Preserved fossil<span> (like a fossil in amber, ice or tar.</span>
Answer:Speed can be defined as
Explanation: B because if you think about it, 50 milers per hour means they are dividing the time it takes to drive a certain distance going that speed. So id has to be answer B! your welcome. Please upvote only if its correct!
Since they are rowing against the current, their velocity realtive to the groud is 20-1.5 = 18.5km/h
So the distance travelled is 18.5km/h*0.5h = 9.25km
Going back with the current their ground realative velocity is 20+1.5 = 21.5km/h so the time it takes them to return 9.25 km is 9.25km/(21.5km/h) = 0.43h ~= 26 min
Assuming it starts at rest, the roller coaster only has potential energy at the top of the hill, which is
When it reaches the bottom, its potential energy will have converted to kinetic energy,
where 
is its velocity at that point. By the law of conservation of energy, assuming no loss of energy due to other sources (e.g. sound, heat), we have
