# Newton's laws and submerged objects.

by Gersty
Tags: laws, newton, objects, submerged
 P: 28 I have access to a remote controlled submersible and I want to use it to do some physics with some students of mine. We are examining Newton's Laws. I intend to use spring scales to determine the mass of the sub and the max force the sub can exert by applying full forward thrust while reading a spring scale attached to the sub. We'll then have the sub push floating containers of various weights across a tank to record the times. We'll use Δx = ½ a (Δt)^2 and F=ma to calculate the acceleration for each trial and then compare the results. My issue is that this is good as far as it goes, but... 1. Δx = ½ a (Δt)^2 will yield bad data because acceleration won't be constant. 2. F=ma will yield bad data because we can't calculate the force of friction (drag) to determine the net force. I think it will be worth doing to get practice collecting data and making calculations as long as we discuss the previous two points, but I'm looking for some help in making the calculations a little more realistic. Any ideas?
 Engineering Sci Advisor Thanks P: 6,039 This is more complicated than you apparently think. For example The thrust of the sub will depend on its velocity relative to the water, and the relationship between thrust and speed will depend on what type of propulsion system it has (e.g. propellor or water jet). The effective mass of the system is not equal to its physical mass, because the objects can't move unless some water moves in the opposite direction. This is hard to quantify because it depends not only on the shape of the objects, but also on the width and depth of the water channel. It's an important reason why fluid flow experimenters prefer to have steady flow conditions, not accelerating ones. If this is part of an "intro to physics" or "intro to mechanics" course, maybe you would be better with a simpler situation, despite the "cool factor" of the submarine, unless you are sure your students can handle so many deviations from the basic notion of "F = ma" without getting lost, and they have the enough math skills to quantify them (which probably means calculus).
 HW Helper Thanks P: 5,166 IDK why you want to use a submersible to experiment with and demonstrate Newton's Laws of motion, unless you just want to play with boats in a bathub. There are simpler experimental setups (land-based, unfortunately) which can be used that don't include all of the uncertainties you mentioned in the OP. Save the boats for when your students have grasped the basics.
P: 28

## Newton's laws and submerged objects.

I agree. I'm trying to do physics with a focus on maritime applications. Thus the difficulty. Another idea would be to use wheeled carts with sails and a fan to investigate how sails work and focus on some net force and acceleration calculations.
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