Physics Help Forum Equations of Motion

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 Oct 24th 2009, 10:19 AM #1 Junior Member   Join Date: Oct 2009 Posts: 24 Equations of Motion Q) A body is thrown upwards with an initial velocity of 15 m/s Calculate: a) time taken to reach the maximum height b) maximum height reached c) total time taken by body to remain in air For the first one I calculated it by using the formula: v = u + at For the second one I'm not sure which formula to use and why? s = ut + 1/2 at^2 OR 2as = v^2 - u^2 Also for the second one what value of a (acceleration) for the upward motion will be -9.81 and for downward 9.81 right? I'm not sure how to solve the third one. Thanks for your help in advance!
 Oct 24th 2009, 11:02 AM #2 Banned   Join Date: Aug 2009 Location: UK Posts: 240 a) time taken to reach the maximum height b) maximum height reached c) total time taken by body to remain in air a) = 15/a = 15/9.8 = 1.5306 secs b) = v^2 = 0 = u^2- (2as) To find s use u^2/(2a)= 225/(2x9.8) = 11.48m (2 d.p) c) = 1.5306 x 2 = 3.0612 secs
 Oct 24th 2009, 11:32 AM #3 Banned   Join Date: Aug 2009 Location: UK Posts: 240 sorry i forgot the other question: Also for the second one what value of a (acceleration) for the upward motion will be -9.81 and for downward 9.81 right? No! other way around.
Oct 25th 2009, 03:56 AM   #4
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 Originally Posted by Paul46 a) time taken to reach the maximum height b) maximum height reached c) total time taken by body to remain in air c) = 1.5306 x 2 = 3.0612 secs
Could you tell me why did we just double the time it took for the object to reach maximum height to get this answer? Is it because the time it take a thrown object upwards is equal to the time it takes that object to come back to ground?

For my second question I think you have made a mistake. For an object moving in space, it is moving against the force of gravity, thus it's acceleration should be -9.81 and for an object which is falling towards earth, it is moving with the force of gravity, thus it's acceleration is +9.81. Am I right?

 Oct 25th 2009, 04:19 AM #5 Banned   Join Date: Aug 2009 Location: UK Posts: 240 I think the answers i have gave you are correct, i have just checked in my physics books and they state upwards is positive, downwards is negative. Any one else reading this thread just to confirm who is right?
 Oct 25th 2009, 06:01 AM #6 Junior Member   Join Date: Jul 2009 Posts: 7 HI I agree with Paul . When an object goes against gravity , its -ve simply because this object is moving against its desired motion . For (3) , the time taken to reach maximum height is the same as the time taken for the object to fall from maximum height back to the ground provided that air resistance is neglected . This is because the gravity acting on the object is constant .
Oct 25th 2009, 06:17 AM   #7
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I think you guys are a bit confused and making me confused as well. Have you read my last post properly guys?

Here check this out

WikiAnswers - When is acceleration due to gravity negative and positive

That states exactly what I've said in my post:

 For my second question I think you have made a mistake. For an object moving in space, it is moving against the force of gravity, thus it's acceleration should be -9.81 and for an object which is falling towards earth, it is moving with the force of gravity, thus it's acceleration is +9.81. Am I right?
It's completely opposite of what Paul is stating in his post.

I've always used the acceleration as +9.81 when the ball is moving upwards and -9.81 when the ball is falling down. I've always found the answer to be correct.

On a second note, all of the books and references I have studied, state the same thing as I've stated earlier.

Last edited by unstopabl3; Oct 25th 2009 at 06:22 AM.

 Oct 25th 2009, 06:35 AM #8 Banned   Join Date: Aug 2009 Location: UK Posts: 240 This is a quote from one of my books: Velocity time graphs: " it is possible to represent the same motion in different ways. Think of the journey of a stone that is flicked upwards. Firstly, think in terms of velocity. The initial flick gives it a large upwards velocity but this decreases as the stone rises. For one instant at the top of the journey, the velocity is zero and then it becomes a downwards velocity. The upwards velocity is positive and the downwards velocity is negative. The downwards velocity increases in size until the stone has fallen back to where it started." Am i thinking wrong in some way?
 Oct 25th 2009, 06:46 AM #9 Banned   Join Date: Aug 2009 Location: UK Posts: 240 Mathaddict, You have me thinking on something else also, the air resistance would not matter, i think because velocity pressure = (1/2)pv^2 so the air resistance up would equal air resistance down. I think it may be best to wait for one of the forum helpers, moderators to sort this out, otherwise we could be here all day disagreeing. Last edited by Paul46; Oct 25th 2009 at 06:52 AM.
 Oct 25th 2009, 06:58 AM #10 Junior Member   Join Date: Oct 2009 Posts: 24 You are talking about and referring to "velocity" whereas I was referring to "acceleration due to gravity". Two different things bro

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