Physics Help Forum Quick Relative motion with Vectors Question!

 Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum

 Sep 21st 2009, 03:39 PM #1 Junior Member   Join Date: Aug 2009 Posts: 18 Quick Relative motion with Vectors Question! Three forces acting on an object are given by F1 = ( - 2.05 i - 2.40 j ) N, F2 = ( 5.50 i ) N, and F3 = ( - 46.0 i + 46.0 j ) N. The object experiences an acceleration of magnitude 3.75 m/s2.(a) What is the direction of the acceleration? 1-45.7 (from the positive x axis) I got this answer but is wrong? can someone tell me why? (b) If the object is initially at rest, what is its speed after 12.0 s? 3 ??m/s (c) What are the velocity components of the object after 12.0 s? ( ?? i + ?? j ) m/s Someone please help me with b and c. thanks
 Sep 21st 2009, 11:15 PM #2 Physics Team   Join Date: Feb 2009 Posts: 1,425 Add the components to get the resultant R = -42.55 i + 43 j. Note F2 = 5.5 i + 0 j If you draw this you will find it lies in the second quadrant as the x coord is - ve and y +ve . Note that 43 / 42.55 gives tan of the angle between R and the -ve y axis. Thus tan inv (43 / 42.55) will give you this angle in radians. (Pi -this angle ) will give you the angle (+ve) with respect to the +ve X axis say theta. The net force on the object, R will be given by $\displaystyle R\ =\sqrt {(-42.55)^2\ +\ (43)^2}$ . (b) You have the accln a (3.75) with initial vel u = 0 as it starts from rest and the time t Use $\displaystyle v\ =\ u\ +\ a\ t$ to get the final speed. (c) The x component will be v cos theta and the y component will be v sin theta. Last edited by physicsquest; Sep 21st 2009 at 11:18 PM.
 Sep 22nd 2009, 09:31 AM #3 Junior Member   Join Date: Aug 2009 Posts: 18 Thanks for helping me and explaining the problem.

 Tags motion, question, quick, relative, vectors

 Thread Tools Display Modes Linear Mode

 Similar Physics Forum Discussions Thread Thread Starter Forum Replies Last Post apple123 Waves and Sound 1 Mar 11th 2010 02:56 AM KaylaN Kinematics and Dynamics 1 Sep 22nd 2009 04:56 AM KaylaN Kinematics and Dynamics 1 Sep 22nd 2009 04:55 AM bonnie.huang Kinematics and Dynamics 1 Jul 7th 2009 04:57 AM F19 Kinematics and Dynamics 0 Feb 24th 2009 05:10 PM