In theory you need to apply 7.8 km/s of deltaV to an object to put it into orbit. In practice, you also need to use more to overcome to factors. There's gravity drag, the action of gravity on your rocket while it's speeding up to orbital velocity. There's also atmospheric drag, the resistance of the atmosphere to your rocket when it's traveling up to space. The faster you accelerate the less gravity drag but the more atmospheric drag you experience since you spend less time accelerating but are traveling faster lower in the atmosphere, and drag is proportional to the square of the velocity.

As you can see, using an electromagnetic cannon where you are at full speed leaving the barrel of the gun, you will have zero gravity drag but an immense amount of atmospheric drag. This means both that you will have to apply much more deltaV and that the heating caused by the passage of your capsule through the atmosphere will be enough to destroy any known material.

Finally, if you launch at full velocity from the surface of the Earth then the rules of orbital mechanics mean that, if you are going into an elliptical orbit, your orbit will intersect the launch point eventually and your capsule will lithobrake on the ground, also destroying it.

Neither of these problems apply to launching from the Moon back to Earth, for the record. The moon has no atmosphere to apply drag and heating. And if you're leaving the Moon's orbit for Earth you're on a hyperbolic orbit rather than an elliptical one and won't be coming back around in any case. So for getting off of airless Moons into orbit around their primary electromagnetic launchers are an excellent idea.