Need help with these two questions! Thanks for ur help! 1.) An object oscillates

Question

Need help with these two questions! Thanks for ur help!

1.) An object oscillates back and forth along the x-axis in simpleharmonic motion. Its displacement as a function of time is given byx = Acos(omega* t + phi) . At time t = 0 the object has a positivex-acceleration.

A.) phi = + pi /4 radians
B.) phi = + 3pi /4 radians
C.) phi = 0
D.) phi = + pi /2 radians
E.) more than one of the above

2.) Two identical thin rods, each of mass m and length L, arejoined at right angles to form an L-shaped object. This object isbalanced on top of a sharp edge. If the object is displacedslightly, it oscillates.
Assume that the magnitude of the acceleration due to gravity isg.

a.) Find omega, the angular frequency of oscillation of theobject?
Note: answer for the angular frequency may contain the givenvariables m and L as well as g.

thanks again!

Explanation / Answer

x = Acos(omega* t + phi) to get acceleration take two deriviatives with respect to time v=dx/dt=-Asin(t+)* a=dv/dt=-Acos(t+)*^2 a(t)=-A*^2*cos(t+) at t=0 we have a(t)=-A*^2*cos() so the only value of that gives a positive acceleration is=3/4 so the answer is B) 2) not sure about this one yet

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