Finding the Right Words
One skill that it is important to master is that of using words to describe what is happening to a system, or to interpret the values calculated from an equation. Physics is not just about ‘getting the right answer’...
In this exercise, we give you practice at expressing graphs in words. In each of the following three cases, you are asked to decribe the motion of the physical system implied by the graph; the alphabetical labels are there as reference points.
System : Object being moved Plot : gravitational potential energy U(x) versus time (t) from some reference point System : Ball being thrown vertically upwards Plot : velocity versus time System : 2 cars. You, represented by the blue line and a police patrol car, the red line. Plot : distance (from some origin) versus time
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Solution
- Decrease in gravitation potential energy (A-B) as object is lowered. B-C is no change in the object’s p.e., faster rate of change of grav p.e. (C-D) returns object to original height.
- Ball thrown up at 4ms-1, reaches maximum height at B, falls back to below launch point and rebounds off the ground inelastically at C, rises back up past launch point, stationary again at D and then stopped at ground level on the way down (E).
- When you set off, at A, and travel at constant velocity, the police car is ahead of you. You pass it at B, and at C it moves off and catches up with you at D, goes past you and then stops. You pass it again at E.