A person stands on the edge of a high cliff that is next to the sea. The person throws a stone vertically upwards. Air resistance acts on the stone.
The stone eventually hits the sea.
Which velocity–time graph best shows the motion of the stone from when it is released until it hits the sea?
ANSWER: B
The Physics behind the answer:
- A very important note here is that air resistance acts on the stone.
- Since air resistance is acting, then the stone is experiencing a decreasing resultant force.
- A decreasing resultant force means that the stone's acceleration is also decreasing.
- In a velocity-time graph, the acceleration is the gradient of the graph.
- Among the 4 options, B shows a deceasing gradient.
Other concepts that are worth looking at:
- The options don't use the Cartesian plane as a convention for directions. It looks like, in this case, the negative y-axis represents upwards direction.
- If the air resistance is negligible, then the acceleration of the stone is constant and the answer would have been A.
- If the graph is a speed-time graph [magnitude only i.e. positive values] instead of a velocity-time graph, then the answer would have been D. You may want to compare the second portion of the graph when the y-axis value is zero between option D and B.
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