The three standard equations for uniformly accelerated motion use the variables…
PHYSICS · Multi-part · A Level · Physics · Mechanics and materials · Mechanics and materials
Problem
The three standard equations for uniformly accelerated motion use the variables displacement, velocity, acceleration and time. A classmate often gets confused about how to assign positive and negative signs while applying these equations.
Answer the following in the context of one–dimensional motion along a vertical line (upward taken as positive):
(i) Explain, with reason, why the numerical value of the acceleration due to gravity has a negative sign in this convention.
(ii) A ball is projected vertically upward, rises to its highest point and returns to the point of projection. Describe qualitatively how the velocity changes during ascent and descent, indicating the sign of velocity in the two parts of the journey.
(iii) During the above journey the average speed is not zero but the average velocity is zero. Justify this statement using suitable reasoning based on displacement and distance.