Session 196

This is the final portion of Chem/Phys, where we dive into a discrete passage talking about ramps, friction, and how long it would take for the force to stop the object.

We’re joined by Phil from Blueprint MCAT, formerly Next Step Test Prep. If you would like to follow along on YouTube, go to premed.tv.

Get your FREE copy of Blueprint MCAT’s Full-Length 1 to follow along: Go to https://medicalschoolhq.net/bp3. In the menu, click “MCAT,” then “Free Resources.” (That’s an affiliate link, so if you end up making a purchase from Blueprint later on, I get a small commission at no extra cost to you.)

Listen to this podcast episode with the player above, or keep reading for the highlights and takeaway points.

[02:44] Question 57

If a child with a sliding toy block (with mass = m), and sliding the toy block down the ramp, the coefficient of static friction between the block and the ramp is 0.25. When the block is halfway down the ramp, the child pushes down on the block perpendicular to the plane holding it. What’s the minimum force that the child must apply to keep the block from starting to slide down the ramp?

1. mg sin ፀ
2. 0.25 mg cos + mg sin ፀ,
3. mg sin ፀ – 0.25 mg cos ፀ
4. mg sin ፀ / 0.25 – mg cos ፀ

Thought Process:

F = mass x gravity

ፀ is the angle of the ramp, which is 45 degrees. And it doesn’t really matter because we don’t have numbers.

If the friction was high enough, the kid wouldn’t have to push down at all. It would stop on its own and so it’s obviously going to play some role in this.

Note a couple of forces here. There’s the friction force that’s going to be holding this on to the ramp. There’s also going to be a force pulling it down the ramp in the opposite direction. And that’s going to be due to gravity and that’s going to be mg sin ፀ. sin is for sliding so the one that’s going down the ramp is going to be the sin.

If this kid’s holding it stationary, that force of friction has got to be equal to the force or the mg sin ፀ. So there’s already a good relationship here: mg sin of ፀ is equal to the force of friction. 

Now the other force that we have going on here, which is the all classic, inclined plane question. We’re going to have gravity pointed into the plane, which is going to be mg cos ፀ. And then there’s going to be a normal force that’s going in the opposite direction. Under normal conditions, that’s pretty much it.

However, we got this kid here who is pushing this block into the inclined plane. And so that’s an added force pushing it into the plane. And so because this object is not like smashing into the plane or rocketing off, we know these forces all have to be equal, and then we cancel out.

We know that there are two forces going into the plane: the force of the kid pushing it and the force of gravity, mg cos ፀ. Those added together are going to be equal to the normal force. So normal force is equal to mg cos ፀ plus the force of the kid pushing.

Now, we’re trying to solve for this force of the kid pushing. Let’s try to rearrange that equation to say that the force of the kid pushing is going to be the normal force minus mg cos beta. The problem is, there’s no normal force in these equations. And there’s nothing we can do with that. So we need to get rid of this normal force.

The force of friction = coefficient x normal force. And so we know that the force of friction is equal to mg sin ፀ which is equal to 0.25 x normal force. So that means that the normal force = mg sin ፀ /0.25.

So if we go back, we know that:

Force of pushing = normal force – mg cos ፀ

and,

Normal force = mg sin ፀ / 0.25. 

Therefore,

Force of pushing = mg sin ፀ / 0.25 – mg cos ፀ.

Correct answer: D

[09:51] Question 58

Tucson, Arizona is well known for its Sunny dry weather. A 33 degrees Celsius day in Tucson can feel relatively cool and pleasant compared to a similar temperature in a human city like Tampa. The phenomenon of “dry heat” feeling subjectively cooler is a result of:

1. decreased evaporation from the skin helping to keep the body cool
2. increased evaporation from the skin helping to keep the body cool
3. decreased cardiac output in response to dehydration lowering cutaneous perfusion
4. increased cardiac output in response to dehydration lowering cutaneous perfusion.

Thought Process:

Evaporation is that as water goes from a liquid to a gas, it’s an endothermic reaction. It absorbs energy and it absorbs heat so it makes you feel cooler.

C and D is like you’re having a heart attack or something. Your heart was working. And maybe it’s cooler because you’re dead.

When you are dehydrated, your cardiac output does go up because your heart rate is going up to get the blood going. But that has nothing to do with cutaneous perfusion or not. Because your body is going to shunt blood where it needs to go anyway. So it just doesn’t make sense.

Correct answer: B

[11:36] Question 59

Vinblastine is a microtubule-disrupting drug that inhibits tubulin polymerization. Which of the following processes would be directly inhibited upon vinblastine treatment?

1. Phagosome transport to the lysosomes
2. Mitosis

III. Meiosis

1. Electron transport.
2. II only
3. I and IV only
4. II and III only
5. I, II, and III only

Thought Process:

Microtubules are like the little highways inside the cells. That’s how we move stuff from the golgi to the lysosomes and vice versa. So moving phagosomes to the lysosomes, those little highways are probably going to be useful, which are made from tubulin.

On a side note, mitosis and meiosis are pretty similar. So just off of that, if you find yourself really lost here, remember tha if it’s something important for mitosis, it’s probably also important for meiosis. So it means that the answer has to also have both II and III, and not just II or not just III, because they’re so similar.

In mitosis and meiosis, you do use microtubules to push and pull the chromosomes during metaphase. They push them onto the metaphase plate. Then in anaphase as you pull them apart, they’re still moving things around inside the cell. And so that’s where tubulin comes in because tubulin messes with mitosis, or cell division. That’s why it is used sometimes as an anti-cancer medication because it’s makes it difficult for cells to divide and split, which is important for cancer.

Correct answer: D

[14:48] Tips Before Moving on to the Next Section

Even if you have a little time left, we encourage students not to go back to the passages. Just take a deep breath. And be sure to use the 10-minute break.

Even if you’re not sure what to do, even if you’re a little bit lost, you can still figure your way to an answer. Critical thinking skill is very important. Obviously, there’s a lot of content that you need to have mastered as well. But even if you’ve mastered all the content, there’s still a lot of figuring and weighing and finangling your way towards a correct answer. And that sort of thing tends to become difficult if you get a little burnt out and fried and exhausted.

[16:43] Buckle Up for CARS Section

As you’re coming out of the Chem/Phys section and going into cars, the landscape is changing drastically.

You need to prepare your brain like you’re switching gears so that you’re ready to tackle the challenges that are coming up in the CARS section. 

Links:

Meded Media

Blueprint MCAT