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Homework Help: SUVAT questions, Projectile motion?

  1. Dec 26, 2008 #1
    Hey guys,
    I'm taking A Level Physics, at the moment in AS level (lower level) studying kinematics, pretty straight forward but I am having trouble with projectile motion.

    I can't think of a question but this is like the question I had on my physics paper the other day.

    A ball is thrown with a certain velocity (neglecting air resistance) and comes in contact with a wall, whilst in contact for 0.4s the ball bounces back'

    And I have to work out the velocity at which the ball rebounds of the wall, the change in kinetic energy but I'm hoping you guys have come across these sort of questions before and that you'll be able to help me :P

    I know you have to work out the horizontal and vertical components of the projection but I always get confused with some of the questions.

    I'm sorry if I can't provide enough info. but basically questions which involve SUVAT equations with projectile motion.

    Thanks for any help, much appreciated,
    Revising for Chem., Maths and Physics woo!

  2. jcsd
  3. Dec 26, 2008 #2
    Hi, I'm doing further maths at A-level, and so I know what you need to know.

    The question that you have given is incomplete like you said; you need to know the constant force that the wall is exerting on the ball, but never mind. It also involves impulses and forces, not just SUVAT.

    You are right in saying you need to resolve the velocity/acceleration/forces in the two perpendicular directions (usually horizontal and vertical). The way to do this is to use the cosine/sine functions.

    Then you write down all the information for the vertical motion, and then all the info for the horizontal motion.

    The horizontal acceleration is always zero, and the downward acceleration is always 9.8m/s² (unless otherwise stated). You then choose the correct equation from the SUVAT equations (the one that only includes a value you don't know and you want to work out.)​

    e.g. "A Particle is projected at 30 degrees to the horizontal at v m/s, it lands 30m away at the same height that it was projected." find v.

    u = v*sin(30)
    v = (we dont know, and dont care)
    a = -9.8 (note the negative, because it is downwards)
    s = 0 (because it has no vertical displacement)
    t = t (we want to know this)

    u = v*cos(30)
    v = v*cos(30)
    a = 0
    s = 30
    t = t (the same t that is in the verticle)

    so for vertical, we choose the equation that doesn't have the final velocity in it, because we dont know it and we dont need to know it. this is:

    S = ut + (1/2)*a*t²

    Put the values in, rearrange for t.

    We get: t = 0.1767v

    then look at horizontal (replacing "t" with "0.1767v")

    We use any equation with S and t in it.

    S = ut + (1/2)a*t²

    30 = [v*cos(30)]*[0.1767v]

    v = [(30)/(cos(30)+0.1767)]

    v = 28.8 m/s (I haven't checked through all this, so there m ight be a mistake, just wait for someone to confirm it)

    For the Impulse/kinetic energy part of the question:

    there are four formulas you might need for something like what you have asked:

    F = ma
    Impulse = force*time
    Impulse = mv - mu
    KE = (1/2)mv²

    Here is a question you might get: A ball of 0.25kg hits a wall, and is travelling at 10 m/s. It is in contact with the wall for 0.4 seconds, and leaves the wall with a velocity of 5 m/s. What is the impulse exerted on the ball, and KE lost in the collision, and the constant force that was applied to the ball, from the wall.
    (with the initial direction as positive)
    Impulse = mv - mu
    -Impulse = -(0.25*5) - +(0.25*10) [they all end up negative, and so they can all be changed to positive]
    Impulse = 3.75 Ns (they almost always ask you about the unit "newton seconds" and you always get a mark for it, so get it right)

    For the KE lost, just work out the KE before and then the KE after, and take the final one from the Intial one. NOTE: The direction of the ball doesn't matter, so the kinetic energy at one point should always end up positive.

    For the constant force: just divide the impulse by the amount of time that the ball was touching the wall.

    3.75/0.4 = 9.325 N.

    Any questions? I probably just put much to much time into this answer, but never mind. If any of the calculations are wrong then tell me quickly.
  4. Dec 26, 2008 #3
    Thanks alot for that, I really just needed some clearing up on the topic like a worked example, and the example you gave was alot like the question on my previous physics paper, I just need to apply some logical thinking to the question (well more than most questions which just require inputting values into a formula).

    I sort of forgot about how when an object is projected at an angle you multiply sin/cos angle by the velocity (may be a very simple thing, but I always forget stuff like that).

    Then the substituting of the horizontal/vertical value into the opposite I didn't think you could do, well wasn't sure and that's been cleared up.

    Thanks, I do think I understand this alot more clearly now :)
    Luckily enough for myself this year in A level physics the school is using a new exam/spec. so apparently the test will be easy due to it being new and the school wanting to get as much people to pass the test as possible... still no excuse to revise as much as possible :p

    Once again thanks,
  5. Dec 29, 2008 #4
    No problem, If you have any more ponders or questions, then I will be glad to help.

    Btw, I haven't got to the physics mechanics module yet, I just answered the questions using the mechanics I have learnt in M1 and M2 in F-maths. Are you doing the test when you get back to school after xmas hols?
  6. Dec 29, 2008 #5
    Yeah, first week I get back I have 4 exams, Chemistry, Maths, Physics and General Studies (not really a chosen subject, not optional). Each exam is based on one module which consists of 2 'sub-modules' you could say.

    I can't seem to find my physics book as well, oh joy, so revision through the internet.
  7. Dec 29, 2008 #6
    well your doing the same subjects as me, just I'm also doing F-maths.

    If you need any physics past papers:

    http://www.egsphysics.co.uk/files/web_pages/exams.html [Broken]

    That site has all of them back to 2001 in .pdf.
    Last edited by a moderator: May 3, 2017
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