Answer:
Displacement is the shortest distance travelled from the initial point to the final point. After half revolution the displacement is the length of the diameter which is equal to 2r. Hence the displacement is 2r.
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The first experiment that was made to attempt to measure the speed of light involved detonating gunpowder by Isaac Beeckman. He declared that his experiment was inconclusive. Galileo also tried to measure the speed of light using two lanterns placed across each other. The next experiment involved planets where Ole Romer based his calculation on its movements. The final calculations were derived from different theories by different scientists including Maxwell until it ended up with the exact value for the speed of light.
Answer:
1) 
2) 
3) 

Explanation:
Given:
width of river, 
speed of stream with respect to the ground, 
speed of the swimmer with respect to water, 
<u>Now the resultant of the two velocities perpendicular to each other:</u>



<u>Now the angle of the resultant velocity form the vertical:</u>



- Now the distance swam by the swimmer in this direction be d.
so,



Now the distance swept downward:



2)
On swimming 37° upstream:
<u>The velocity component of stream cancelled by the swimmer:</u>



<u>Now the net effective speed of stream sweeping the swimmer:</u>



<u>The component of swimmer's velocity heading directly towards the opposite bank:</u>



<u>Now the angle of the resultant velocity of the swimmer from the normal to the stream</u>:



- Now let the distance swam in this direction be d'.



<u>Now the distance swept downstream:</u>



3)
Time taken in crossing the rive in case 1:



Time taken in crossing the rive in case 2:



Gravitational is the force of 2 object or body like the sun a earth and gravity is the force on 1 object or body for instance an apple falling