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Penny drop experiment step 5

British Science Week: Penny drop science experiment

Penny drop experiment step 5

If you’ve ever wanted to put a scientific theory into practice at home then now’s your chance. Sir Isaac Newton’s first law of motion helps explain the motion of conventional physical objects and systems. He implies that any object at rest will remain at rest unless an unbalanced force acts upon it.

So, if you were to place a tennis ball in space and give it a bit of a shove, its momentum will keep it moving at the same speed and in the same direction unless something bumps into it, and if left untouched its inertia will keep it in the same place.

On Earth, however, Newton’s law is seemingly complicated by the permanent forces of gravity and friction, the former constantly pulling them towards the ground while the latter slows them down. Does this disprove Newton’s law? No, far from it. In fact, these forces help demonstrate its high probability, as can be seen at home in a simple and easy-to-construct experiment. Let’s get started…

What you’ll need:

1 sheet of card
1 glass jar
1 cup of water
1 penny
1 pencil
1 selection of other coins of various sizes
Sticky tape

Step 1

Penny drop experiment step 1
Penny drop experiment step 1

Take your card and cut it into long thin strips vertically roughly 2cm wide, then tape the ends together so it forms a hoop. This experiment works best when the hoop is 8-10cm across. However, for variables to the experiment, take another two strips and make one smaller hoop and one larger.

Step 2

Penny drop experiment step 2
Penny drop experiment step 2

Next, take your glass jar and fill it with water roughly 2/3rds up. The water adds an extra level of data return, as we shall see later, so it is best used.

Step 3

Penny drop experiment step 3
Penny drop experiment step 3

Third, put your water-filled glass on a level surface and then place the hoop on top of it, so that it radiates out from the centre of the jar like the face of a fan. Finally, place your penny on top of the hoop so it is directly above the glass jar. The card hoop should support the penny and maintain its form if done correctly. If the hoop deforms, you need thicker card.

So how does this relate to Newton’s first law? Well, currently the penny is at rest, its inertia keeping it in the same place. Gravity, one of Earth’s meddling forces, is also being counteracted by the hoop, which itself is fixed in position by the neck of the jar. With gravity taken out of the equation and friction negligible, Newton’s law is currently ringing true.

Step 4

Penny drop experiment step 4
Penny drop experiment step 4

Okay, action time. Take your pencil and hook it through the hoop. Now move your hand so the pencil is hovering by either the right or left side of the hoop at is equator. Now in one swift movement, whip the hoop to the side – just like a waiter whipping a tablecloth away – and watch the results. If you have performed this step correctly the penny should drop straight down and land in the glass of water, eventually resting at its bottom. If this does not happen – ie, the penny falls to one side of the jar – try again with a faster hand movement.

Step 5

Penny drop experiment step 5
Penny drop experiment step 5

Right, before anything else, repeat the last step but this time use either the smaller or larger hoop, or a smaller or larger coin. If replicated correctly, you should notice how the success rate of the coin dropping straight down into the jar when using the larger hoop/coin is less than before, while greater if using the smaller hoop/coin. Finally, notice how the coin’s speed decreases as it travels through the water – this shows an increase in friction over Earth’s standard atmosphere.

When the original hoop was whipped away, the force counteracting gravity was removed, allowing it to exert its influence on the coin. The speed of the hoop’s withdrawal also mitigated the effects of friction on the penny’s centre of mass. Consequently, the coin was left suspended in the air with just the force of gravity to pull it down in a straight trajectory into the glass. If the coin were in the vacuum of space, however, with no gravitational force impressed upon it, this would not happen.

The increased/decreased contact area between the hoop and the coin affects the level of trajectory-altering friction, with the larger hoop inflicting more and the smaller one less. Consequently, if there did not have to be any contact between the coin and the hoop, there would be no physical friction – atmospheric drag remains though – and the coin’s straight course would not be altered.

Conclusion

While Newton’s first law of motion may initially seem inconsistent with our experience on Earth, those experiences are in fact consistent. Simply put, an object will remain stationary or moving in a straight line, providing no auxiliary forces act upon it. For us on Earth, though, any object (including human beings) will always naturally be impressed upon by the forces of gravity and friction.

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