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Physics/Newtons Law Problem


Three blocks, with masses m1 = 5.90 kg, m2 = 5.50 kg, and m3 = 7.10 kg, are pull on a horizontal frictionless surface by a 18.00 N force that makes a 39 o angle (θ) with the horizontal (see figure). What is the magnitude of the tension between the m2 and m3 blocks?

Hello Neal,

The applied force is not horizontal, so we need to calculate what component of the 18 N is in the horizontal direction.
F_horiz = 18.00 N*cos39 = 13.99 N

The sum of the 3 masses to be accelerated is 18.5 kg. They will accelerate according to Newton's 2nd Law:
F = m*a so in this case
a = F_horiz/m = 13.99 N/18.5 kg = 0.7562 N/kg = 0.7562 m/s^2

So what is the tension in the cord pulling m3? It is enough to accelerate a 7.10 kg mass at a rate of 0.7562 m/s^2.

You can do the rest of this! Use Newton's 2nd again, where this time F is the tension, T, and m is the mass m3.

I hope this helps,


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Steve Johnson


I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.


I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University

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