<< Chapter < Page Chapter >> Page >

We can see Newton’s third law at work by looking at how people move about. Consider a swimmer pushing off from the side of a pool, as illustrated in Figure 04_04_swimmer. She pushes against the pool wall with her feet and accelerates in the direction opposite to her push. The wall has thus exerted on the swimmer a force of equal magnitude but in the direction opposite that of her push. You might think that two forces of equal magnitude but that act in opposite directions would cancel, but they do not because they act on different systems .

In this case, there are two different systems that we could choose to investigate: the swimmer or the wall. If we choose the swimmer to be the system of interest, as in the figure, then F w a l l o n f e e t is an external force on the swimmer and affects her motion. Since acceleration is in the same direction as the net external force, the swimmer moves in the direction of F w a l l o n f e e t . Since the swimmer is our system (or object of interest) and not the wall, we do not need to consider the force F f e e t o n w a l l because it originates from the swimmer rather than acting on the swimmer. Therefore, F f e e t o n w a l l does not directly affect the motion of the system and does not cancel F w a l l o n f e e t . Note that the swimmer pushes in the direction opposite to the direction in which she wants to move.

Alt text
A swimmer is exerting a force with her feet on a wall inside a swimming pool

When the swimmer exerts a force F f e e t o n w a l l on the wall, she accelerates in the direction opposite to that of her push. This means that the net external force on her is in the direction opposite to F f e e t o n w a l l . This opposition is the result of Newton’s third law of motion, which dictates that the wall exerts a force F w a l l o n f e e t on the swimmer that is equal in magnitude but that acts in the direction opposite to the force that the swimmer exerts on the wall.

Other examples of Newton’s third law are easy to find. As a teacher paces in front of a blackboard, he exerts a force backward on the floor. The floor exerts a reaction force in the forward direction on the teacher that causes him to accelerate forward. Similarly, a car accelerates because the ground pushes forward on the drive wheels in reaction to the drive wheels pushing backward on the ground. You can see evidence of the wheels pushing backward when tires spin on a gravel road and throw rocks backward.

Another example is the force of a baseball as it makes contact with the bat. Helicopters create lift by pushing air down, creating an upward reaction force. Birds fly by exerting force on air in the direction opposite that in which they wish to fly. For example, the wings of a bird force air downward and backward in order to get lift and move forward. An octopus propels itself forward in the water by ejecting water backward through a funnel in its body, which is similar to how a jet ski is propelled. In these examples, the octopus or jet ski push the water backward, and the water in turn pushes the octopus or jet ski forward.

Practice Key Terms 4

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




Source:  OpenStax, Updated tutor hs physics content - legacy. OpenStax CNX. Mar 16, 2015 Download for free at https://legacy.cnx.org/content/col11768/1.4
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Updated tutor hs physics content - legacy' conversation and receive update notifications?

Ask