Physics Help Forum Pendulum In a Car

 Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum

 Oct 29th 2011, 02:02 PM #1 Junior Member   Join Date: Aug 2011 Posts: 23 Pendulum In a Car A pendulum of mass 1.0 kg is suspended from the roof of a car travelling on a level road. An observer in the car notices that the pendulum string makes an angle of 10 deg with the vertical. What is the acceleration of the car? I drew the FBD diagram (attached) and I wrote the vertical and horizontal force equations: Vertically: Tcos 10 deg=mg Horizontally: Fc=mv^2/r=Tsin10deg. So I got he equation mgtan10deg=mv^2/r after putting the equations together. But I don't know the radius of the pendulum, so I don't know how to solve the problem. Thanks! Never mind, I'm not supposed to use centripetal force since the pendulum is not going in a circle. Any hints on how to start the problem? Last edited by teddybear9; Oct 29th 2011 at 04:52 PM.
 Oct 29th 2011, 10:56 PM #2 Physics Team     Join Date: Jun 2010 Location: Mauritius Posts: 609 Hmm, you don't have circular motion here. The bob remains in that position for some time. So that: Along the vertical: vector sum of forces on the bob = 0 N Along the horizontal: vector sum of forces on the bob = 0 N Can you do that from this? Or, do you maybe need some more guides? __________________ Jerry (Got my results!) It is easier to protect your feet with slippers than to cover the earth with carpet. No one can go back and change a bad beginning; but anyone can start now and create a successful ending. If a problem can be solved, no need to worry about it. If it cannot be solved what is the use of worrying?
 Oct 30th 2011, 07:32 AM #3 Junior Member   Join Date: Aug 2011 Posts: 23 Oh I got it! Tcos10=mg Tsin10=ma Sub T=mg/cos10 into Tsin10=ma, cancel out the m's and solve for a. But what's the use of the mass given in the problem? Am I supposed to cancel it out?
 Oct 30th 2011, 07:35 AM #4 Physics Team     Join Date: Jun 2010 Location: Mauritius Posts: 609 Sometimes they give you values which you can very well do without Someone else might have solved for the value of T, and used that in the second equation to get the same answer. But I would say you did a good job here, well done! __________________ Jerry (Got my results!) It is easier to protect your feet with slippers than to cover the earth with carpet. No one can go back and change a bad beginning; but anyone can start now and create a successful ending. If a problem can be solved, no need to worry about it. If it cannot be solved what is the use of worrying?
 Oct 30th 2011, 08:42 AM #5 Junior Member   Join Date: Aug 2011 Posts: 23 Thank you so much!

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