Star thrower

Classical Mechanics Level 1

Dominic is the star discus thrower on South's varsity track and field team. In last year's regional competition, Dominic whirled the $1.6\text{ kg}$ discus in a circle with a radius of $1.1\text{ m}$ , ultimately reaching a speed of $52\text{ m/s}$ before launch. Determine the net force acting upon the discus in the moments before launch.

4.1 \times 10^4 \text{ N}

3.9 \times 10^3 \text{ N}

3.91 \times 10^3 \text{ N}

1 solution

Marcin Popławski
May 11, 2020

From Newton's Second Law of Motion, we know that:

$F = ma$

The primary acceleration here is the one of circular motion given by the equation:

$a = \frac{v^2}{r}$

Substituting it into the first equation and plugging given values we obtain:

$F_{centripetal} = \frac{mv^2}{r} = \frac{1.6 \times 52^2}{1.1} = 3933.1 N$

The gravitational force is present as well, though its magnitude is negligible compared to centripetal force:

$F_{gravitational} = mg = 1.6 \times 10 = 16 N$

Hence, the net force acting upon the discus right before the launch equals:

$F_{net} = \boxed{3.9 \times 10^3 N}$

How could there be any net gravitational force before the launch when his hand is holding it up; it's not accelerating downwards. Also, why does it specify between 3.9 and 3.91 if neither answer is correct and 3.91*10^3 is actually closer to the correct answer in spite of being considered incorrect?...

Tristan Goodman - 7 months ago

Star thrower

1 solution

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