Question

a fullback having a mass of 9.0x10^1 kg is running at 4.5 m/s [E]. He is hit by a linebacker and 1.2 s later is moving backwards at 1.5 m/s [W].Find:
a) the impulse imparted to the fullback
B) the average force exerted on the fullback by the linebacker

my work:
impulse is p
so p= p2-p2
p=m2v2-m1v1
=m(v2-v1)
=9.0x10^1(1.5 [w] - 4.5 [E])
=-270 kg x m/s [w]
but its all wrong because the answer is 5.4 x 10^2 kg.m/s [w] apparently. Please help!

Answer 1

watch your signs
east defined as +
initial v = +4.5
final v = -1.5
we want final - initial
or
(-1.5 - 4.5)


impulse = change in momentum
90 (-1.5 - 4.5) = -90(6)
= -540 kg m/s or 540 west

F = rate of change of momentum
= -540/1.2 = - 450 N
or 450 N west