Application of Newton's Laws

The greatest difficulty for high school physics students is related to problem solving, on the most diverse fronts of the discipline.
I will present here the solution of an exercise on block interaction, related to the application of Newton's Laws.
Example 1) (UF-PE) The figure below shows three blocks of masses mA = 1.0 kg, mB = 2.0 kg and mc = 3.0 kg. The blocks move together under the action of a constant horizontal force F of 4.2 N modulus.

Disregarding friction, what is the magnitude of the net force on block B?
a) 1.0 N
b) 1.4 N
c) 1.8 N
d) 2.2 N
e) 2.6 N
Solution.
First we must know that F = m.a (Newton's second law). We must also know that the three bodies move with the same acceleration, and that this acceleration has the same direction and direction as the force F.
So we can calculate the acceleration of the blocks by the Isolated Body System (SCI)
In the figure below we represent the forces acting on A, B and C

Where F is the applied force.
FThe in the first block is the reaction of

B in The due to F.
Fa in the second block is the action of The in B due to F
Fc in the second block is the reaction of ç in B due to F
Fc in the third block is the action of de B in ç due to F
n is the normal force and p is the weight force in the three cases

Simplifying the weights with the normal reaction forces in each case we have to:

As F = 4.2 N, we have:
4.2 = (1.0 + 2.0 + 3.0).a
a = 4.2/6
a = 0.7 m/s2
Once the acceleration is found, we must find the resultant value in B.
The resultant in B is: FB = FTHE – FÇ
Subtracting equations B and C we have:
FTHE = (mB + mÇ). The
FTHE = (2,0 + 3,0). 0,7
FTHE = 5,0. 0,7
FTHE = 3.5 N
Finding the F valueÇ
FÇ = mÇ. The
FÇ = 3,0. 0,7
FÇ = 2.1 N
Thus:
FB = 3,5 – 2,1
FB = 1.4 N
We then conclude that the net force on B is equal to 1.4 N.

By Kléber Cavalcante
Graduated in Physics
Brazil School Team

mechanics - Physics - Brazil School

Source: Brazil School - https://brasilescola.uol.com.br/fisica/aplicacao-das-leis-newton.htm

Torque: what is it, unit, formula and solved exercises

Torque: what is it, unit, formula and solved exercises

Torque, or moment of a force, is the tendency that a strength it has to rotate a body on which it...

read more
Exoplanets: concept and how they are detected

Exoplanets: concept and how they are detected

exoplanets are planets that are outside the Solar system, in orbit of others stars, that is, they...

read more
Nature of a wave. Propagation and nature of a wave

Nature of a wave. Propagation and nature of a wave

To begin the study of a wave, it's good to remember the basic definition of a wave: wave is calle...

read more