Answer: 4
The mechanical advantage is the ratio of the force exerted by the object to the force applied to do work on it.
Here, Jeff tried to lift a rock weighing 600 pounds by wedging board under the rock. Jeff who weighs 150 pounds uses all his weight to exert force on lever and lift rock.
Mechanical advantage, 
Therefore, the mechanical advantage that lever provided to Jeff in lifting rock is 4.
According to law of conservation of energy,
<span>Energy can neither be constructed nor be destroyed but can be transformed from one form to another.
</span>
<span>At the highest point of the pendulum(point b), pendulum is associated with potential energy only and no kinetic energy.
</span><span>Therefore total energy at point b = potential energy = 711 J.... i
</span>
<span>At the bottom most point(point a), pendulum is associated only with kinetic energy and no potential energy.
</span>Therefore total energy at point a = kinetic energy ---- ii
<span>From i and ii,
</span>Kinetic energy = potential energy = 711 J.(Conserving energy)
Hence kinetic energy at the bottom most point is 711 J.
Hope this helps!!
It would be A:facing disagreement forces scientist to prove their theories more consistently :)
