The potential energy (in joule) of a body of mass 2 Kg moving in the x-y plane is given by
U=6x+8y
Where the position coordinates x and y are measured in meters.If the body is at rest at point (6m,4m) at time t=0,it will cross the yaxis at time t equal to
a. 2s
b. 1s
c. 3s
4. 4s
sir i donot have a slightest idea from where to begin

To answer this you need to find the equation of motion. This is given by the expression

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Since U=6x+8y we have

F = -6i - 8j

Equate this to ma and then find the differential equation for the y-component of accleration. This will give you a differential equation. Use the initial conditions v(0) = 0, r(0) = (6, 4), or v[sub]y[/sub](0) = 0, y(0) = 0. Then find the value of t so that y(t) = 0.