MCAT Physics Question — Flotation Device
- by
- Dec 02, 2014
- MCAT Blog, MCAT Physics, MCAT Question of the Day
- Reviewed By: Liz Flagge
A child’s flotation device (ρ = 250 kg / m3) is held underwater at the beach (ρseawater = 1025 kg / m3). The toy is spherical and has a radius of one meter. When the toy is released, what is the magnitude and direction of the toy’s acceleration? (assume g = 9.8 m / s2)
A) 2.45 m / s2 downwards
B) 0 m / s2
C) 29.4 m / s2 upwards
D) 39.2 m / s2 upwards
Explanation
This problem asks us to solve for the net acceleration on a submerged object that is less dense than seawater. Before doing any calculations, we can see that the object’s density is approximately one fourth that of seawater. As a consequence, the object will float in seawater. Thus, when released, it will accelerate towards the water’s surface, where it will then float.
That lets us immediately eliminate choices (A) and (B). If you’re pressed for time, or are unsure how to proceed, then you can make an educated guess and move on.
To solve the problem, use Newton’s 2nd law:
Fnet = ma
The net force will be the buoyant force minus the force of gravity:
Fb – Fg = mtoya
We can then substitute for the forces:
ρseawaterVtoyg – mtoyg = mtoya
Because this is a buoyancy problem it’s often easier to work in terms of density than mass. We can substitute ρV for m in our equation:
ρtoy = mtoy / Vtoy
ρtoyVtoy = mtoy
ρseawaterVtoyg – ρtoyVtoyg = ρtoyVtoya
Factor out the left side of the equation:
(ρseawater – ρtoy)Vtoyg = ρtoyVtoya
(ρseawater – ρtoy)g = ρtoya
(1000 – 250)(10) = (250)a
(750)(10) = (250)a
(3)(10) = a
30 = a
The closest answer choice is (C).
Want more MCAT practice?
We’ve got options for every schedule and learning style!
From the best online MCAT course created by top instructors with 524+ MCAT scores to the most representative full-length practice exams and private tutoring, we can custom tailor your MCAT prep to your goals!
Not sure which option is right for you? Schedule a free MCAT consultation with an MCAT Advisor using the form below. No obligation, just expert advice.
Search the Blog
Free Consultation
Interested in our Online MCAT Course, One-on-One MCAT Tutoring or Med admissions packages? Set up a free consultation with one of our experienced Senior Student Advisors.
Schedule NowPopular Posts
-
MCAT Blog What's on the MCAT?
-
MCAT Blog How to Review MCAT Full Lengths

Free MCAT Practice Account
Need great MCAT practice?Get the most representative MCAT practice possible when you sign up for our free MCAT Account, which includes a half-length diagnostic exam and one of our full-length MCAT practice exams.
Learn More