Question

A body of mass $5kg$ has a momentum of $10kgm/s$. When a force of $0.2N$ is applied on it for $10s$, the change in its kinetic energy is:

• A. $1.1J$
• B. $2.2J$
• C. $3.3J$
  
• D. $4.4J$
  

Answer 1

The correct option is D $4.4J$

Given,

Mass, $m=5kg$

Momentum, $p=10kgm/s$

Force, $F=0,2N$

Time, $t=10seconds$

According to Newton’s second law,

Force = Rate of change of momentum

$F=(p2-p1)/t$

$0.2=(mv-10)/10$

Or $mv–10=2$

Or $5\times v-10=2$

Or $v=12/5=2.4m/s$

p2 = mv = 5 × 2.4 = 12 kg m/s

Increases in kinetic energy = $({p_2}^2-{p_1}^2)2m$

$=({12}^2-{10}^2)/(2\times 5)=4.4J$