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## Session 83

Physics is one of the harder sections of the MCAT for many students. If you are one of those students, you’re not alone. Check out our foundational series now.

We take questions from Blueprint MCAT (formerly Next Step Test Prep) resources and we cover those with Bryan Schnedeker. By the way, check out all our other podcasts on the MedEd Media.

This week as we talk about the foundation of Physics, it would have to mainly deal with movement and energy.

### [01:35] Knowing the SI System

Question 4: Which of the following are base units in the SI system?

- Kilometers
- Grams

III. Seconds

- Kilograms

- (A) I andII
- (B) II and III
- (C) III and IV
- (D) II, III, and IV only

*Bryan’s Insights:*

If there’s one thing Americans buy every single day in metric is soda. Remember that the question asked for the base unit. In the case of grams and kilograms, one of them is the base unit and the other one is the derived unit.

Kilogram is actually the base unit, and the actual standard. Then you get to narrow down your answer choices to C and D. Obviously, C is the right answer here since D includes grams.

Kilometer is not a base unit but the meter is. This is important since the MCAT wants you to know the difference between base and derived units. This is as fundamental as you can get in measuring the world.

### [05:25] Potential Energy

Question 8: A 12-kilogram bag of clothes is lifted 360J of potential energy. Approximately how long will it take to hit the ground of dropped?

- (A) 0.3 seconds
- (B) 0.6 seconds
- (C) 0.8 seconds
- (D) 6 seconds

*Bryan’s Insights:*

The equation you have to know for gravitational potential energy is MGH. M is 12, G is 10 (for the MCAT), and H is the unknown.

So 360 divided by 10 is down to 36. Divide it by 12 and it’s down to 3. So the 12-k bag of clothes is 3 meters off the ground.

Since you’re dropping the object from 3 meters, you should recognize that everything falls at the same acceleration, which falls at 1G (10 m/s2). So if you drop from 3 meters, how long would it take to hit the ground?

The kinematic equation you need to know here is Distance = V initial x Time + 1/2 AT2.

The nice thing about this equation is that if you’re just dropping an object, it has no initial velocity. You simply have to open your hand and initial velocity starts at 0. So you can drop that whole chunk of the equation.

So the equation is now simplified to Distance = 1/2 AT2.

Here, Distance is 3 and acceleration is 10. So to solve for T2, you get 0.6. Again, with MCAT, you don’t have to be super precise with the math, but what you need to have is a really pretty solid number sense. You have to know what happens to a decimal when you take the square root of it. 0.6=T2, means the square root of 0.6. The MCAT wants you to know that if you take out the square root of a decimal, it gets bigger. So the square root of 0.6 is 0.8.

To summarize, there are two equations involved here.

Potential Energy = MGH

Distance = VT + 1/2 AT2

### [10:24] Greatest Horizontal Distance

Question 9: Jessie’s high school Physics class is running a potato cannon competition. The goal is simple, shoot a potato the greatest possible horizontal distance. Right now, Jessie’s cannon shoots potatoes at a 30-degree angle from the ground with a total velocity of 14 m/s. What changes can Jessie make to increase for potatoes’ travel.

- Increasing the velocity to 80 m/s while keeping all other factors constant.
- Decreasing the masses of potatoes to make them fall more slowly.

III. Changing the angle to 45 degrees with respect to the ground.

- Changing the angle to 90 degrees with respect to the ground.

Notice that cos30 = 0.87, sin30 = 0.5, cos45 = 0.71, and sin45 = 0.71.

- (A) I only
- (B) I and II only
- (C) I and III only
- (D) I, III, and IV only

*Bryan’s Insights:*

You want the greatest distance, so IV doesn’t sound right since if you shoot it up, it’s not going to go anywhere. So I’d get rid of the D right off the bat. The question asks for the greatest possible horizontal distance so shooting it straight up is no way to get any horizontal distance.

The right answer here is C. (II) says decreasing the masses of potatoes to make them fall more slowly is not right since everything falls the same. Everything falls at G.

### [13:07] Potential, Kinetic, and Total Energies

Question 11: Consider a positively charged particle is experiencing a force due to an external electric field. Which of the following are conserved for the particle?

- Potential energy
- Kinetic energy

III. Total energy

- Momentum

- (A) I
- (B) III only

Force creates acceleration, not velocity. If you’re going to start moving faster and faster as you push on it, then momentum is not going to be conserved. It’s going to go up. So IV is out.

Kinetic energy is out here as well since you’re not going to conserve kinetic energy here since it’s going to go up. II is out as well. This leaves us to C and D. This is just one of those foundational concepts of the universe – the Law of Conservation of Matter and the Law of Conservation of Energy

Potential energy is just the kind of energy. Total energy III is always conserved. So B is the right answer.

### [16:05] Blueprint MCAT (formerly Next Step Test Prep)

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