a force of 6 N through a distance of 0.9 m, opposite to the motion. How much work do you do in all? 129.6 130 J Solution or Explanation W = 1 · 1 + 2 · 2 + 3 · 3 Be careful about signs: 3 · 3 = F x 3 x 3 is negative , since the force has a negative x component. 2. 2/4 points | 4/4 submissions A crate with a mass of 120 kg glides through a space station with a speed of 3 m/s. An astronaut speeds it up by pushing on it from behind with a force of 180 N, continually pushing with this force through a distance of 5 m. The astronaut moves around to the front of the crate and slows the crate down by pushing backwards with a force of 150 N, backing up through a distance of 3 m. What is the total amount of work done by the astronaut in these two maneuvers? W = 450 450 J After these two maneuvers, what is the speed of the crate? v = 6.245 4.06 m/s Solution or Explanation K f- K i = work = F x 1 x 1 + F x 2 x 2 Pay attention to the sign of the x component of the force! 4/25/08 10:29 AM Ch5 HW3 S2008 Page 2 of 6 http:[email protected][email protected] 3. 2/4 points | 3/4 submissions A mass of 0.088 kg hangs from a vertical spring in the lab room. You pull down on the mass and throw it vertically downward. The speed of the mass just after leaving your hand is 4.00 m/s. (a) While the mass moves downward a distance of 0.09 m, how much work was done on the mass by the Earth? Include the appropriate sign. Work done by Earth = .0776 0.0776 J (b) At the instant in part (a) when the mass has moved downward a distance of 0.09 m, the speed of the mass has decreased to 1.60 m/s. How much work was done on the mass by the spring? Include the appropriate sign. Work done by spring = 3.907E-13-0.669 J Solution or Explanation The work done by the force exerted on the mass by the Earth is + mgh , where h is the distance the mass moves downward. The work is positive, tending to increase the kinetic energy of the mass. Note that the work is F y y , which is a positive number (the y component of the force is negative, and the y component of the displacement is also negative). The work done by the spring tends to decrease the kinetic energy....

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