Look at the ball rolling code I posted a couple forums up. If you take out the part about rolling, and subsitutue a chair for the ball, and subsitute the amount of force the player is applying to the object for the force the player applies with the arrow keys to the ball, then you'll have a pushable object simulator.
When I say subsitute the amount of force the player applies to the object, I mean detect that a collision has occured, and then take the player's velocity vector, the speed/direction they are moving on each axis of the world, and then add that, to the velocity of the object. You can also make the object harder to push by say, dividing the force in half, and having the chair push back on the player with an equal amount of force so they move more slowly when they are pushing against it and it is pushed more slowly.
When I say subsitute the amount of force the player applies to the object, I mean detect that a collision has occured, and then take the player's velocity vector, the speed/direction they are moving on each axis of the world, and then add that, to the velocity of the object. You can also make the object harder to push by say, dividing the force in half, and having the chair push back on the player with an equal amount of force so they move more slowly when they are pushing against it and it is pushed more slowly.