Results 1 to 8 of 8

Math Help - Physics Clarification

  1. #1
    Newbie
    Joined
    Sep 2009
    Posts
    21

    Physics Clarification

    A 100 N force applied by the brakes stops a car traveling at an initial velocity of 25 km/ hr. Solve for mass.
    _

    I'm getting a negative mass, so I must be doing something wrong.

    v = V_0 t + 1/2 a t^2
    <br />
0= (6.94 m/s * 15 seconds) + (.5)(a)(15)^2

    104.1 + 125a

    Which simplifies to -.93 m/s

    I plug that into F= ma

    so...

    100= (m)(-.93)

    Which comes out to -107.53
    Last edited by mr fantastic; December 18th 2009 at 02:18 AM. Reason: Fixed some exponents
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor
    Joined
    Sep 2008
    From
    West Malaysia
    Posts
    1,261
    Quote Originally Posted by Caturdayz View Post
    A 100 N force applied by the brakes stops a car traveling at an initial velocity of 25 km/ hr in 15 seconds. Solve for mass.
    _

    I'm getting a negative mass, so I must be doing something wrong.

    v = V0t + 1/2at^2
    <br />
0= (6.94 m/s * 15 seconds) + (.5)(a)(15)^2

    104.1 + 125a

    Which simplifies to -.93 m/s

    I plug that into F= ma

    so...

    100= (m)(-.93)

    Which comes out to -107.53
    HI

    using v=u+at

    0=6.94+a(15)

    a=-0.463 ms^{-2}

    Negative here means deceleration .

    F=ma

    100=m(0.463)\Rightarrow m=216 kg

    Force here is used to produce a deceleration .
    Last edited by mr fantastic; December 18th 2009 at 02:44 AM. Reason: No edit - just flaggng reply as having been moved from another thread.
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Flow Master
    mr fantastic's Avatar
    Joined
    Dec 2007
    From
    Zeitgeist
    Posts
    16,948
    Thanks
    5
    Quote Originally Posted by Caturdayz View Post
    A 100 N force applied by the brakes stops a car traveling at an initial velocity of 25 km/ hr. Solve for mass.
    _

    I'm getting a negative mass, so I must be doing something wrong.

    v = V_0 t + 1/2 a t^2
    <br />
0= (6.94 m/s * 15 seconds) + (.5)(a)(15)^2

    104.1 + 125a

    Which simplifies to -.93 m/s

    I plug that into F= ma

    so...

    100= (m)(-.93)

    Which comes out to -107.53
    Do you realise that a = {\color{red}-} \frac{100}{m} ....?
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Grand Panjandrum
    Joined
    Nov 2005
    From
    someplace
    Posts
    14,972
    Thanks
    4
    Quote Originally Posted by Caturdayz View Post
    A 100 N force applied by the brakes stops a car traveling at an initial velocity of 25 km/ hr. Solve for mass.
    _

    I'm getting a negative mass, so I must be doing something wrong.

    v = V_0 t + 1/2 a t^2
    <br />
0= (6.94 m/s * 15 seconds) + (.5)(a)(15)^2

    104.1 + 125a

    Which simplifies to -.93 m/s

    I plug that into F= ma

    so...

    100= (m)(-.93)

    Which comes out to -107.53
    Post the entire question before proceeding with you work. That way we don't have to work out what you have omitted ourselves.

    CB
    Follow Math Help Forum on Facebook and Google+

  5. #5
    Newbie
    Joined
    Sep 2009
    Posts
    21
    Sorry I thought I had come back and put the time back into the question.


    Regardless, Mr. Fantastic answered my question, thanks.

    For some reason the fact that this was deceleration completely slipped my mind.
    Follow Math Help Forum on Facebook and Google+

  6. #6
    Senior Member
    Joined
    Feb 2008
    Posts
    383
    Quote Originally Posted by Caturdayz View Post
    A 100 N force applied by the brakes stops a car traveling at an initial velocity of 25 km/ hr. Solve for mass.
    _

    I'm getting a negative mass, so I must be doing something wrong.

    v = V_0 t + 1/2 a t^2
    <br />
0= (6.94 m/s * 15 seconds) + (.5)(a)(15)^2

    104.1 + 125a

    Which simplifies to -.93 m/s

    I plug that into F= ma

    so...

    100= (m)(-.93)

    Which comes out to -107.53
    v = V_0 t + 1/2 a t^2
    0= (6.94 m/s * 15 seconds) + (.5)(a)(15)^2

    this is wrong.

    it should be

    x = u t + 1/2 a t^2

    where
    x= displacement
    u= inital velocity.

    and you cant have x=0 because the car was initally moving.
    Follow Math Help Forum on Facebook and Google+

  7. #7
    Flow Master
    mr fantastic's Avatar
    Joined
    Dec 2007
    From
    Zeitgeist
    Posts
    16,948
    Thanks
    5
    Quote Originally Posted by BabyMilo View Post
    v = V_0 t + 1/2 a t^2
    0= (6.94 m/s * 15 seconds) + (.5)(a)(15)^2

    this is wrong.

    it should be

    x = u t + 1/2 a t^2

    where
    x= displacement
    u= inital velocity.

    and you cant have x=0 because the car was initally moving.
    The required equation is most likely meant to be v = u + at.
    Follow Math Help Forum on Facebook and Google+

  8. #8
    Newbie
    Joined
    Dec 2009
    Posts
    16
    Wow! Mr.Fantastic your my inspiration!

    It's time to play the music
    It's time to light the lights
    t's time to meet the Muppets on the Muppet Show tonight.

    It's time to put on makeup
    It's time to dress up right
    It's time to raise the curtain on the Muppet Show tonight.

    Why do we always come here
    I guess we'll never know
    It's like a kind of torture
    To have to watch the show

    And now let's get things started
    Why don't you get things started
    It's time to get things started
    On the most sensational inspirational celebrational Muppetational
    This is what we call the Muppet Show!

    (Gonzo blows his trumpet)
    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. Need some clarification
    Posted in the Geometry Forum
    Replies: 2
    Last Post: April 28th 2010, 01:54 AM
  3. Clarification of PDE.
    Posted in the Differential Equations Forum
    Replies: 2
    Last Post: February 10th 2010, 03:35 PM
  4. 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
  5. physics, acceleration, physics problem
    Posted in the Math Topics Forum
    Replies: 3
    Last Post: September 29th 2007, 03:50 AM

Search Tags


/mathhelpforum @mathhelpforum