Отрицательная масса не может быть. Что же не так?
Ускорение отрицательное, потому что оно направлено вниз.

Трение воздуха создает силу, противодействующую движению. Перья имеют большую площадь поверхности по сравнению с их небольшой массой, поэтому у них больше трения о воздух.

An object's mass doesn't change when it is falling.

Weight just means the force of gravity, which also doesn't change in a short freefall.

Weight and force of gravity mean the same thing. A planet's gravity field determines your weight, but not your mass.

• $$\text{Earth}$$ $$F_{g}=mg$$ $$F_{g}=(0.95)(9.8)$$ $$F_{g}=9.31 \, \mathrm{N}$$
• $$\text{Moon}$$ $$F_{g}=mg$$ $$F_{g}=(0.95)(1.6)$$ $$F_{g}=1.52 \, \mathrm{N}$$

Mass doesn't depend on gravity, so it's the same everywhere.

• $$\text{weight on Earth}$$ $$F_{g}=mg$$ $$F_{g}=(0.\overline{54})(9.8)$$ $$F_{g}=5.35\, \mathrm{N}$$
• $$\text{weight on Mars}$$ $$F_{g}=mg$$ $$F_{g}=(0.\overline{54})(3.711)$$ $$F_{g}=2.02\, \mathrm{N}$$

In the simple case of a flat horizontal surface with no vertical acceleration the force of gravity will always be equal and opposite to the normal force.

• $$F_{g}=mg$$ $$F_{g}=(20)(9.8)$$ $$F_{g}=196 \, \mathrm{N}$$
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Separate the gravity vector into components parallel and perpendicular to the ground. Acceleration is zero in the perpendicular direction.

• $$\text{perpendicular to ground}$$

  $$F_{g\perp}=F_{g}\cos(20)$$ $$F_{g\perp}=(196)\cos(20)$$ $$F_{g\perp}=184 \, \mathrm{N}$$
  $$\sum F_{\perp}=ma$$ $$-F_{g\perp}+F_{N}=ma$$ $$F_{N}=ma+F_{g\perp}$$ $$F_{N}=(20)(0) + 184$$ $$F_{N}=184 \, \mathrm{N}$$
  

• $$\text{parallel to ground}$$

  $$F_{g\parallel}=F_{g}\sin(20)$$ $$F_{g\parallel}=(196)\sin(20)$$ $$F_{g\parallel}=67 \, \mathrm{N}$$
  $$\sum F_{\parallel}=ma$$ $$F_{g\parallel}=ma$$ $$(67)=(20)a$$ $$3.35 \, \mathrm{\tfrac{m}{s^{2}}}=a$$

The tension force is equal and opposite for the person and crate.

The 100 N is only the x-part of the tension force vector. We can find the total tension force with the Pythagorean theorem. At 45° the x and y parts of the force are the same.

The tension force on the string is equal but opposite for the squirrel and box.

The negative sign tells us that the squirrel's force is pointed left.

Both free body diagrams share the same tension and acceleration. Build two equations with Newton's second law and solve a system of equations for T and a. While T and a are the same magnitude they have different directions, so watch the sign of acceleration in particular.

This Khan academy video covers this problem in more depth.

We have two variables and two equations, this means we can plug one equation into the other. The Tension is the same for each body. The accelerations are the same, but in opposite directions, so we need to make one acceleration negative.

The 25kg object is falling down at 4.2 m/s². The 10kg is rising up at 4.2 m/s².