this is simple the word your looking for is 8 letters long and the definition is key to the answer
You can solve this by using Newton's First Law or Newton's Second Law.
1) Newton's First Law or Inertia Law states that in the abscense of a net force acting over an object, this will not chage its state of movement, i.e it will remain at rest (if it is no moving) or with uniform movement (if the object is moving).
2) Newton's Second Law: Net force = mass * acceleration => acceleration = net force / mass = 0 / mass = 0.
Zero accelerations means rest or uniform movement.
First Law is implicit in Second Law.
Answer:
I learned this last year but I don't remember. Sorry
The rock's height <em>y</em> at time <em>t</em> is given by
<em>y</em> = 45 m + (7.2 m/s) <em>t</em> - 1/2 <em>g</em> <em>t </em>²
where <em>g</em> = 9.80 m/s² is the magnitude of the acceleration due to gravity. Set <em>y</em> = 0 and solve for <em>t</em> :
0 = 45 m + (7.2 m/s) <em>t</em> - 1/2 <em>g</em> <em>t </em>² → <em>t</em> ≈ 3.9 s