Two identical blocks, A and B, of mass M, are stacked in an elevator as shown to the right. The elevator accelerates upwards with acceleration a . What is the magnitude of the force that block A exerts on block B?

M (g+a)
M (g-a)
2 M (g+a)

https://online-s.physics.uiuc.edu/cgi/courses/shell/common/showme.pl?courses/phys100/fall10/preflights/9/fig23a.gif

2. Three boxes are arranged as shown. The middle box has a mass of 2 kg and accelerates to the right at 5 m/s2 on a horizontal frictionless table. The boxes to the left and right hang freely, suspended by strings over massless, frictionless pulleys. The tension in the left string is T1 = 10 N.

What is the mass of the box, M1, on the left?

M1 = 0.675 kg
M1 = 1.91 kg
M1 = 2.13 kg
M1 = 3.75 kg
M1 = 4.16 kg

https://online-s.physics.uiuc.edu/cgi/courses/shell/common/showme.pl?courses/phys100/fall10/preflights/9/fig10.gif

Force that A exerts on B = ma

m is the mass of A,
a is the relative acceleration of A to B.

Or if you prefer:

W_A - R_A = m_A a

Weight of A is Mg
Reaction of A is what you are looking for,
m_A is the mass of A, which is M,
a is the acceleration of the system, that is a.

2. What have you tried for this one?