# Need some help guys .

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#1
a rocket of mass 550 kg blasts vertically from the launch pad at an acceleration of 4.2ms^. calculate :
The weight of the rocket?
the thrust of the rocket engines?

(What would be the sign of g here as if you take it negative your answer would be completely different with taking the g sign positive. G is acting downward so obviously it's negative but text book took as positive . can someone explain to me this as i don't get it at all. thanks guys .)
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6 years ago
#2
(Original post by Alen.m)
a rocket of mass 550 kg blasts vertically from the launch pad at an acceleration of 4.2ms^. calculate :
The weight of the rocket?
the thrust of the rocket engines?

(What would be the sign of g here as if you take it negative your answer would be completely different with taking the g sign positive. G is acting downward so obviously it's negative but text book took as positive . can someone explain to me this as i don't get it at all. thanks guys .)
It doesn't matter if g has positive or negative sign. If positive: F = m(a + g), if negative, F = m(a - (-g)). By convention, g is the magnitude of the gravitational field strength at the surface of the Earth, so is positive.

As long as your model is correct, you can take minor freedoms in exactly the details of the model.
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#3
so if i get G as a positive value in order to sort out different type of questions, will i get the right answer?
thanks
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6 years ago
#4
(Original post by Alen.m)
so if i get G as a positive value in order to sort out different type of questions, will i get the right answer?
thanks
Yes, if you know what you're doing, but you have to be careful. A better way explaining the previous question is that the thrust(upwards) has to give the acceleration ma(upwards) and also has to have the extra magnitude(mg up) to cancel the weight(mg down), hence ma + mg. The weight is downwards and the extra magnitude of the thrust is upwards, so this can cause confusion.

Note also that you need to work on your notation. g denotes the gravitational field strength at the earth's surface(9.81ms-2), which is the constant you need to use. G denotes the gravitational constant(6.67x10-11m3kg-1s-2), which you don't need to know at this stage.
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