The Brainliest Answer!
According to me , Velocity doesnot change , Because our earth is also a sphere & our speed doesn't get changed or reduced automatically . according to Newton's first law of motion , " A body in state of uniform motion will continue in its motion unless & untill an external force acts on it " , it means Ant will still remain in its same speed unless ant doesn't want to increase or decrease its speed .

These were the conditions of we neglect resistance(friction) of ball , if the resistance of ball is very high to resist motion automatically ant's speed will be reduced & they have to apply a large force to get walk on its surface .

I explained you both cases , hope it helped you :)
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Thank u sooooooo much u have cleared my doubt ur answer helped me alot once again thank you sooooooo much
:) mah pleasure , click on thanks button below .. ^_^