Gravity is the force that is pushing the backpack down into the table and there is also a force that pushes against the backpack. A balanced force.
The car with the larger mass has a larger force than the smaller car but the smaller car is going faster because it has less weight than the truck does.
Her hand smacks the counter and the same force pushes back as her hand comes in contact the counter. The action of smacking the counter made the counter have a reaction with the same amount of force the hand applied.