Good old Isaac Newton developed a law or two that we use to describe motion in physics.
Newton’s First Law of Motion:
Objects in motion tend to stay in motion, objects in rest tend to stay at rest.
Basically, Newton claims that objects are stubborn - they like to keep doing what they’re doing. If they’re moving, they want to stay moving; if resting, they want to stay at rest.
The resistance to a change in motion is called inertia.
The greater the mass of an object, the greater the inertia; the lower the mass, the lower the inertia.
So the motion of lower mass objects will be easier to change.
Examples of inertia and Newton’s 1st Law are everywhere – consider what happens when you slam on the brakes of a car…inertia causes you to slam forward as the car stops.
Notice the motion of the crash test dummy in the video above. When the cart crashes, it stops because it reaches a barrier. Inertia causes the dummy in the cart to continue moving.
Real world connection: as you can see, inertia is the main reason we use safety belts in automobiles. In an accident without them, your inertia could cause you serious injury (e.g.: flying out of the front window!).
Newton’s Second Law of Motion:
Formally: the acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.
Otherwise: For an object with a specific mass, the more force is applied to it, the greater it will accelerate.
Also, when a set force is applied to two objects of different masses, the object with the lowest mass will accelerate the greatest.
This law is expressed mathematically as F = m/a, which can be conveniently (and algebraically!) rearranged to find the acceleration of a object given force and mass (a = F/m), and the mass of an object involved in a motion (m = F/a).
Newton’s Third Law of Motion:
Every action has an equal and opposite reaction.
This statement means that in every interaction, there is a pair of forces acting on the two interacting objects.
The size of the forces on the first object equals the size of the force on the second object.
The direction of the force on the first object is opposite to the direction of the force on the second object.
In other words: all forces act in pairs.
The pennies in the video below hit the desk with a downward force. The desk also pushes back up on the pennies with equal force. Thus, the bounce.
How this works for you: every time you push on a door to open it, the door pushes back on you with equal force. The reason why this doesn’t appear to be so is because your mass is combined with the mass of the building during the push. In space, however, it’s another story (see the video).
Often, we have difficulty working with physics concepts because of the “reality” factor. On earth, many of the concepts we cover are “sticky” because forces like friction and combined masses of objects muddy concepts. In space, however, pure physics concepts are reality, as in the video above.
