PPT ActivityInertia

Developer Notes

Need drawings/directions for the ramp demo.

We may want to have the vocabulary and definitions on a separate sheet somehow from the activity sheet so the students don’t just look them up instead of thinking about them.

Goals

Students should understand that motion is relative. That your reference point can change.

Students should understand and get a feel for inertia.

Students should understand that mass and inertia are equivalent.

Concepts & Skills Introduced

Area / Concept
physics / motion is relative
physics / Newton’s 1st Law of Motion – inertia
physics / mass measures inertia

Time Required

min

Warm-up Question

Place a rock in the middle of the floor. The warm-up question: Describe the rock’s motion.

Presentation

As an introduction to the dynamics unit, note that in the motion section we were able to describe the way things moved - speed, acceleration. In statics, we talked about their strength. Here in dynamics we’re going to look at WHY things move.

Start with the warm-up question. Have a short discussion. Most of the kids will be stumped.

Answers could include: It’s not moving. Gravity is pulling it down. The floor is holding it up. The whole earth is moving. There’s friction. Nothing is making it move.

Ask What is motion? Get the class to define it. The definition should be something like a changing distance between two things. So if the distant isn’t changing, that’s still motion, it’s just zero motion, analogous to 0 being a number, just not positive or negative.

So, what is changing distance with the bowling ball? The moon, the sun, the cars on the street. Walk past the ball. Is the ball moving relative to you? Or you relative to it? This a rich area for discussion.

  • If one person is standing still, and another walks by, who is moving?
  • If two people are walking, who is moving?
  • If two people are riding in cars going the same speed and direction, are they moving?
  • If you go up the down escalator, are you moving? How about someone going up the up escalator white you’re going up the down escalator – who’s moving?
  • Does Earth move? Of course not! Are you crazy? Obviously it doesn’t move.
  • Have you ever looked up at a building and it looks like it’s moving because of the clouds?
  • Have you ever sat at a stop light and suddenly felt like your car was moving, but it turned out to be the car next to you that was moving?

It all depends on your reference point!

Motion is relative. If I ask you how fast something is moving, the answer is “Relative to what?” There’s a good social lesson here, too. You need to be able to look at things from someone else’s point of view.

Fun question: Is there ever a case of zero motion?

Perform the demonstrations shown.

Have the students write answers to the summary questions in groups, then have them discuss and define what they’ve discovered as a class. Remind them of the bowling ball in the middle of the floor, too. When they’ve come up with a definition, give them the term inertia. Compare their definition to Newton’s statement of the first law, then to the definition of inertia.

Friction may come up somewhere in the discussion. That’s good, tell them they’re on exactly the right track, but they should make this an ideal situation and ignore the friction (which we’ll discuss in the next lesson).

Do the exercises.

Introduce the definition of inertia and Newton’s 1st. Now that we know what inertia is, we can analyze it some more. Basically, things don’t want to change how they’re moving.

Things don’t want to start moving.

  • Put an index card over a cup, with a penny on it. Flick the index card out, and the penny will drop in the cup.
  • Kick a bowling ball to get it rolling. (Don’t actually do this! Maybe ask a student to, but don’t let them. The idea is to get the feel that the bowling ball is hard to get moving.)
  • From Paul Hewitt – Form a wire coat hanger in an M shape, put a clay ball on each end, then put it on your head so you’re looking at one ball. Spin around quickly and you’ll be looking at the other one.
  • Pull a tablecloth out from under some dinnerware (I haven’t actually done this).
  • Compare to pulling a bag off the bag roll at the supermarket – the roll doesn’t want to start rolling.

Things don’t want to stop moving.

  • Roll a bowling ball slowly toward a table leg, or a block in the floor. If you’re comfortable with it, roll it toward someone’s feet – the kids will get a better feel for what’s happening.

Things don’t want to change direction.

  • Again, roll the bowling ball toward something.
  • Throw a ball.
  • Shoot a dart – why don’t they just keep going?

That’s inertia. Objects want to continue their state of motion (or non-motion).

Assessment

Writing Prompts

Relevance

Summary

Exercises

Challenge/ extension

Background / History

Remember that scientists try to simplify principles to their ideal form so that they can understand them. That is exactly what Galileo did in his experiments. Some of them were done qualitatively, with a great deal of insight, just from thinking. Newton took the principle that Galileo discovered and made it the first of his three Laws of Motion.

Problem

Your teacher will demonstrate some of the experiments that Galileo ran. See what you can figure out logically about the ideal motion of objects.

Materials

1ramp

1steel ball

Procedure

  1. Answer the questions in your lab book.
  2. Height
  3. Roll a ball down one ramp and up another, steeper one. How high does the ball go?
  4. Make the second ramp a little less steep. How high does the ball go?
  5. Make the second ramp even less steep. Now what?
  6. Time
  7. Roll a ball up a steep ramp. How quickly does it stop?
  8. Roll a ball up a less steep ramp. Does it take more or less time to stop?
  9. Make the ramp even less steep. More or less time?
  10. Speed
  11. Roll a ball up a ramp. As it goes up the ramp, what happens to its speed?
  12. Roll a ball down a ramp. As it goes down the ramp, what happens to its speed?

Summary

  1. Height – What happens to how high the ball goes as the ramp gets less and less steep? If the second ramp were level and long, how far would the ball go?
  2. Time – What happens to the time as the ramp gets less and less steep? What must happen as some point if the pattern continues? If the ramp were level long, when would the ball stop?
  3. Speed – What happens as the ball goes up? Down? Logically, what must happen somewhere in between? If the ramp were level long, what would happen to the ball’s speed?
  4. What do these tell you about the ideal motion of objects?

Exercises

  1. If you’re riding in a car and you decelerate quickly, which direction does your body go? Why?
  2. When the light turns green and the car accelerates, which direction does your body go? Why?
  3. When you turn a corner in a car, which direction does your body go? Why?
  4. If you’re riding along on a bicycle and you quit pedaling, what happens? Why?
  5. If you threw a ball into the air, and there was no gravity and no air resistance, what would happen to the ball?
  6. When you start running, why is it hard to get going?

Challenge/ extension

Vocabulary

Inertia – the property of matter which resists changes in motion. Inertia is measured by mass.

Newton’s 1st Law of Motion - Every object continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it.

dynamics act inertia 030130 dk03.doc1Printed: 2/2/20199:41 AM