1.

A stone is thrown horizontally with an initial speed of 30 m//s from a bridge. Find the stone 's total speed when it enters the water 4 seconds later. (Ignore air resistance).

Answer»

`30 m//s`
`40 m//s`
`50 m//s`
`60 m//s`

Solution :After 4 seconds, the stone's vertical speed has changed by `Delta v_(y)=a_(y t)=(10 m//s^(2))(4 s)=40 m//s`. Since `v_(0_(y))=0`, the value of `v_(y)` at t=4 is `40 m//s`. The horizontal speed does not change. Therefore, when the rock hits the water, its velocity has a horizontal COMPONENT of `30 m//s` and a vertical component of `40 m//s`.

If you recognize this as a 3 : 4 : 5 triangle that has been scaled up by a factor of 10, you can easily USE the Pythagorean Theorem to find that the magnitude of the total velocity v, is `50 m//s`.


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