A0.5 kg ball moves in a circle that is 0.4 m in radius at a speed of 4.0 m/s. calculate the centripetal force exerted on the ball.

M = 0.5 kg, the mass of the ball
r = 0.4 m. the radius from the center of the circle to the ball
v = 4.0 m/s, the tangential velocity of the ball as the ball rotates
      counterclockwise about the center of the circle.

Refer to the diagram shown below.

By definition, the centripetal acceleration of the ball is
a = v²/r = (4 m/s)²/(0.4 m) = 40 m/s²
The centripetal force exerted on the ball is
F = m*a =  (0.5 kg)*(40 m/s²) = 20 N

  20 N

That depends on what kind of environ meant it is in  in space it has the same force that was applied when the velocity was gained

Answerboth technicians (a) and (b);