Atoy cart at the end of a string 0.70 m long moves in a circle on a table. the cart has a mass of 2.0 kg and the string has a breaking strength of 40.n. calculate the maximum speed the cart can attain without breaking the string

Given that the mass of the toy cart is 2.0 kg and and the acceleration is unknown, the normal formula would be a=f/m where a is acceleration, f is force and m is mass but the string's breaking strength is 40n so I think the formula in this case will be f is greater than m*a
40 is greater than 2a
40 is greater than 2a
40/2 is greater than 2a/2
20m/s² is greater than a 
Therefore the maximum speed the toy cart should have should be less than 20m/s²

If football was tossed in air without gravity or friction the ball would go into space