This is Newton's second law of motion.
It states that Force exerted on a moving object is directly proportional to its product of mass and acceleration.
i.e Force(F) = Mass(M) × Acceleration(A)
Because Force is constant here, reducing the weight of the body the force is applied to consequently reduces its mass.
If mass is reduced, the body's acceleration increases hence increasing its speed.
That is why the 40N dumbbell move faster.
