I'm trying to figure out how useful weapons would be in a world where everyone has superpowers. Specifically for this question, I'm trying to figure out the effects of increasing body strength. How much stronger then an ordinary human would someone have to be to be bulletproof? (or highly resistant) Would this also help for nonprojectile weapons? (like how Kevlar won't stop a knife)
Bodily strength wouldn't inherently give you resistance to damage. It's more a matter of how you got that strength. Is your strength from a power-enhancing forcefield? From denser muscles? From redefining the local laws of physics? "superpowers" covers a wide range of possible explanations. Many of them would give you some additional resilience with your strength, but not all, and certainly not equally.
Real answer: come up wiht an explanation for your your superstrength works, and work it from there. Alternately, decide how you want the strength-to-resilience thing to line up, and then come up with a form of superstrength that makes that make sense.
Being super strong does not equal invulnerability. Most superheros that have super strength have invulnerability because super strength without invulnerability is very impractical. Without invulnerability super strength would probably get you killed.
So I would just give you a second power.
I have to disagree with the other answers.
Just imagine this as a car. Yes you can install a massive supercharged motor but such as motor would start to rip the car to pieces. You need to reinforce the body to handle the power. You need stronger brakes, better gearbox perhaps an improved electrical system.
All of this would help in an accident. Sure the motor might mean a much bigger accident but in a regular accident it has to help.
Super strength would require the strengthening of the whole body. Bones would be denser or have a different stronger structure to prevent them from breaking under extreme loads.
Muscles and tendons also need to be stronger to prevent them from tearing. Perhaps the body incorporates a bio form of carbon nano tubes to prevent tearing.
Skin also needs to be strengthened so it doesn't tear off. Super strength doesn't work if your palms rip off while trying to lift a bus.
All of this would increase resistance to bullets. You can't re-enforce everything without increasing resistance to damage.
It's a mini verses a truck thing.
Without further explanation, strength is just better muscles. Muscles can be cut with a knife, penetrated by bullets or projectiles, and bruised, no matter how big they are.
When bullets are fired into ballistic jelly designed to simulate the resistive power of flesh or muscle, they can penetrate 12 inches, easily, even 24 inches. Obviously the majority of muscles in your body could not be large enough to completely surround you with 24 inches of muscle. Your head, for example, cannot have muscles that big. Even 24 inches of muscle would not pose a barrier to a sharp 36 inch sword (or spear).
Invulnerability is a different super power that prevents penetration (and bruising or damage) at the surface. Like the Superman movie that starts with Superman taking a bullet in slow motion to his open eye, the bullet flattens and drops to the ground.
As other have mentioned, strength does not equal protection, but let me try to interpret "strength" as "toughness". This way, humans have a long way to go before they would become bulletproof.
Human skin has ultimate tensile strength of only 20 MPa, while for Kevlar it's 3757 MPa.