The 3rd law says that for an action there is an opposite and equal reaction. For every interaction that object always exerts force on the first object. This law is used based on the law of conservation of momentum. The formula for the 3rd law is P=mv which stands for momentum=mass times velocity. For example, when someone pushes you on a skateboard the person on the skateboard will move the direction the person pushed. The 3rd law is also known as the action-reaction law. This uses mass, acceleration and momentum.