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Often known as Newton's laws of motion, these tell us how objects move.
Things behave strangely when in deep space - far away from any gravity or atmosphere. Well, it seems strange to us 'Earthlings'. That's because our brains expect objects to fall to the floor, and to be slowed by the air, or blown by the wind.
But the way things behave in space tells us the true behaviour of objects - without those confusing effects of gravity or air.
An astronaut (on a space walk) letting go of a ball would see it just float in front of them.
If they throw the ball they will see it fly off in a straight line and keep moving at a constant speed without stopping.
Our experiences in daily life (on Earth) give us a false idea of the laws of motion. Objects don't automatically fall to the floor, and don't always have to come to a stop.
First law
First law
An object remains at rest, or continues to move at constant velocity unless acted upon by a force.
Let's think about that astronaut on a 'spacewalk', a long way from Earth's gravity.
If he lets go of the ball it will just remain in place (at rest), floating in front of him.
If he throws the ball it will fly off in a straight line, and stay at the same speed forever, or until another force acts upon it.
Second law
Second law
The vector sum of the forces on an object is equal to the mass m of that object multiplied by the acceleration a of the object: F = ma
F = ma is an equation we have seen before.
'The vector sum' just means we need to add up all of the forces to find out the strength and direction of the total force, which then tells us the amount and direction of acceleration.
Third law
Third law
When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.
Let's think about our astronaut again. This time he's connected to another astronaut by a wire. Let's assume both astronauts are the same mass.
What happens when he pulls on the wire?
Does one astronaut stay put and the other one move?
No, actually we would see that they both get pulled towards each other, meeting in the middle.
This shows that any force he applies to the other astronaut, is also applied to him.
The same is true if he then pushes the other astronaut away - they will both move at the same speed in opposite directions.
Example
Example
To show these laws in action, let's play a game (below) where we control a spaceship. Press and hold 'Fire Engines' for a few seconds. The rocket motor pushes a jet of hot gas out of the back of the ship, this applies an equal forward force (thrust) to the ship (due to the 3rd law). The thrust causes the ship to accelerate at a constant acceleration (since F = ma, the 2nd law).
As long as you hold down the button the ship will continue to accelerate, i.e. building speed. When the button is released, the acceleration stops and the ship will then move at a constant velocity (the 1st law), with no sign of slowing down.
If we want to stop the ship we need to rotate the ship by 180 degrees and apply thrust in the opposite direction. See if you can make it come to a complete stop. You'll notice that the thrust time needed to slow down the ship exactly equals the thrust time used to speed it up.
This kind of motion seems very strange to us 'Earthlings', but it is really the way things behave when we don't have to worry about gravity or friction.
The 'two astronauts' and the 'spaceship motion' questions are examples of 'thought experiments'. Physics students should use thought experiments to strip away anything which complicates the problem - simplifying it to get a better understanding of the physics involved.
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