Which characteristics is common in mature rivers river

When you drop a pebble from height a it reaches the ground with speed v if there is no air resistance. from what height should you drop it so it will reach the ground with three times the speed? (a) 9m (b) 3m (c) 6m (d) 18m

Given the position vector of the particle r(t)=(t+1)i+(t^2−1)j+2t k, find the particle's velocity and acceleration vectors at t=1

If a tank filled with water contains a block and the height of the water above point a within the block is 0.8 meter, what is the pressure at point a?