Which change would result in a stronger electromagnet?

a) reducing the number of batteries
b) increasing the voltage of the battery
c) substituting another metal for the iron core
d) decreasing the number of coils around the iron core.

when two balls of the different masses are dropped from equal height, they reach the ground at the same time .this is because they experience the same acceleration.

the acceleration is independent of mass.

thus when they are dropped they will always maintain the same velocity and travel the same distance (neglect air resistance of course) at the same time.