To calculate the force needed, we can use Newton's second law of motion:

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

Here's the breakdown:

* Mass (m): 70 kilograms

* Acceleration (a): 4.2 meters per second squared (m/s²)

Now, plug the values into the equation:

* F = 70 kg * 4.2 m/s²

* F = 294 Newtons (N)

Therefore, a force of 294 Newtons is necessary to accelerate a 70 kilogram object at a rate of 4.2 meters per second squared.