Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Force questions?

  1. Jun 22, 2004 #1
    I'm not sure what equation is used on this particular problem.

    How much time would be requird to stop a 2,000 kg car that is moving at 80.0 km/hr if the braking force is 8,000 N?

    I know.....
    Mass 2000 kg
    Acceleration is 80.0 km/hr
    Breaking force is 8,000 N

    What tension must a 50.0 cm length of string support in order to whirl an attached 1,000.0 gram stone in a circular path at 5.00 m/s?

    I used the formula F= mv^2/ r

    = (1,000.0 g) (5.00 m/s)/ 50.0 cm
    = 1,000.0 x 5.00 g*m/s

    I wanted to know if I was tackling this question correctly..if not what am I doing wrong????

  2. jcsd
  3. Jun 22, 2004 #2
    80 km/hr is the velocity, not the acceleration. Just use Newton's second law and some kinematics and you should solve this problem easily.
    You might want to check the units on that again. I suggest you stick to kg, m, and s.
  4. Jun 26, 2004 #3
    terpsgirl...for the first problem....use the equation that states that impulse = change in momentum.....the equation is:
    Force * time = change in velocity * mass.

    Think of what the original velocity is, and the final velocity...and u should be able to find the change in the velocity. Basically plug in your results...and solve for time.

    for the second problem.....i would check ur units....notice what unit ur expressing r in....and look at the units ur measuring speed in. Remeber that u should measure everything in kg's, meters and seconds.
    Last edited: Jun 26, 2004
  5. Jun 26, 2004 #4
    First convert to the correct units.
    Car = 2000kg.
    Speed = 22.22... m/s.
    Braking force = 8000N.

    Using F=MA. 8000=2000A, the 8000N force acting against the car will create a deceleration of magnitude 8000/2000, thus, a = -4m/s/s.

    U = 22.22....., V = 0, a = -4, t = ?
    V = u + at
    0 = (200/9) + (-4t)
    (200/9) = 4t
    t = 5, 5/9 seconds
    = 5.56 seconds (2dp)

    I haven't studied the concept used in question 2, but the equation looks like it would be effective. However, [always] convert to more useful units. You should be working in m/s, kgs, Ns and M.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook