First slide

Collisions

Question

A gun which fires small balls of mass 20 g is firing 20 balls per second on the smooth horizontal table surface ABCD. If the collision is perfectly elastic and balls are striking at the centre of table with a speed of 5 m/s at an angle of 60° with the vertical just before collision, then force exerted by one of the legs on ground is (assume total weight of the table is 0.2 kg)

Moderate

A

0.5 N
B

1 N
C

0.25 N
D

0.75 N

Solution

Force on table due to collision of balls:
$F_{dynamic} = \frac{dp}{dt} = 2 \times 20 \times 20 \times 10^{- 3} \times 5 \times 0 .5 = 2 N$
Net force on one leg $= \frac{1}{4} (2 + 0 .2 \times 10) = 1 N$

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