# Physics problem from MCAT Question of the Day?

#### SupremeDoc

##### Gunner
2+ Year Member
The problem is:
"A hollow rubber ball, whose volume is 3 x 10-3m3 and has a mass of 1.0 kg) is held by a 1-meter-long string attached to the bottom of a large container of water. What is the tension in the string?"

I imagined the string to be hanging down from the bottom of the container. But judging from the correct answer, the string is holding the ball upright from the bottom of the container. So my forces were completely upside down and needless to say, I couldn't do the problem. Is there something I'm missing? Isn't this question a bit ambiguous? Are questions on the MCAT really like this?

#### Cornfed101

##### SDN Bronze Donor
Bronze Donor
2+ Year Member

I can't answer if this is similar to what is on the MCAT because I haven't taken it, but it seems like fair game.

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#### Zenabi90

2+ Year Member
@Cornfed101 got it right. Look at his quick illustration and force diagram.

This is a basic physics discrete question. You should have handled plenty of these during high school physics and college physics. It's like a buoy attached to an anchor. It seems you had a problem visualizing the situation.

The ball is stationary so Fnet = 0.

Just think about it generally. We know the forces on the ball include Fgrav, Ftension, and Fbuoyancy.

Fg is pulling it down, Ft is pulling it down, and Fb is pulling it up. Since the net force is 0, we can derive the equation:

Fb = Ft + Fg. To isolate Ft, we rewrite as Ft = Fb - Fg. We can expand that out as:
Tension = Ft = (density of fluid displaced x volume fluid displaced x gravity vector) - (mass x gravity vector)

Ft = (1E3 kg/m^3 x 3E-3 m^3 x 10m/s^2) - (1kg x 10m/s^2) = 30 - 10 = 20 kgm/s^2 = 20N.

SupremeDoc

#### Cornfed101

##### SDN Bronze Donor
Bronze Donor
2+ Year Member
The best advice for solving problems like this is to draw a force diagram EVERY TIME. Then it just becomes a simple algebra problem.

OP

2+ Year Member

#### Zenabi90

2+ Year Member
@Cornfed101 @Zenabi90 Makes sense, thank you guys!
You're welcome.

When I took the MCAT in 2012, there were definitely similar discrete-type questions in the chemistry/physics section. So the possibility is there.

I assume, by the amount of people still advocating for section bank/discrete type practice questions during review, this remains the case. I will take it this Saturday, so we'll see!

SupremeDoc
OP

#### SupremeDoc

##### Gunner
2+ Year Member
@Zenabi90 The discrete-type questions don't seem that hard, so that's good. Good luck on the MCAT!

#### Zenabi90

2+ Year Member
@Zenabi90 The discrete-type questions don't seem that hard, so that's good. Good luck on the MCAT!
Yup. If you check out the timing, its 95 minutes for 59 questsions, 10 passages of 4-7 questions and 15 discretes. At most, that's 1.5 min per question. Figure since there's passages to read, each discrete should be easy enough to read, think through, complete, and select an answer within 60 seconds. They won't be too complex in concept or calculation.

Thanks!

SupremeDoc

#### Coltuna

2+ Year Member
The problem is:
"A hollow rubber ball, whose volume is 3 x 10-3m3 and has a mass of 1.0 kg) is held by a 1-meter-long string attached to the bottom of a large container of water. What is the tension in the string?"

I imagined the string to be hanging down from the bottom of the container. But judging from the correct answer, the string is holding the ball upright from the bottom of the container. So my forces were completely upside down and needless to say, I couldn't do the problem. Is there something I'm missing? Isn't this question a bit ambiguous? Are questions on the MCAT really like this?
I saw nothing near this complex for caluclations on my MCAT in June but N=1

SupremeDoc

#### acetylmandarin

5+ Year Member
I got this incorrect because I used 1 as the density of water . I don't remember constants very well.....

#### Zenabi90

2+ Year Member
I got this incorrect because I used 1 as the density of water . I don't remember constants very well.....
It IS 1. But the units are different. 1 g/cubic cm or roughly 1g /mL or 1000 kg/cubic m. All equivalent. it units matter.

#### acetylmandarin

5+ Year Member
It IS 1. But the units are different. 1 g/cubic cm or roughly 1g /mL or 1000 kg/cubic m. All equivalent. it units matter.
Yeah, I know now haha. I've actually made this error multiple times since I began studying, and it never seems to stick with me