Calculate the final speed of a 100-kg rugby player who is initially running at 7.00 m/s but collides head-on with a padded goalpost and experiences a backward force of 15 kN for 50 ms.

Given Information:  

Mass = m = 100 kg

Time = Δt = 50 ms

Initial speed = Vi = 7.00 m/s

Force = F = 15 kN

Required Information:  

Final speed = Vf = ?

Vf  = -0.5 m/s

Explanation:  

As we know the Impulse is given as

I = -FΔt

Where negative sign indicates backward force

I = -15,000*0.050

I = -750

From the impulse-momentum relation, we can write

I = mVf - mVi

Where Vf is the final speed of the rugby player and Vi is his initial speed

mVf = I + mVi

Vf = (I + mVi)/m

Vf = (-750 + 100*7)/100

Vf = -0.5 m/s

Therefore, the final speed of the rugby player is -0.5 m/s

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