Question

Two metal bricks are held off the edge of a balcony from the same height above the ground. The bricks are the same size but one is made of Titanium (density of 4.5 g/cm%) and one is made of Lead (density of 11.3 g/cm3) so the Lead is about twice as heavy as the Titanium. The time it takes the bricks to reach the ground will be:________.
a. less but not necessarily half as long for the heavier brick
b. about half as long for the lighter brick
c. less but not necessarily half as long for the lighter brick
d. about half as long for the heavier brick
e. about the same time for both bricks

Answer 1

Answer:

e.

Explanation:

• Assuming that the air resistance is neglectable, both bricks are only accelerated by gravity, which produces a constant acceleration on both bricks, which is the same, according  Newton's 2nd Law, as we can see below:
• $F_{g} = m*g = m*a (1)$
• ⇒a = g = 9.8m/s² (pointing downward)
• Since acceleration is constant, if both fall from the same height, we can apply the following kinematic equation:

       $\Delta y = v_{o} * t - \frac{1}{2} *g*t^{2} (2)$

• Since both bricks are held off the edge, the initial speed is zero, so (2) reduces to the following equation:

        $h =\frac{1}{2} *g*t^{2} (3)$

• Since h (the height of the balcony) is the same, we conclude that both bricks hit ground at exactly the same time.
• If the air resistance is not negligible, due both bricks have zero initial speed, and have the same shape, they will be affected by the drag force in similar way, so they will reach the ground at approximately the same time.