SAT Physics Subject Test

Chapter 3 Newton’s Laws

THE THIRD LAW


If object 1 exerts a force on object 2, then object 2 exerts a force back on object 1, equal in strength but in the opposite direction.


Newton’s third law was originally stated in the form, “For every action, there is an equal and opposite reaction.” Because of this historical phrasing, these two forces, F1-on-2 and F2-on-1, are called an action/reaction pair.


The Skinny on the Third Law

Two objects must interact for a force to exist. When both objects interact, each body experiences a force due to the other interacting body. If A and B are the two interacting masses, let F1 be the force acting on A due to B; F2 is the force acting on B due to A. F1 and F2 are the same magnitude but have opposite directions. F1 = −F2.


  4. A woman riding a bicycle collides head-on with a parked school bus. Which object feels greater force?

Here’s How to Crack It

Newton’s third law states that the force exerted by the bus on the woman/bike will be equal in magnitude to the force that the woman/bike exerts on the bus. What is different is the effect of the force. Since Fnet = ma, the bus (with larger mass) will experience a smaller acceleration than the woman on the bike.