Question

What is the correct equation to determine the net force of the object below?

The object has a mass of 3.5 kg and is slowing to a stop at a rate of 1.5 m/s2.

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Ff - Fapp - FT = 3.5 x 1.5


Ff - Fapp + FT = 3.5 - 1.5


Ff - Fapp - FT = 3.5 + 1.5


Ff + Fapp + FT = 3.5 x 1.5

Answer 1

Use ΣF = ma. If you take the positive direction as the object's motion (so applied force Fapp is +), then

Fapp - Ff - FT = m a = 3.5(−1.5) = −5.25 N.

Equivalently, the net force has magnitude 5.25 N opposite the motion, so Ff + FT - Fapp = +5.25 N.