First slide

Second law

Question

A force of 25 N acts on a body at rest for 0.2 s and a force of 70 N acts for the next 0.1 s in opposite direction. If the final velocity of the body is 5 ms^–1 in opposite direction, the mass of the body is

Moderate

A

1 kg
B

2 kg
C

0.8 kg
D

0.4 kg

Solution

F = P/t
P₁ = F₁t₁ = 25 x 0.2 = 5 kg-m/s
P₂ = F₂t₂ = (-70) x 0.1 = -7 kg-m/s
Net impulse = (-7) + 5 kg-m/s = -2 kg-m/s
Now, mv_f - mv_i = Net impulse
$∴$ m x (-5) - 0 = -2 ⇒ m = 0.4 kg

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