Newton’s first law of motion states that an object at rest stays at rest, and an object in motion stays in motion, until acted upon by an external force. As you have learned, the resistance of any object to a change in its motion is called inertia, so it’s not surprising that this first of the Newton principles is also known as the “law of inertia.”
Watch the following animation, which describes this law, using baseball plays as examples.
As you watch the segment, you may want to check the definitions of specific terms. In addition to inertia, the terms unbalanced force, friction, and gravity will be mentioned.
Stop the animation at any time, watch it again, or read the transcript.
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Manager:
The first part of Newton’s law of inertia is pretty straightforward. An object at rest will stay at rest, unless and until it comes in contact with an unbalanced force.
That means that this baseball isn’t going anywhere, unless it encounters another force. Say a wind gust knocks it over, or someone picks it up, like this.
So the ball stays at rest, until someone or something moves it, or hits it.
The second part of the law—an object in motion will stay in motion, until and unless it comes into contact with an unbalanced force—might take a bit more imagination.
There are many unbalanced forces in baseball: friction, gravity, air currents, to name a few. For this example we can use friction. A grassy infield has a great deal of friction, since grass growing out of dirt is rough and can be a little bumpy.
So if I hit a ground ball, the friction from the field is gonna stop the motion, pretty quickly.
Okay, let’s say we were playing this game on ice.
Ice is a smooth surface and slicker than grass and dirt, so it has a lot less friction, though it still has some.
Less friction, so the ball goes farther before it stops.
So what if I hit this ball, but in a place where there is no unbalanced force at all—no gravity, no air currents, and no friction?
With nothing there to stop it, the ball’s inertia continues, and the ball stays in motion, forever.
In the space below, write one or two sentences in which you apply Newton's first law to another sport, such as football, hockey, or soccer, or to another real-life situation. You can choose such actions as moving a heavy object across the floor, skateboarding down a hill, or any another activity that involves forces and motion.