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# Re: force

Joseph Bellina wrote:

It seems to me that acceleration is an observable, but you have to be
taught how to observe it, whereas force is unfortunately, a bit of a
figment of our imagination. I go for the observable first.

On Wed, 30 Oct
1996, Roger A. Pruitt wrote:

I can't comment with any personal experience, but I have a colleague who
firmly believes that students don't fully understand when there will be
accelerations and in what direction until they fully understand the net
force acting. Note that I wrote NET force. Too often the book, and we as
teachers, just say "the force acting on the object" when we should
stress that it is the net force acting.

How would you feel about starting with the concept of force? Is there
anything sacrosant about starting with kinematics other than that is the
way beginning textbooks always start.

Roger

It seems to me that force is far more observable than is acceleration. I
experience and feel forces--run into a wall or sit in a car and have it
speed up, for example. Yes, there are accelerations involved, but what I
feel are forces on me. These seem to me to directly observable--very
tangable.

Here's a problem. Drop a ball on a coil spring. At what point will the
acceleration on the ball be zero? Most students and many physicists will
say that it is when the spring is fully compressed. WRONG! It is when
the net force on the ball is zero. At that instant the ball is still
moving downward and compressing the spring.

I think there might be some advantage, therefore, to teaching forces
first then kinematics. However, a better approach might be to integrate
them more than is presently done.

Roger