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Three Laws of Motion · Acedra
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Three Laws of Motion
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0:16
There are three laws that govern the motion of everything in the Universe.
0:32
The first law states that a body at rest will stay at rest…
0:38
and that a body in motion will stay in motion.
0:51
Without a force, all objects will continue moving in the same direction with the same speed forever.
1:05
Objects here on Earth slow down only because of the force from friction and air resistance.
1:21
The second law states that objects change their motion when a force is applied.
1:55
An object’s change in motion is the strength of the force divided by the mass of the object.
2:12
This means that it is harder to change the motion of objects with more mass.
2:40
When all forces are removed, the object’s speed will stay constant.
2:53
If we watch the previous scene again, we will see that the moment Victoria exerts a force against the train, the train instantly also exerts a force on her.
3:08
This is what causes Victoria to move backwards.
3:18
The two forces are exactly equal in strength, and they are in exactly opposite directions.
3:36
The third law states that for every action, there is an equal and opposite reaction.
4:02
When Victoria’s feet move against the Earth, she is creating the force pushing her forward.
4:10
Victoria’s feet moving against the Earth also creates an equal and opposite force pushing the entire Earth in the opposite direction.
4:27
But, because the mass of the Earth is so large, the impact of this force on the Earth’s motion is extremely small.
4:38
For larger forces, the impact on the Earth’s motion is no longer negligible.
4:48
According to the first law of motion, the Earth will want to continue moving in a straight line.
4:58
According to the second law of motion, the gravitational force from the sun will want to cause the Earth to fall directly into the center of the Sun. Note: In Einstein’s Theory of Relativity, gravity is not a force but a curvature in Space Time. This will be discussed later in this video.
5:21
According to the third law of motion, the Earth will exert an equal and opposite gravitational force on the Sun.
5:31
Since the Sun’s mass is much bigger than the Earth’s mass, the impact on the Sun’s motion will be negligible, but the impact on the Earth’s motion will be significant.
5:44
The first law of motion wants the Earth to continue moving in a straight line.
5:54
The second law of motion wants the Earth to fall directly into the Sun.
6:02
The result is that the Earth follows a path in between these two options.
6:22
This is what causes the Earth to orbit the Sun.
6:37
Just as the Earth’s forward motion prevents it from falling directly into the sun, this is also what allows a roller coaster to travel upside down.
7:07
The ball’s forward motion allows it to continue traveling upward, against the force of gravity.
7:21
If the ball’s forward motion is fast enough, the ball will continue moving along the track, and gravity will not have enough time to cause the ball to fall off.
7:37
Suppose we apply a force to a rotating object.
7:53
Each point on the object will want to continue moving in the same direction as before.
8:04
These two points will cause the object to move like this.
8:22
And these two other points will cause the object to move like this.
8:43
The events shown here all happen at the same time, and this is the result.
8:54
This means that when we apply a force to a rotating object, its rotation is changed to look like this.
9:10
This creates the illusion that the force was really applied 90 degrees ahead in the rotation.
9:28
This has profound implications for our everyday lives.
9:37
Let us consider the rotating wheel of a bicycle.
9:45
When Victoria leans in one direction, this creates a force on the wheel.
9:55
However, because the wheel is rotating, the result is that it will appear as if the force is applied 90 degrees ahead in the rotation.
10:11
This will cause the entire bicycle to rotate like this.
10:23
This is why leaning causes a bicycle to change direction.
10:33
This is also why objects that normally fall over almost instantly…
10:44
are easy to balance when they are spinning.
10:53
This happens because the gravitational force always appears to be applied 90 degrees in advance of the direction in which the object is about to fall.
11:14
But this will not work if the object stops spinning.
11:25
This is the reason why it is easy to balance a bicycle when the wheels are spinning rapidly, but nearly impossible to balance when the bicycle is standing still.
11:38
The purple sphere at the North Pole is standing still.
11:53
If we apply a force, and ignore friction and air resistance, the purple sphere will follow this path.
12:06
Since the earth is rotating relative to this path, the sphere’s path will look like this to people on the ground.
12:20
The spheres at the equator are moving faster than all the other spheres.
