In the meantime, we are starting unit 2...
Unit 2: Forces
Here's what movies like Star Wars thinks a "force" is,
What are forces in real life? Watch this,
Here are the notes I gave in class:
What is a force?
- A push or a pull on an object.
- Cause objects to “move” (acceleration).
Four Fundamental Forces
1. Gravity
- weakest
- can only attract (pull)
- acts on anything with mass
2. Electromagnetism (EM)
- electricity and magnetism
- stronger than gravity
- can attract or repel (push or pull)
- holds atoms and molecules together
- most forces you feel on a everyday basis are electromagnetic!
3. Weak Force
- stronger than EM
- only affects radioactive nuclei
4. Strong Force
- strongest of forces
- holds together protons to create nuclei inside atoms.
Forces are vectors!
- have magnitude and direction
- we can represent them with arrows
- we organize forces with Free Body Diagrams (FBD)
- draw all the forces acting on an object
Examples:
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| An object falling due to gravity. |
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| An object falling due to gravity with air friction included. |
Forces add like any vectors would.
The total force on an object is called NET Force (the sum of all force vectors), ∑F.
In the above example: ∑F = F(gravity) + F(air)
NEWTON’S FIRST LAW
(Law of Inertia)
"An object in motion (or at rest) tends to stay in motion (or at rest) unless acted upon by an external force"
More precisely, we can say:
When the net force on an object is zero, the object will move at constant (or zero) velocity.
If ∑F = 0, then a = 0.
or
When an object is moving at constant (or zero) velocity, the net force is zero.
If a = 0, then ∑F = 0.
Example:
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| An object sitting on the ground. |
Using Newton's First Law, the object is not accelerating, therefore, the net force must be zero. We see that there must be a force opposing gravity, otherwise the object will fall right through the Earth!
This force is 90° from the ground, therefore it’s called the Normal Force. (Normal = 90°)
The Normal Force comes from contact with a surface (contact force).
Other common forces…
- Tension: pulling force from a string or rope.
- Friction: contact force that opposes the direction of motion.
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