Whenever an object feels a force that force must be due to another object, but Newton's first two laws don't tell us much about how objects interact. Newton's third law helps complete the picture. It tells us that when objects exert a force on something then they must feel an equal and opposite force exerted back on them. So if I push on a car with a certain force then the car must push back on me with an equal and opposite force to the one I applied. In that case I don't accelerate because the earth's friction also exerts a force on me so my net force is zero.
© 2008 Kansas State University Physics Education Research Group.