12:34
If we apply a force away from the equator, the first law of motion will want the sphere to keep moving around the earth faster than the other spheres.
12:53
From the perspective of someone on the ground, the sphere will appear to veer towards the east.
13:14
Therefore, objects moving from the equator to the North Pole will veer in the opposite direction as objects moving from the North Pole to the equator.
13:34
Suppose we have a low pressure area, and the air molecules around this area experience a force towards the center.
13:50
The air molecules experiencing a northern force will veer in the opposite direction of the air molecules experiencing a southern force, causing a rotation.
14:05
This is the reason why hurricanes rotate counterclockwise in the Northern Hemisphere, and clockwise in the Southern Hemisphere.
14:23
All the laws of motion are always riding with us, including the third law.
14:40
As the helicopter exerts of force on the blades, the blades instantly exert an equal and opposite force on the helicopter.
14:51
This causes the helicopter to want to rotate in the opposite direction of the blades.
15:05
This is why a tail rotor is needed.
15:14
The force from the tail rotor can be adjusted to cause the helicopter to turn left or right, or to keep the helicopter pointed in a steady direction.
15:33
The tail rotor exerts a force on the helicopter by creating an equal and opposite force on the air molecules.
15:54
This is the same way in which the main rotor creates an upward force against gravity.
16:06
Birds and airplanes also fly by creating a force against the air.
16:22
The upward force against gravity is caused by the way air flows around the wings.
16:32
Suppose that the air around a wing is standing still.
16:45
The air pressure from above and below the wing will cancel out, and there will be no net force.
16:59
As air flows around the wing, the wing’s shape forces the air above the wing to flow faster than the air below.
17:18
The air pressure below the wing is now creating a greater pressure than the air above.
17:34
This creates a net upward force on the wing.
17:52
This also creates an equal and opposite downward force on the air.
18:08
Birds, airplanes, and helicopters work by pushing against the air.
18:23
But this will not work in space, where air does not exist.
18:39
In space, we need another way to create a force.
19:06
As the walls exert a force on the particles, the particles exert a force on the walls.
19:19
If there is a hole in one of the walls, then the forces on the walls will no longer cancel each other out.
19:39
This is how rockets create a force to travel through space.
19:54
Although the force of gravity gets weaker as we move further away from the Earth, this is not the reason astronauts feel weightless when in orbit.
20:08
In Earth’s orbit, the force of gravity is still very strong.
20:22
The first law of motion wants Victoria to keep moving in a straight line, and her forward motion is the only thing preventing her from falling directly into the Earth.
20:39
The first law of motion also wants the box to continue moving in a straight line, and the box’s forward motion prevents the box from falling directly into the Earth.
20:53
Victoria and the box both end up orbiting the Earth following the exact same path.
21:07
Since Victoria and the box are always moving together, this creates the illusion that she is weightless inside the box.
21:24
This is the same way in which Victoria would appear weightless if the box were to fall from the sky.
21:47
This happens because gravity is different from all other forces.
22:02
If two objects experience the same force, the lighter object will accelerate faster.
22:14
In the case of gravity, however, gravity exerts a greater force on more massive objects than on lighter ones.
22:25
This causes all objects to fall with the exact same acceleration, regardless of their mass.
22:36
Some objects fall faster than others only because of air resistance.
22:47
Air resistance affects different objects differently.
22:58
This is why parachutes work, and why feathers fall slower than bowling balls.
23:09
In space, there is no air resistance, and all objects react to gravity in the same way, regardless of their mass.
23:25
Victoria and the box therefore follow the same path and stay together.
23:41
No other force behaves this way.
23:49
Albert Einstein discovered that there is a profound reason for this.
23:58
It is because gravitational forces don’t actually exist.
24:14
Here, there is no force and Victoria is actually still just following a straight line.
24:28
Objects with mass cause a curvature in space and time, causing straight lines to appear curved to us.
24:42
This creates the illusion that there is a gravitational force.
24:53
Different objects travel through the same curved space-time.
25:03
This is why unlike all other forces, gravity causes all objects to follow the exact same path, regardless of their mass.
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