First slide

Third law

Question

A rocket of mass 1000 kg is to be projected vertically upwards. The gases are exhausted vertically downwards with velocity 100 ms^–1 with respect to the rocket. What is the minimum rate of burning of fuel, so as to just lift the rocket upwards against the gravitational attraction? (take g =10 ms^–2)

Moderate

A

50 kgs^–1
B

100 kgs^–1
C

200 kgs^–1
D

400 kgs^–1

Solution

Here $Mg = u \frac{dm}{dt}$
$\begin{array}{l} 1000 \times 10 = 100 \times \frac{dm}{dt} \\ \frac{dm}{dt} = 100 kg / s \end{array}$

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