Question

With what speed in miles/hour ($1m/s=2.23mi/h$) must an object be thrown to reach a height of $91.5m$ (equivalent to one football field)? Assume negligible air resistance.

• A. $94.4mi/h$
  
• B. $84.4mi/h$
  
• C. $74.4mi/h$
  
• D. $94mi/h$
  

Answer 1

The correct option is A $94.4mi/h$

As we assume negligible air resistance, so the acceleration of the object is due to gravity, i.e. $a=-g=-9.8m/s^2$

where minus sign for motion against gravity.

When the object reaches top most height, the final velocity will be zero i.e $v=0m/s$

Maximum height reached $S=91.5m$

Let $u$ be the initial velocity of the object.

Using formula $v^2-u^2=2aS$

$\therefore$ $0^2-u^2=2(-9.8)\left(91.5\right)$

$⟹$ $u=42.35m/s=42.35\times 2.23mi/h=94.4mi/h$