In air, the guinea and the feather fall at different speeds due to air resistance. To show this, turn the tube upside down so both items fall from one end of the tube to the other. As you will see, the guinea will fall much faster than the feather because it is affected less by air resistance. (The feather has a greater surface area which comes in contact with the surrounding air).
Turn the vacuum pump on and expel as much air from the tube as is safely possible. When you have done this, tighten the Hoffman clip and turn off the pump. Inside the tube you should now have a near vacuum. Turn the tube upside down once again and you will see that the guinea and feather arrive at the other end at the same time. With no air in the tube, both objects fall at the same speed as there is no air present (therefore no or negligible resistance) to affect their descent.
When finished, carefully unscrew the Hoffman clip fully to let air into the tube again, only then will it be safe to remove eye protection.
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