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Homework Help: 4 homework problems (static coefficient, acceleration/direction, power, ect)

  1. Oct 23, 2005 #1
    #1) The coefficient of static friction bwtween the block and the board is 0.6. If the blocks weight is 75 N, what minumum weight attached to the pulley will cause the block to move?

    #2)Find the direction and acceleration of a single pulley with 2 masses (7 kg and 10 kg)

    #3)A machine requires 30J of energy every minute what is the power consumption?

    #4)A car moving north at 60 km/h collides and lockes bumpers with a large trunk going 50 km/h West. If the mass of the car is only 1/3 that of the truck mass, what is the magnitude and direction of velocity of the combined masses?

    1~ I don't even know where to start on #1...

    2~ I think you start with the force equation F= ma

    3~ I know the power equation is P= W/t, but how do you get work when all we are given is 30 J

    4~ I know the direction is northwest, but i'm not sure how to find the magnitude of the combined masses??
  2. jcsd
  3. Oct 23, 2005 #2
    1.) My guess is; this is the situation: one block is resting on the board some horizontal distance from the pulley which has the other block hanging vertically from it.

    F = F_f : impending motion
    Belt Friction equation for a pulley system is: F_tight = F_loose * e ^ (u * theta)
    F_tight is the force on the taut rope hanging vertically, or intuitively the side with the least slack.
    F_loose is the force on the rope that has slack, that is, the side where the rope has the least amount of surface contact with the pulley.
    theta (in radians) is the wrap angle defined as the angle between the contact of the tight and loose sides of the rope on the pulley. So if one rope hangs vertically and the other is horizontal, theta is pi/2. If they are both vertical then theta is pi.

    2.) The Force on the pulley is equivalent to the sum of the tensions. T_pulley = T_1 + T_2. Setup equilibruim equations for the tensions.

    3.) W = delta(E) = E_2 - E_1 pick sample points and times plug into P = dW/dt

    4.) Draw out velocities as vectors, and use conservation of momentum to find the final velocity of the combined truck and car mass.

    - Arman Khos.
    Last edited: Oct 23, 2005