Whisper wrote:
On 17/05/2021 10:12 pm, wrote:

Sorry didnt read it.

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[What everyone knows about science:]
It's movie time in Science Class.
Popeye sits in the back of a boat with an electric fan
playing on the sail, apparently moving the boat forward.
The science teacher says this is of course impossible
because the momentum imparted to the air by the fan is "cancelled"
when the sail captures the air. The boat cant be propelled by the method
shown in the cartoon.
But the science teacher has swallowed a simplified version
of the science and overlooked something.
If you sit in the back of the boat and throw a small ball mass m toward
the sail at velocity v the boat is pushed backward with change in momentum mv
and the ball moves forward with momentum -mv.
If the ball then "sticks" to the sail then that momentum mv is absorbed and
the net momentum of the boat returns to 0 and the boat stops moving.
But if the ball hits the sail and bounces off backwards with speed v
then the change in momentum is 2mv and the net momentum imparted to
the boat is mv.
Popeye is right, the science teacher is not.