Question

The average resisting force that must act on a 5kg mass to reduce its speed from 65 to 15 cm/s in 0.2 second is

Answer 1

Acceleration = Mass × Acceleration

Acceleration = Change in Velocity / Time

Change in Velocity = Final Speed - Initial Speed

⇒ Initial Speed = 65 cm/s

⇒ Final Speed = 15 cm/s

⇒ Change in Speed = 15 - 65 = -50 cm/s

Negative sign indicates acceleration direction is opposite to motion of body.

Acceleration = -50 cm/s / 0.2 s =  -250 cm/s²

In m/s², we can write it as:

Acceleration = 250/100 m/s² ⇒ -2.5 m/s²

So Force = -2.5 × 5 = -12.5 N

Here negative direction indicates force is applied on opposite direction to motion.

Hope it helped !!

Answer 2

F*dt = mv - mu

F = m (v - u) /dt = 5*(65-15)/0.2*100=5*50/20=12.5N