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# Newton's Third Law & Rockets watch

1. Hey guys I've gotten my self confused (again).

If the force of an object acting on a surface will have an equal and opposite reaction force. How come it takes so much energy for a rocket to take off?

If the forces are balanced when it's stationary, surely just the smallest amount of thrust will allow it to take off?

I know as I'm typing this it makes no sense but I thought I'd exaggerate my confusion as to get a better answer, thanks in advance!
2. (Original post by Retsek)
...
Perform a force balance on the rocket in the upwards direction:

Your pitfall is assuming that the reaction force remains of constant magnitude - whereas it decreases as thrust increases.

Rest your palm on the table and lift your wrist up gradually with your other hand. The upwards force (thrust) gradually overcomes the weight of your hand (or rocket) until it is greater than the weight, and only then will your hand start to rise upwards.
Perform a force balance on the rocket in the upwards direction:

Your pitfall is assuming that the reaction force remains of constant magnitude - whereas it decreases as thrust increases.

Rest your palm on the table and lift your wrist up gradually with your other hand. The upwards force (thrust) gradually overcomes the weight of your hand (or rocket) until it is greater than the weight, and only then will your hand start to rise upwards.
Sorry I'm not familiar with this 'force balance' thing, but I am beginning to understand, you say that before it takes off it must first overcome it's force on the surface, and as it overcomes this force the reaction force (which is equal and opposite to the weight) will decrease too as it's directly proportional, so until it overcomes the force of it's weight, it stays still.
(I probably over explained that but it was consolidation for me too).
4. (Original post by Retsek)
Sorry I'm not familiar with this 'force balance' thing, but I am beginning to understand, you say that before it takes off it must first overcome it's force on the surface, and as it overcomes this force the reaction force (which is equal and opposite to the weight) will decrease too as it's directly proportional, so until it overcomes the force of it's weight, it stays still.
(I probably over explained that but it was consolidation for me too).
The reaction force pushes the rocket upwards.
The thrust force pushes the rocket upwards.
The weight force pushes the rocket downwards.

For the rocket to move upwards, the sum of the reaction and thrust forces must be greater than the weight force.

In equilibrium (when the rocket has yet to move off the ground), the forces must be balanced:

If the rocket is moving upwards:

...but right at the instant when the rocket is just about moving upwards, it breaks contact with the ground and hence the reaction force is zero.

Hence, you can probably see more clearly that the reaction force decreases as the thrust increases.

From the first equation, you can see that the reaction force is only equal to the weight force if there are no additional forces acting e.g. thrust.
The reaction force pushes the rocket upwards.
The thrust force pushes the rocket upwards.
The weight force pushes the rocket downwards.

For the rocket to move upwards, the sum of the reaction and thrust forces must be greater than the weight force.

In equilibrium (when the rocket has yet to move off the ground), the forces must be balanced:

If the rocket is moving upwards:

...but right at the instant when the rocket is just about moving upwards, it breaks contact with the ground and hence the reaction force is zero.

Hence, you can probably see more clearly that the reaction force decreases as the thrust increases.

From the first equation, you can see that the reaction force is only equal to the weight force if there are no additional forces acting e.g. thrust.
You have opened my eyes, absolute lifesaver, thank-you!

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