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# Tutor profile: Steve H.

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Steve H.
High school science teacher for 37 years
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## Questions

### Subject:Chemistry

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Question:

Why do chemists work with moles? Wouldn't it be easier if we didn't have to change quantities into moles to do calculations?

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Steve H.

Actually it would be much more cumbersome and complicated if we didn't work with moles. When one is working with things as small as atoms, the number of atoms present in a sample are incredibly large! For example, 1/1000th of a gram of neon gas would have almost 3 x 10 to the 19th power atoms in it. That would be a 3 followed by 19 zeros! This would be an awkward number to work with and it would not be easy to convert this to other units, such as liters. By working with moles, we have very easy ways to convert a quantity of a substance to its mass in grams, its volume in liters (if it is a gas) or to even determine how many atoms are present! We could never hope to count the number of atoms in a sample, but by simply measuring its mass or its volume (if it is a gas), we can easily calculate how many atoms are present. When we work with balanced chemical equations, moles allow us to easily take the amount of one substance involved in the reaction and calculate the amounts of the other substances involved. In summary, moles give chemists numbers that are easy to work with and also easy to convert to other units, once you learn the easy steps involved.

### Subject:Biology

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Question:

How do organisms use sugar to get energy?

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Steve H.

(This is just a basic description that would be appropriate for middle school students or an introduction in a biology class). Living things on this planet primarily rely on a process called cellular respiration in order to get energy from sugar. Most organisms require a sugar called glucose to start this process, and they have the ability to transform other sugars into glucose as well. The overall equation for cellular respiration is: glucose reacts with oxygen yielding carbon dioxide, water and energy. Animals have to eat other organisms in order to get glucose but plants are able to make their own glucose in a process called photosynthesis. Using humans as an example, the foods that we eat that are rich in carbohydrates provide us with glucose. This glucose makes its way into our bloodstream and is carried to our cells. We get the oxygen gas by breathing air and the oxygen works its way from our lungs to our blood and also ends up in our cells. Inside of our cells, tiny cell parts called mitochondria carry out a series of reactions that convert the glucose and oxygen into carbon dioxide (which we end up exhaling), water and energy. Now, we don't actually use that energy to do our daily activities. That energy is used to make another chemical called ATP, which stands for adenosine triphosphate. ATP is the main source of energy for our cells to do their different jobs that are essential for life. So to summarize, organisms use glucose in order to produce energy to make ATP, the main chemical energy source.

### Subject:Physics

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Question:

What is the difference between centripetal force and centrifugal force?

Inactive
Steve H.

The biggest difference between these two forces is the frame of reference. Centripetal force is described by an observer OUTSIDE of a rotating system. The observer sees an object moving in a circle and states that there must be some force present that is causing that circular motion, since in the absence of any forces an object will move in a straight line with constant velocity. They will also state that this "centripetal" force is acting inwards towards the center of the circle at all times and is causing an accompanying "centripetal" acceleration that is also towards the center of the circle. Centrifugal force is a fictitious force only described by an observer that is actually moving in a circular path. They "feel" like there is an outside force upon them that is flinging them outwards. The only reason they feel this is because they are in a non-inertial reference frame, one that is accelerating. It is very similar to a person in a car that suddenly comes to a stop. The person feels like there is a force that flings them forward, but to an outside observer, they see no force. The person's inertia simply keeps them moving forward while the car stops. Centrifugal force is much the same, a person's inertia tends to keep them moving in a straight line outwards as they circle but to the person moving in a circular path, it feels like a force is pulling them outwards. Another reason that centrifugal force is a fictitious force is that it does not obey Newton's 3rd Law of motion. It is impossible to state the object that is exerting the centrifugal force on you, and you can't name the object that you are exerting an equal and opposite force upon. One never sees both acting at once. Centripetal force is always described by an outside observer as an inward force acting on the object in circular motion. Centrifugal force is only brought up by the person that is actually moving with circular motion and is not a true force.

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