|
Prerequisite knowledge and skills
|
Basic knowledge of forces and free-body
diagrams, as well as concepts of kinematics (displacement, velocity, and
acceleration). This lesson is designed to fit before Newton's 2nd
or 3rd laws.
|
|
Lesson Goals and New Content
|
Newton's 1st law. The goal
is to guide the students through an exploration, leading to a
formulation of Newton's 1st law in their own words. The
students should also realize that a change in motion of an object is
related to the forces acting on it.
The lesson also touches upon the independence of the directions of
motion, although this is not the central topic.
|
|
Procedure
|
Opener (~5 min.)
Ask the students whether they have ever tried throwing a
ball into the air while sitting in a moving car or train, and if they
know why the ball doesn't just fly to the back of the vehicle when
leaving your hand. In my experience, explanations from the
students include that the air pushes the ball along after it leaves your
hand, as well as other explanations involving forces pushing or pulling
the ball along with you. If no one suggests that, try suggesting
it yourself and see what they say. (This explanation will serve as
part of a discrepant event during the development phase, and it is in my
experience a common misconception among students at this point that
forces are needed to maintain motion, so be sure to bring it up!
If they feel stupid for suggesting this idea because they sense it might
be wrong, you can tell them that it was the prevailing explanation among
ancient Greek scholars for many centuries!). Play the role of the
unknowing, not giving away any hints as to the correct answer just yet,
and use the question to lead you into the demonstration and worksheet.
Development (~30 min.)
Distribute the worksheets and call on two volunteers. Do
part 1 with the class and give them about 5 minutes to discuss the
questions in groups of 3-5 before moving on to part 2. Note that
they should be able to see what the acceleration is based solely on
whether the ball is changing velocity or not (i.e., not by thinking of
Newton's 2nd law!). Now do part 2, and let the students
discuss the questions as before.
This is a good point for the teacher to interject for a few minutes and
check the progress of the students. Ask them as a class what they
got for their free-body diagrams, and show the correct versions on the
board. Ask whether anyone identified any forces that could push
the ball along. Their free-body diagrams should contradict
the idea of a force maintaining the motion of the ball. Do not
elaborate more on this just yet, but continue to part 3. As
before, give them 5 minutes to discuss the questions. Then do part
4, followed by another 5 minute discussion.
Closure (~10 min.)
Poll the groups to see what their explanations were for the
different parts. At this point, the class should (perhaps with a
little guidance) be able to dispel the explanation that a force is
pushing the ball along to keep it with the chair. Hear the ideas
the groups came up with. If needed, guide them towards the correct
idea by asking them whether their free-body diagrams showed any force
pushing the ball along. Together, you should reach the conclusion
that no forces were acting on the ball to slow it down, and that forces
are needed to alter its motion. At this point, ask them to
formulate this idea in their own words. (See bottom of
worksheet.) You might even give a suggestion of how you would
formulate it to get them started, depending on how well they grasped the
demonstration.
|
|
Extensions
|
Discussions of Newton's 2nd and
3rd laws are easily linked to this lesson. In
particular, it should now be familiar to the students when seeing
Newton's 2nd law that when F is 0, a is 0, and
the object is either at rest or moving at a constant velocity.
|