Question

If a certain mass has its velocity changed from 6.00 m/s to 7.50 m/s when a 3.00 n force acts for 4.00 seconds, find the mass of the moving object.

Answer 1

By definition we have that the force for time is equal to the product of the mass for the change in speed.
 We have then that
 F * (delta t) = m * (delta v)
 Clearing the mass
 m = (F * (delta t)) / (delta v)
 Substituting the values
 m = ((3.00) * (4.00)) / (7.50-6.00) = 8
 answer
 The mass of the moving object is 8Kg

Answer 2

We know the relation between force and mass and acceleration follows this relation: F = m * a
   And we can calculate the acceleration as:
 Vf = Vi + a * t
  where Vf is final speed, Vi is initial speed, a is acceleration and t is time
 7.5 = 6 + a * 4
 7.5 - 6 = a * 4
 1.5 / 4 = a
 a= 0.375 m2/s
   The the mass is F = m * a 3 = m * 0.375 m = 8
   Then the mass is 8 kg