1.  how much force is required to keep a body moving with a constant speed on a frictionless surface?
2. give an example in which momentum changes even through the speed and mass remain the same?
3. if the mass of a body and the force acting on it are both doubled what happens to the acceleration?
4. is any force required to keep a body moving in a circular path?

1-No forces are required to keep a body moving with a constant speed on a frictional surface.

2-Momentum=Mass x Velocity

If mass and velocity of a body are constant then it's momentum will also remain same.

3-We know that,

Force (F)=Mass( m) x Acceleration (a)

Or a=F/m

If m'=2m and F'=2F

Then a'=2F/2m=F/m=a

Acceleration will remain same.

4-Yes,The centripetal force is required to keep a body moving in a circular path.