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Homework Help: Draw the v(t) and a(t) graph

  1. Apr 7, 2015 #1
    • HW Template missing as it was moved from another forum
    1. problem
    Two forces, F1 and F2(t), act on a mass m = 1.0 kg. Force 1 F1 is to the left in the negative x-direction and force F2(t) is to the right in the positive x-direction. F1 is constant with a magnitude of 1.0 N whereas F2(t) has a magnitude of 2.0 N for the time interval 0.0 s ≤ t ≤ 5.0 s and drops to zero after 5.0 s. The mass is at rest at time t = 0.0 s.

    a) Sketch the acceleration versus time graph for the time interval 0.0 s ≤ t ≤ 15.0 s for the mass m.
    b) sketch the v(t) for the same time as question a.

    2. attemps:
    I am confusing about the time t=0s, when the object is at rest, that means a=0 and v=0? no unbalanced forces?
    Since f2=2N and f1 =1N during the interval time [0,5]. netforce would be 1N. Because F2(t)=2N during [0,5] and it comes to 0 after 5s. for example t=6s.7.,,,. I believe that i shoud calculate the friction force cos It will be 1N in magnitude and the same negative direction as F1.
    Now when t>5s, t=6s, Fnet is now missing F2. Will the mass start acceleration? although I have considered the FFric.? If it does, a=-1m/s/s. so velocity will be -1 at t=6, hence -2,-3... at t=7,8... while a is now constant and has the value of -1?
    and if there is friction consideration, fnet =0 cos friction force will resist the tend to accelerate by F1
    3. thank in advance
  2. jcsd
  3. Apr 7, 2015 #2

    Simon Bridge

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    no. "At rest" means the speed is instantaneously zero.

    the net force is different for different times. Force is a vector, dont forget to say which direction the net force points in.

    ... do not assume what is not in the problem statement. Friction is not mentioned anywhere so what makes you think it is 1N?

    ... does it have an unbalanced force acting on it?

    ... no friction is mentioned, acceleration is constant inside both time intervals of interest.
  4. Apr 7, 2015 #3
    Thank for your guiding Simon, So I am still in trouble with the time t=0s, does F2 has magnitude of 2N and right ward direction?
    if mass is at rest means it has V instantaneously =0, that means since F2 applied forced, There is unbalanced force act on mass, Fnet=F2-F1, a=1 and in positive direction. Velocity has changed to 0 m/s when F2 started?

    before we consider the F2 started. time before we call it 0s, Object is moving with only Fnet=F1=ma1, a1=-1, negative positions ? again, velocity changed to 0 and changed also direction to positive direction when F2 applied?
  5. Apr 7, 2015 #4

    Simon Bridge

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    Consider: what dfference does it make to your graph?

    velocity is equal to 0 at t=0s ... it is not changed to that.
    We are not told what happened at t<0s so don't assume.

    When does the problem say that F2 started?

    At t=0, what is the position of the object? What is the net force? If you don't answer questions I cannot help you.
    This problem is very straight forward and you are over-thinking it.
    Last edited: Apr 7, 2015
  6. Apr 7, 2015 #5
    Hum, I think that at t=0, the object is at rest, Velocity equal 0 and Netforce is 1N following F2 direction, with the acceleration of 1m/s/s ? right
  7. Apr 7, 2015 #6

    Simon Bridge

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    Directions are "left" and "right" or "x direction".
    So at t=0, v(0)=0 a(0)=F(0)/m and F(0)=F2-F1 in the +x direction. ... well done.

    I used brackets since v a and F are functions of time... so is x. v(0)=v(t=0) see?

    In fact. for ##0\leq t\leq5## (seconds) ##F=+1##N ... the ##+## indicates the ##+x## direction.

    Take this one step at a time:
    Using the same notation, what is the acceleration for ##0\leq t \leq 5##s?
  8. Apr 7, 2015 #7
    Hi, since Fnet unchanges during interval time above, a=Fnet/m=+1, so what about velocity ? does it follow the equation v=vo+ a.t?
    v(0)=0, v(5)= 0+1x5=5? I wonder why the answersheet covers only maximum of +2 of velocity ( in vertical line) for the graph to be drawed

    After 5s, a(t>5)=Fnet(t>5)/m Fnet now is -1 since Only F1 act on the mass. negative direction. so a=-1 negative direction.?
    again, how about velocity, if it was previously 5 m/s, now with the acceleration -1, velocity changes direction and its value decreases by 1 m/s per second?
  9. Apr 7, 2015 #8
    I have got some ideas, can it is possible to assume that only during 5 seconds, a =+1, so velocity changes to +1. and from 5-10, it drops to 0 since a=-1 and from 10-15s, a remains -1 and velocity is now -1., if 15-20s, velocity -2...
  10. Apr 7, 2015 #9

    Simon Bridge

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    ... well done.
    For the interval in question, v(t)=t so v(5)=5m/s

    No idea - I didn't set the question.
    Velocity should be on the vertical axis and the line of constant velocity should be horizontal. You will need to extend the graph or relabel the axes (I'd suggest the latter), but check you have read the question properly ... maybe you missed out something. It could also be an edit of an earlier question and the editor forgot to change the graph axes values.

    Well done ... so:
    ... correct: for ##t>5## the velocity is given by ##v(t>5)=v(5)-t##. The graph just decreases with the same slope that it increases before.

    Taking things one step at a time stops different stages of the problem from confusing you.
    You can test your understanding by working out how long before the object returns to x=0 if you like.

    While I was typing you wrote:
    ... and you were doing so well too :(
    You cannot just pluck numbers out of the air - you have to use physics.
    If the acceleration is 1m/s2 then v=1m/s after 1s has passed.
  11. Apr 7, 2015 #10
    oh yes, Thank a lot. I just think crazy out of a box when velocity was too high than the graph expected to be drawn.
    I attached the area for the graphs.
    they would draw the wrong coordinate for velocity

    Attached Files:

  12. Apr 9, 2015 #11

    Simon Bridge

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    Like I said - just relabel the axes after checking that you have read the question correctly.
    The top one (that goes to 6) looks like it has the correct range for the v(t) graph and the bottom one looks correct for a(t)... so maybe they just got them switched.
  13. Apr 9, 2015 #12
    yes, I got it. thank so much for your help Simo
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