Physics - drwls can you help me with this?

A 2.00kg block hangs from a rubber cord, being supported so that the cord is not stretched.
The unstretched length of the cord is 0.500m and it's mass is 5.00g.The "spring constant" for the cord is 100N/m. Block is released and stops at lowest point.

a) determine the tension in cord when block is at lowest point.
I'm not sure but I do know that
Sum F= T-mg= 0 at lowest point.
but is the T= mg and that's it?

b) what is length of cord in stretched position?
I think but I'm not sure that I can find it by using

v= rad(T/mu)= rad(mgL/mblock)

However I don't think I can use this since I don't have v or L.

c) find speed of transversal wave in cord if block is at lowest position.

what I thought transverse speed was was from the equation:
vy= -omega*A cos(kx-omega*t) but I don't think I can use k that was given since It's not the same and I don't think I have omega either so how would I find transversal wave speed?

Thanks alot

  1. 👍
  2. 👎
  3. 👁
  1. a) The block stops and turns around when it has traveled twice the equilibrium deflection.
    When the block has reached its lowest elevation, lost gravitational potential energy of the block and cord is converted to potential energy of the stretched cord. Let X be the deflection from the starting position at that time.
    (1/2) k X^2 = M g X + m g X/2
    The second term is the loss in PE due to the lowering of the center of gravity of the cord. m is the cord's mass and M is the block's mass.
    X = 2(M + m/2)g/k = 0.393 m
    The cord tension is the spring constant times that deflection, or kX = 39.3 N
    You cannot apply your T = Mg equation because the Mass is accelerating at the lowest position.
    (b) Use the value of X I derived in (a).
    (c) The speed of a transverse wave in a stretched cord is
    V = sqrt [T/(m/L)], where
    m/L is the cord mass per unit length. At lowest position, L = 0.500 + 0.393 = 0 893 m
    m = 0.005 kg and T = 39.3 N
    Solve for V

    1. 👍
    2. 👎
  2. Oh..I think one of the biggest problems with this question was my ability to actually visualize what was happening with the block. I thought they meant that it was at rest at it's lowest postion - thus it stopped moving and therefore SumF= 0.

    Thanks very much for your help drwls =D

    1. 👍
    2. 👎
  3. The block IS temporarily at rest at its lowest position, but it is accelerating up again at that time. It is not in force equilibrium there.

    1. 👍
    2. 👎
  4. So technically it would be this:

    SumF= T-mg= ma right?

    1. 👍
    2. 👎
  5. Yes. But you don't need to use that equation to get Tension or position vs. time. I used an energy method

    1. 👍
    2. 👎
  6. Yes, I saw that. But how do know to use energy equation and when do I use that one?

    1. 👍
    2. 👎
  7. Using the energy conservation is always a good shortcut to get conditions at turnaround points, or maximum velocities, when there is oscillatory motion. You would used Newton's law of acceleration if you wanted to solve for the equation of motion vs. time. The quickest way to get the tension T in your problem is just to use T = kX and solve for the maximum X.

    1. 👍
    2. 👎
  8. Oh okay.

    Thanks very much drwls =)

    1. 👍
    2. 👎

Respond to this Question

First Name

Your Response

Similar Questions

  1. physics

    A block of mass m1=3.70kg on a frictionless plane inclined at angle 30 deg is connected by a cord over a massless, frictionless pulley to a second block mass m2= 2.30kg. What is... a) The magnitude of the acceleration of each

  2. physics

    A pulley is suspended by a Cord (C). on one end of the pulley there is a 1.2 kg block and on the other is a 3.2 kg block.Determine the tension in this cord after the masses are released and before one hits the ground. Ignore the

  3. Physics

    A solid, uniform, frictionless cylindrical reel of mass M = 3.00 kg and radius R = 0.400 m is used to draw water from a well.? A bucket of mass m = 2.00kg is attached to a cord that is wrapped around the cylinder. (a) Find the

  4. physics

    Figure shows a block (mass ) on a smooth horizontal surface, connected by a thin cord that passes over a pulley to a second block which hangs vertically. (a) Draw a free-body diagram for each block, showing the force of gravity on

  1. Science

    An existing cord that you use for a lamp is much thinner than an extension cord recommended for use with large appliances. if u plugged in a refrigerator using a lamp extension cord, the plastic coating of the cord could melt and

  2. physics help plz

    One 3.5 kg paint bucket is hanging by a massless cord from another 3.5kg paint bucket, also hanging by a massless cord, as shown in the figure . Part a) If the buckets are at rest, what is the tension in lower and higher cord ?

  3. 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

  4. 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

  1. Physics

    One 2.5 kg paint bucket is hanging by a massless cord from another 2.5 kg paint bucket, also hanging by a massless cord, as shown in Fig. 4-44. (a) If the buckets are at rest, what is the tension in each cord? (lower cord)________

  2. Physics

    A cord has two sections with linear densities of 0.10 kg/m and 0.20 kg/m respectively. An incident wave, y(x,t) = 0.050sin(6.0x - 12.0t), where x is in meters and t in seconds, travels from the lighter cord to the heavier one.

  3. 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

  4. physics

    An elastic cord is 65 cm long when a weight of 75 N hangs from it but is 85 cm long when a weight of 189 N hangs from it. What is the “spring” constant k of this cord

You can view more similar questions or ask a new question.