Answer:
1 second
Explanation:
The impulse exerted on the boy is equal to its change of momentum:
[tex]I = \Delta p\\F \Delta t = m \Delta v[/tex]
where
F = 500 N is the push on the boy
[tex]\Delta t[/tex] is the contact time
m = 25 kg is the mass of the boy
[tex]\Delta v = 20 m/s[/tex] is the change in velocity of the boy
Solving the formula for the contact time, we find
[tex]\Delta t=\frac{m\Delta v}{F}=\frac{(25 kg)(20 m/s)}{500 N}=1 s[/tex]
