Results 1 to 4 of 4

Math Help - Anyone good with Physics?....im sure!

  1. #1
    Member
    Joined
    Mar 2007
    Posts
    80

    Anyone good with Physics?....im sure!

    force acting on a mass of 50kg when it will will accel' it from 2m/s to 9m/s in 10s?.......I got 9-2 = 7, f=ma, (50x7) = 350n Is this right?

    the one im stuck on though, is a ball of mass 5kg launched upwards with 800n force gets to height 'H'. What is 'H' and what is the time from launch until ball returns to ground. (when g = 9.8m's^2)

    I know this doesn't quite belong in maths forum but you guys have ben a big help in the past, any input would be much appreciated. THANKS!
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Grand Panjandrum
    Joined
    Nov 2005
    From
    someplace
    Posts
    14,972
    Thanks
    4
    Quote Originally Posted by bobchiba View Post
    the one im stuck on though, is a ball of mass 5kg launched upwards with 800n force gets to height 'H'. What is 'H' and what is the time from launch until ball returns to ground. (when g = 9.8m's^2)
    It depends on how long the force acts for.

    800n will accelarate a 5kg mass upwards at a=800/5 -g m/s^2

    Now if we were talking about an impulse of 800 newton seconds we could do
    something.

    RonL
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Member
    Joined
    Mar 2007
    Posts
    80
    Whooops....sorry, your right, it acts for 20 seconds.
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Forum Admin topsquark's Avatar
    Joined
    Jan 2006
    From
    Wellsville, NY
    Posts
    9,853
    Thanks
    321
    Awards
    1
    Quote Originally Posted by bobchiba View Post
    the one im stuck on though, is a ball of mass 5kg launched upwards with 800n force gets to height 'H'. What is 'H' and what is the time from launch until ball returns to ground. (when g = 9.8m's^2)
    Quote Originally Posted by bobchiba View Post
    Whooops....sorry, your right, it acts for 20 seconds.
    First a note: The unit "Newton" is named after someone, so it should properly be abbreviated with "N," not "n."

    F = ma
    800 N = (5 kg)*a

    a = 800/5 m/s^2 = 160 m/s^2

    The net acceleration is then
    160 m/s^2 - 9.8 m/s^2 = 150.2 m/s^2 (upward)<-- Since the object's weight still acts on it.

    So, what is the equation of motion of an object under a constant acceleration?
    y = y0 + v0*t + (1/2)a*t^2
    I'll set the origin at the point where the ball lifted off and +y upward. So y0 = 0 m and v0 = 0 m/s (presumably it was launched from rest, the problem never actually stated this.) So at the end of 20 s the height H will be:

    H = (1/2)a*t^2
    H = (1/2)*150.2*(20)^2 m = 30040 m.

    To find out the time from launch to when it hits the ground we need two pieces of information: how long it takes for the ball to reach the ground from the height H, and what the velocity is at the end of the 800 N acceleration period. We'll do this part first.

    While the ball is accelerated upward we have:
    v = v0 + at <-- The ball starts from rest, so v0 = 0 m/s.
    v = 150.2*20 m/s = 3004 m/s
    (Note: since the sign on this velocity is + the velocity is upward.)

    Now I'm going to reset the problem such that the time starts at the end of the 800 N acceleration phase. The ball starts at a height of 30040 m with an (upward) initial velocity of 3004 m/s and an acceleration only due to gravity of -9.8 m/s^2 (negative because it's acting downward.) The equation of motion is:
    y = 30040 + 3004t - (1/2)*9.8*t^2

    We wish to know when y = 0 m again. (Then add 20 s to this time and we get our final answer.) So:
    -4.9t^2 + 3004t + 30040 = 0
    Using the quadratic equation I get:
    t = 622.903 s or t = -9.842 s
    The negative time is clearly a ridiculous answer, so it takes 622.903 s to get back to the ground from this height.

    Thus the total time of flight is
    622.903 s + 20 s = 642.903 s

    -Dan
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Got a physics question or interested in physics?
    Posted in the Math Topics Forum
    Replies: 16
    Last Post: February 3rd 2014, 01:56 AM
  2. good books forbiing really good at math and physics
    Posted in the Math Topics Forum
    Replies: 1
    Last Post: August 22nd 2009, 03:26 PM
  3. how do solve quantum physics or physics equations
    Posted in the Advanced Applied Math Forum
    Replies: 2
    Last Post: March 26th 2008, 04:33 AM
  4. physics, acceleration, physics problem
    Posted in the Math Topics Forum
    Replies: 3
    Last Post: September 29th 2007, 03:50 AM

Search Tags


/mathhelpforum @mathhelpforum