Answer:
T - M g - Ff = M a describes the acceleration of the object
T = M g + Ff if the object moves at constant speed (a = zero)
Ff = μ M g = .3 * 40 * 9.8 = 118 N the force of friction
Ff will be in a direction opposite to the motion
M g = 40 kg * 9.8 m/s = 392 N
a) T = M g + Ff for an object moving upwards
T = 392 + 118 = 510 N
b) T = M g - Ff for an object moving downwards
T = 392 - 118 = 274 N
Answer:
This is a conceptual problem so I will try my best to explain the impossible scenario. First of all the two dust particles ara virtually exempt from any external forces and at rest with respect to each other. This could theoretically happen even if it's difficult for that to happen. The problem is that each of the particles have an electric charge which are equal in magnitude and sign. Thus each particle should feel the presence of the other via a force. The forces felt by the particles are equal and opposite facing away from each other so both charges have a net acceleration according to Newton's second law because of the presence of a force in each particle:

Having seen Newton's second law it should be clear that the particles are actually moving away from each other and will not remain at rest with respect to each other. This is in contradiction with the last statement in the problem.
The correct answer is A. Most reactive non metals.
Firstly if some one knows how to read the periodic table he would have no confusion in deciding whether group 7 has metals or non metals. Group 7 contains non metals so basically we can easily cancel out two options of metals.
Secondly group 7 non metals are the most reactive non metals as they need only one electron in order to complete their valence shell and become stable.
The correct answer for the question that is being presented above is this one: "a. the pressure decreases." As the speed of a fluid increases, the pressure decreases." The relationship of the speed and the pressure is inversely proportional. As the pressure increases, the speed decreases.