Physics

posted by .

A small block on a frictionless horizontal surface has a mass of 2.50×10−2 Kg. It is attached to a massless cord passing through a hole in the surface. The block is originally revolving at a distance of 0.300 m from the hole with an angular speed of 1.75 rad/s. The cord is then pulled from below, shortening the radius of the circle in which the block revolves to 0.150 m. You may treat the block as a particle.

What is the new angular speed?

Find the change in kinetic energy of the block.

How much work was done in pulling the cord?

(I'm having problems setting up/figuring out which equations to use)

  • Physics -

    Don't think about equations. Think about conservation laws.

    They expect you to treat this as a conservation of angular mometum problem. The block will speed up so that the product M R^2 w is conserved (constant). Calculate the new w (angular speed).

    Once you have the new w, calculate the new kinetic energy (1/2)M(rw)^2

    r decreases by a factor of 2
    w increases by a factor of 4
    KE increases by a factor of ?

    The change in KE equals the work done.

  • Physics -

    I'm on the change in KE part and this is what I get:

    KEf=(1/2)(2.5*10^-2)(.150*7)^2
    =.01378125

    KEi=(1/2)(2.5*10^-2)(.300*1.75)^2
    =.0034453125

    I'm not quite sure what to do with these two numbers...
    I subtracted KEi from KEf, but was told I had "rounding error" with my value of .01

  • Physics -

    Yes, the KE increases by a factor of 4. I have not checked your numbers, but the difference between the KE values will be the work done. You should only carry three significant figures.

    The work done should be three times the initial kinetic energy.

Respond to this Question

First Name
School Subject
Your Answer

Similar Questions

  1. Physics

    A small block on a frictionless horizontal surface has a mass of 2.50×10−2 Kg. It is attached to a massless cord passing through a hole in the surface. The block is originally revolving at a distance of 0.300 m from the hole …
  2. physics

    A small block on a frictionless horizontal surface has a mass of 2.50×10−2 . It is attached to a massless cord passing through a hole in the surface. (See the figure below .) The block is originally revolving at a distance of …
  3. physics

    A mass of 0.04 kg is attached to a cord passing through a small hole in a frictionless, horizontal surface. The mass is initially orbiting with speed 4.9 m/s in a circle of radius 0.4 m. The cord is then slowly pulled from below, decreasing …
  4. physics

    A block of mass 3.62 kg lies on a frictionless horizontal surface. The block is connected by a cord passing over a pulley to another block of mass 7.8 kg which hangs in the air, as shown. Assume the cord to be light (massless and weightless) …
  5. Physics

    A block of mass m1 is on top of a block of mass m2. Block 2 is connected by an ideal rope passing through a pulley to a block of unknown mass m3 as shown. The pulley is massless and frictionless. There is friction between block 1 and …
  6. physics

    A block of mass 4.44 kg lies on a frictionless horizontal surface. The block is connected by a cord passing over a pulley to another block of mass 6.55kg which hangs in the air , as shown above. Assume the cord to be light (massless …
  7. Physics

    A block of mass 5.53 kg lies on a frictionless horizontal surface. The block is connected by a cord passing a. over a pulley to another block of mass 2.25 kg which hangs in the air. Assume the cord to be light (massless and weightless) …
  8. Physics

    A small block on a frictionless surface has a mass of 70 g. It is attached to a massless string passing through a hole in a horizontal surface (see diagram). The block is originally rotating in a circle of radius 45 cm with angular …
  9. Physics

    A small block with a mass of m = 0.120 kg is attached to a cord passing through a hole in a frictionless, horizontal surface. The block is originally revolving at a distance of 0.40 m from the hole with a speed of 0.70 m/s. The cord …
  10. mechanics and fluids

    a block of mass 6 kg resting on a horizontal surface is connected by a cord passing over a light, frictionless pulley to a hanging block of mass 4 kg. the coefficient of kinetic friction between the block and the horizontal surface …

More Similar Questions