Question

The friction coefficient between an athlete's shoes and the ground is 0.90. Suppose Superman wears these shoes and races for 50 m. There is not upper limit on his capacity of running at high speeds.

Find the minimum time that he will have to take in completing the 50 m starting from rest.

• A. sec
• B. 10 sec
• C. 30 sec
• D. There is no limit to superman's speed. 50 m he can do in the blink of an eye

Answer 1

The correct option is A

sec

What makes you walk forward yes that's right friction.

So, if you are running too there is only force on you in the forward direction i.e., friction which will give you acceleration. Let's assume superman's mass is m.

The maximum friction superman can have on him from ground is $f_r=\mu N=\mu mg$

$\mu mg=ma$

$\Rightarrow a=\mu g$

Now we agree he can't have more acceleration than this.

Distance to run is 50m

s=50m ;u=0 m/s; a=$\mu g$=0.9$\times$10=9 m/$s^2$

t=?

s= ut + $\frac{1}{2}a{t}^{}2$

50=$\frac{1}{2}$ $\times$ 9 $\times$ $t^2$

$\frac{100}{9}=t^2$ $\Rightarrow t=\frac{10}{3}=3.33sec.$