Question

The average resisting force that must act on a $5Kg$ mass to reduce its speed from $65cm/s$ to $15cm/s$ in $0·2s$ is

• A. $12·5N$
• B. $25N$
• C. $50N$
• D. $100N$

Answer 1

The correct option is A

$12·5N$

Step1: Given data

Mass of moving object$\left(m\right)=5kg$

Time$\left(t\right)=0·2sec$

Initial speed$\left(u\right)=65cm/sec=0.65m/sec$

Final speed$\left(v\right)=15m/sec=0.15m/sec$

Step2: Formula used

We know that the impulse experienced by the object equals the change in momentum of the object. So,

$$\begin{gathered} Impulse=Changeinmomentum \\ Force\left(f\right)\times time\left(t\right)=mass\left(m\right)\times changeinvelocity\left(∆v\right) \end{gathered}$$

Step3: Compute the force

Substitute the known values in the following formula,

$\begin{array}{rcl}f\times t& =& m\times ∆v\\ f\times 0·2& =& 5\times \left(0·65-0·15\right)\\ f& =& \frac{5\times \left(0·65-0·15\right)}{0·2}\\ f& =& 12·5N\end{array}$

So, the average resisting force must be $12·5N$.

Hence, option A is the correct answer.