We don't have the knowledge to manipulate gravity, and I don't think it's possible
High mass elements create more gravity, that being said, even if we manage to figure out how to push a hammer with gravity force, the gravity of the planet you are on will just overwrite it.
Gravity is a very complex force, it's not linear and apparently the only way to create it, it's by having an enormous mass, you don't just have to be big, you need mass.
So let's say with create a hammer with a lot of mass, you wouldn't be able to hold it.
Now, also imagine you hit the car with the hammer, as old Newton said every force creates a reaction, you will be throw in the air as well.
A hammer with a propulsor on the edge, I mean, a kind of like a impulsor that when it hits a target activates, but again, talking in the actual world, if you don't have the strength to hold the reaction, you will get recoil, just like any other weapon. Or in the other edge! A propulsor that boost the hammer swing. Now, if you miss, it would definitely hurt.
I don't think is possible to do just use gravity nor use a somehow propulsor (as spaceships, but of course much more smaller in every aspect), but I'm not sure on the second one.
Hope it helps.