First slide

Newton's laws of motion

Question

A block of metal weighing 2 kg is resting on a frictionless plane. It is struck by a jet releasing water at the rate of 1 kgs^–1 and with a speed of 5 ms^–1. The initial acceleration of the block is

Moderate

A

$2 {ms}^{- 2}$
B

$2.5 {ms}^{- 2}$
C

$3 {ms}^{- 2}$
D

$3.5 {ms}^{- 2}$

Solution

The water jet striking the block at the rate of $1 {kgs}^{- 1}$ with a speed of 5 ms-¹will exert a force on the block
$F = v \frac{d m}{d t} = 5 \times 1 = 5 N$
Under the action of this force of 5 N, the block of mass 2 kg will move with an acceleration given by
$a = \frac{F}{m} = \frac{5}{2} = 2.5 {ms}^{- 2}$

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