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[BL] Review Newton’s First Law. Explain that the property of objects to maintain their state of motion is called inertia.
[OL]
[AL] Take two similar carts or trolleys with wheels. Place a heavy weight in one of them. Ask students which cart would require more force to change its state of motion. Ask students which would stay in motion longer if you were to set them in motion. Based on this discussion, have students speculate what inertia may depend on.
[BL]
[OL] Explain the concepts of mass and weight. Explain that these terms may be used interchangeably in everyday life but have different meanings in science.
Inertia is the tendency for an object at rest to remain at rest or for a moving object to remain in motion in a straight line with constant speed. This key property of objects was first described by Galileo. Later, Newton incorporated the concept of inertia into his first law, which is often referred to as the law of inertia .
As we know from experience, some objects have more inertia than others. For example, changing the motion of a large truck is more difficult than changing the motion of a toy truck. In fact, the inertia of an object is proportional to the mass of the object. Mass is a measure of the amount of matter (or “stuff”) in something. The quantity or amount of matter in an object is determined by the number and type of atoms it contains. Unlike weight (which changes if the gravitational force changes), mass does not depend on gravity. The mass of an object is the same on Earth, in orbit, or on the surface of the Moon. In practice, it is very difficult to count and identify all of the atoms and molecules in an object, so mass is usually not determined this way. Instead, the mass of an object is determined by comparing it with the standard kilogram. Mass is therefore expressed in kilograms.
This video contrasts the way we thought about motion and force in the time before Galileo’s concept of inertia and Newton’s first law of motion with the way we understand force and motion now. Refer to [link] as you watch.
In this simulation, you will first explore net force by placing blue people on the left side of a tug of war rope and red people on the right side of the rope (by clicking people and dragging them with your mouse). Experiment with changing the number and size of people on each side to see how it affects the outcome of the match and the net force. Hit the Go! button to start the match, and the “reset all” button to start over.
Next, click on the Friction tab. Try selecting different objects for the person to push. Slide the applied force button to the right to apply force to the right and to the left to apply force to the left. The force will continue to be applied as long as you hold the button down. See the arrow representing friction change in magnitude and direction depending on how much force you apply. Try increasing or decreasing the friction force to see how this affects the motion.
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