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hamid14

Tue Oct 19, 2010 3:10 pm


physics test

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I have physics tomorrow, so i am trying to do the review questions. I can't quite figure this question out. If someone can tell me how to solve it, I'd appreciate it. 

A car is travelling 50.0 km/h north then makes a turn and heads west at 50.0km/h west, the turn takes 5.0seconds. find change in velocity and acceleration. 

thanks in advance


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DanShadow

Tue Oct 19, 2010 3:30 pm


RE:physics test

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I'm not "great" in physics, but it sounds like your missing some variables.
There is a range of values which could be the velocity when decelerating before the turn, accelerating after the turn, and another range of values for what the velocity the driver is driving at while turning, which could be better defined by knowing the weight/mass of the vehicle, and at what the max velocity the driver could turn at with the given weight/mass without flipping his/her vehicle.

Answering a question with more questions, sorry :)


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DtY

Tue Oct 19, 2010 3:44 pm


Re: RE:physics test

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I'm not "great" in physics, but it sounds like your missing some variables.
There is a range of values which could be the velocity when decelerating before the turn, accelerating after the turn, and another range of values for what the velocity the driver is driving at while turning, which could be better defined by knowing the weight/mass of the vehicle, and at what the max velocity the driver could turn at with the given weight/mass without flipping his/her vehicle.

Answering a question with more questions, sorry :)Given how early in the year it is, the *speed* is probably assumed constant, with only the direction changing.

You have:
http://www.texify.com/img/%5CLARGE%5C%21%5Cvec%7Bv_1%7D%20%3D%2050km/h%5BN%5D.gif
http://www.texify.com/img/%5CLARGE%5C%21%5Cvec%7Bv_2%7D%20%3D%2050km/h%5BW%5D.gif

And you should know the acceleration formula:
http://www.texify.com/img/%5CLARGE%5C%21%5Cvec%7Ba%7D%20%3D%20%5Cfrac%7B%5Cvec%7Bv_2%7D%20-%20%5Cvec%7Bv_1%7D%7D%7B%5CDelta%20t%7D.gif


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hamid14

Tue Oct 19, 2010 3:45 pm


Re: physics test

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that's all the question gives you and it is possible to solve.


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TerranceN

Tue Oct 19, 2010 3:46 pm


RE:physics test

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First you have to set directions to be positive, lets choose North in the North-South direction and West in the east west direction.

Change in velocity is the final velocity (Vf) minus the initial velocity(Vi), just make this easier lets just make it Vf + (-Vi). Now it is just vector addition, and you don't even need to split up your velocity vectors into components either.

When you do the vector addition you are left with this

http://img843.imageshack.us/img843/1126/velocitiesrearranged.png

From there you should know what to do to find the change in velocity.

As for the change in acceleration, it is change in velocity per unit time so a = (Vf-Vi)/t.


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Insectoid

Tue Oct 19, 2010 3:59 pm


RE:physics test

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In 0.5 seconds, velocity in the north direction becomes zero from 50km/h. Similarly, the velocity in the west direction becomes 50km/h from zero. From here it's pretty basic. 

Remember to convert your units to m/s.


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hamid14

Tue Oct 19, 2010 4:00 pm


Re: physics test

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50-50 = 0? i get 0 for acceleration. :o


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TerranceN

Tue Oct 19, 2010 4:13 pm


RE:physics test

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Did you not learn vector addition? When you add vectors you have to split them up into components in directions perpendicular from one another (like x and y axis in cartesian space) In this case its North and West. When you add these you add all the x (or west in this case) components up separately from the y (or north in this case) components. You will end up with something like this (vfx - Vix, Vfy - Viy), or in this case (13.8 - 0, 0 - 13.8) = (13.8, -13.8) (I also converted to m/s). To get the magnitude of this you can use the pythagoras theorom (cause your components will make a right angle, just like in the picture I posted above). Hope that helps.


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andrew.

Tue Oct 19, 2010 4:22 pm


RE:physics test

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Here is my solution (I hope it's correct):
http://dl.dropbox.com/u/2162996/solution2.jpg

