NEWTON'S 3rd LAW OF MOTION
To remind you, here is Newton's 3rd law:
"For every action, there is an equal but opposite reaction."
Newton's third law is based off of the fact that forces come in pairs. One of these forces is called an action force and the other is called a reaction force. [5]
When an object is at rest, the forces are the same size but one force is in one direction and the other is in the opposite direction. [5]
For an object to move, one of these forces must be larger than the other. The object will move in the direction that the bigger force is acting in. [5]
What are Some Everyday Examples of Newton's 3rd Law?
We can explain Newton's third law through the use of some examples.
Swimming:
When a person is swimming they push the water around them away, this is the action force. The water then pushes you forward, this is the reaction force.
Balloon:
If you are holding the end of a balloon that has been blown up and you let it go, the air from inside the balloon will be released out of the opening, this is the action force. The air around the balloon will push back and send the balloon flying in the opposite direction to the action force. [1]
Driving:
When you are driving a car, the tyres push the road back, this is the action force. The road then pushes the car forward, this is the reaction force. [6]
How Does Newton's 3rd Law Apply to Rockets?
Newton's 3rd law can help explain why the rocket stays at rest when it is sitting on the launchpad. From Newton's 1st law, we know that when the forces are balanced, an object will remain at rest.
For the rocket the action and reaction forces are:
-
Action force - gravity pushing the rocket down to the ground
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Reaction force - the ground pushing back up on the rocket
These two forces are equal in size but they are acting in opposite directions. Since the forces are the same size, we know that the rocket will stay at rest.
From what we have learned in the 'Newton's 1st law' section, we know that for the rocket to be launched we must apply an unbalanced force, otherwise the rocket will stay at rest on the ground.
The unbalanced force here, comes from the thrust. The force of thrust comes from switching on the rocket's engines. [1]
When the engines are switched on, gas is pushed out from the bottom of the rocket. The rocket exerts a downwards force on the gas, this is the action force. The gas then exerts an upward force on the rocket, this is the reaction force. These forces are the same size but they act in opposite directions. [4]
For the rocket to launch into the sky, the upwards reaction force due to the thrust must be larger than the downwards force due to gravity (the weight of the rocket). When the thrust is larger than the weight, the forces become unbalanced and since the the thrust is the larger force, and this is acting upwards, the rocket will accelerate upwards into space! [4]
This graphic shows what we have discussed above. The force of thrust is larger than the weight, therefore the forces are unbalanced and the rocket launches upwards, since this is the direction in which the force due to the thrust is acting.
Newton's 3rd Law Simulation
This simulation shows you how Newton's 3rd law can help us understand how a rocket launches!
Rocket Launch Game
Now that you have learned all about how a rocket launches, go ahead and try the rocket launch game!
You can build your very own rocket and launch it into space!
Hint: If you are struggling with the question in the game, remember what we have learned from Newton's 2nd law:
f = m x a
If you are still struggling, click the "skip question" button to skip to the rocket launch!