02 Jul What do you need to know about Forces & Newton’s Laws of Motion?
A force is just a push or a pull
There are lots of ways to push or pull:
- Gravity pulls objects down toward the ground. We call that force the weight of the object.
- A string or rope can pull on an object.
- Springs can do that, too. That’s tension.
- One object can push on another. That’s a contact force. We’ll be particularly interested in the normal force (or the reaction force).
- An object might float in liquid. The object is supported by a force called buoyancy, or upthrust.
- Electric charges attract or repel each other. That’s the electric force.
- Air resistance opposes motion through the air. That’s called a drag force.
- Friction opposes the motion of one object sliding against another. We have two main kinds of friction force: static and kinetic (or dynamic)
Even though there are a lot of ways to push or pull, there are only four FUNDAMENTAL forces in physics. “Fundamental” means we don’t have a deeper explanation for them. The four are:
Strong nuclear force
Weak nuclear force
You have only ever had direct experience with the first two. EVERY force you have EVER experienced or witnessed is one of these two.
You’ve never directly seen the nuclear forces because they operate only inside the nucleus.
But wait: where’s tension, and friction, and contact force on this list?
Those are all instances of the electromagnetic force!
When a solid object pushes against another object, at the atomic level what’s really going on is that the negative electrons of the atoms of one object are repelling the negative electrons of the atoms of the other object. The reason a string can exert tension is because its molecules are held together by electric forces. Drag and friction also arise because of the interactions of atoms bumping into each other, by which we mean experiencing electron vs. electron repulsion.
In high school physics, we’ll learn a lot about gravity and the electromagnetic force, and a little about the other two.
Newton’s Laws of Motion: How to remember which law is which
1) The pen on my desk is not going to move unless I push on it. That’s inertia.
2) If I do push on it, the acceleration will depend on how hard I push and how much mass the pen has. F = ma.
3) However hard I push on the pen, the pen will push back on me. Equal and opposite reactions.
There is no difference between uniform motion and being at rest. Motion is relative. That’s why “an object at rest will stay at rest” is THE SAME AS “an object in uniform motion will stay in uniform motion“. Rest is uniform motion.
Third law force pairs are on different objects, not the same objects. That’s why they don’t “cancel out”.
Third law force pairs have to be the same type. Gravity and gravity, contact force and contact force, electric force and electric force.
An object at rest on a table has a normal force equal to its weight. That’s an example of the second law, F=ma. The forces add up to zero so the acceleration is zero. It is not an example of the third law.
It’s true in this case that the normal force is equal to the weight, but it does not have to be. If the table breaks, the normal force will be less than the weight and the object will accelerate downward, but still the third law will hold: the object will be pushing on the table just as hard as the table pushes on the object. Both of those forces will be less than the object’s weight! What is the third law pair for the weight of the object? It’s the pull of the object on the earth.
Understanding Newton’s First Law (Law of Inertia)
European Space Agency video aboard International Space Station:
Veritasium: Why Does the Earth Spin?
Veritasium: How Does the Earth Spin?
Newton’s Second Law (F = ma)
ESA video aboard ISS
Newton’s Third Law (equal and opposite reaction force pairs)
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