There's a one-question physics test that lets me know if we're making progress. It's the last question of Unit 3. "Consider the ball toss carefully. Assume that upward is the positive direction. Indicate in the table that follows whether the velocity is positive, zero or negative during each of the three parts of the motion. Also indicate if the acceleration is positive, zero or negative."
Why?
Learning physics requires revising preconceptions we come to the class with. The meaning of acceleration illustrates one such preconception. As commonly used, the word acceleration means moving faster. The reasoning goes like this:
If you're not moving then you can't be accelerating. Zero velocity means not moving doesn't it? — therefore if v = 0 then a = 0.
Right?
Wrong!
The precise definition of acceleration in the physics context is the rate of change of velocity: a= dv/dt
The velocity can be instantaneously at zero, but if the velocity is changing then acceleration is not zero.
If you understand this then you're changing your preconceptions based on your physics lessons. If not then either you're not listening, or you hear but don't want to accept something that violates your sense of what's right.
The reason we need to get this is that forces lead to acceleration and physics is all about forces and what they do to things.
If you think you got it then try this question:
Mike jumps out of a tree and lands on a trampoline. The trampoline sags 0.5 m before launching Mike back into the air. At the very bottom, where the sag is the greatest, Mike’s acceleration is:
A. Upward B. Downward C. Zero
Update For answer click here.
A) Upward
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