Pulley Experiment

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In this new experiment we are going to experiment with pulleys. We are going to see how the force needed to lift a weight is reduced thanks to the use of pulleys. It is possibly one of the most instructive experiments for physics, as thanks to this experiment we can see how the forces required to lift a weight are reduced thanks to the use of pulleys. Newton's laws and can be used to explain them.

 

MATERIALS FOR THE EXPERIMENT

Pulleys: http://amzn.to/2yEqr3w
Hooks: http://amzn.to/2kut2rQ
Weights; http://amzn.to/2CyG144 (similar to the one used)
Dynamometer: http://amzn.to/2yF7vlj (similar to the one used)
Rope: Buy fine rope in any shop.

STEP-BY-STEP PULLEY EXPERIMENT

 

STEP 1.

dinamometro polea

In the first step we are going to measure the force needed to lift the weight we have with the dynamometer. We write down the measurement of the dynamometer (which must coincide with the mass).

In this step we take the opportunity to point out the difference between mass and weight. These are two concepts that are often confused. In fact, when we talk about our weight (in Kg) we are actually talking about our mass, since weight is a force and is not measured in Kg but in Newton (N). The reality is that on earth, weight is equal to mass times gravity (9.8m/s2, which we liken to 10m/s), hence when we talk about weight and mass on earth we often use one or the other interchangeably. For example, a mass of 300 grams would have a weight of almost 3N on Earth. On the moon, in space, as it has a lower gravity, this weight decreases for the same mass.

When we say that we weigh 55Kg, we actually mean that we have a mass of 55Kg, and the weight on earth would be 550N (55×10 gravity).

In the example we are talking about, we use a mass of 300 grams. The measurement we get with the dynamometer is 3N, which is equivalent to the weight due to gravity. This relationship between weight and mass comes from the Newton's second lawF=mxa, where F is the weight, m is the mass and the acceleration on earth would be gravity.

In this example of this simple pulley, you can also see the  Newton's third law (action-reaction principle), where the same force that the mass is exerting on the structure is exerted by the structure on the mass.

 

STEP 2

In this second step of the experiment, we intuitively position the pulley to lift the weight.

 

Although placing the pulley in this position makes it easier to lift the weight, we are not really reducing it. If we continue to apply Newton's third law, the principle of action and reaction, we see that the force required to lift the mass of 300 grams is still 3N, as indicated by the dynamometer.

 

 

STEP 3. INVERTED PULLEY

When we now place the pulley inverted, we will see how, without using other elements compared to the previous case, we are going to reduce the force.

polea invertida

 

 

 

STEP 4. DOUBLE PULLEY

This is actually the case with pulleys. The pulleys of cranes and loading systems are not simple pulleys, but rather a sum of superimposed pulleys which is what really lowers the force needed to lift a weight. In this case we are going to see it with only 2 pulleys and how the forces are distributed so that the force needed to lift the same mass is reduced.

 

 

More schematically, this would be the mounting system of these pulleys

Poleas: fuerzas

 

Fuerzas en doble polea

 

This pulley experiment is a seemingly simple experiment, but it brings together the principles of mechanics and Newton's 3 laws. If they have the patience to understand and analyse it, children will make a big step in understanding mechanics.

 

 

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