70 Overcoming Inertia

Typically an RN like Jolene will walk several miles over the course of a 12 hour shift on the MED floor in a large hospital. Jolene’s velocity changes often as she starts, stops and turns corners.  tells us that Jolene must experience a in order to initiate a change in motion. tells us how to calculate the Jolene needs in order to achieve a particular . But Jolene can’t apply a net force to herself, so how exactly does Jolene control how much net force she experiences?

Newton’s Third Law of Motion

The forces that Jolene experiences must be supplied by the objects around her. The size of the force that Jolene receives from another object, such as the floor or wall, is determined by how hard she pushes against that object. In fact, anytime one object puts a force on a second object, the first object will receive an equal force back, but in the opposite direction. This result is known as . The capacity for using the laws of motion to generate, maintain, and change motion is known as .



The astronaut in the video above starts out in relative to the space station. Then she pushed against the wall. The resistance of the wall to being deformed caused it to apply a reactionary back on her. That unbalanced normal force destroyed her state of static equilibrium, overcame her , and caused her to change relative to the station. This example is a unique form of , but all locomotion depends on this same process of pushing on an object in order receive a push back form the object.

Reinforcement Activity

If the astronaut in the previous video pushes against the wall with 3 N of force, what is the force applied back to her by the wall?

If the astronaut has a of 60 kg, what is her ?

Third Law Pair Forces

The equal and opposite forces referenced in Newton’s Third Law are known as (or third law pairs).

Other include:

  • The Earth pulls down on you due to and you pull back up on the Earth due to gravity.
  • A falling body pushing air out of its way and pushing back on the body.
  • You pull on a rope and the rope pulls back against your hand via .
  • You push on the wall, and the wall pushes back with a .
  • A rocket engine pushes hot gasses out the back, and the gasses push back on the rocket in the forward direction.
  • You push your hand along the wall surface, and the wall pushes back on your hand due to .
  • You push your foot against the ground as you walk, and the floor pushes back against your food due to ( if your foot doesn’t slip, if it does).

You may have noticed that in each of the cases above there were two objects listed. This is because Newton’s Third Law pairs must act on different objects.  Therefore, cannot be drawn on the same and can never cancel each other out.  (Imagine if they did act on the same object, then they would always balance each other out and no object could ever have a , so no object could ever accelerate!)

Reinforcement Exercises

Draw the necessary to show each force in the listed above. How many free body diagrams will you need to draw for each Third Law pair? [Hint: keep in mind the rule about free body diagrams and Third Law pairs.]

Falling as Locomotion

Notice that the list of third-law pair forces includes the force of gravity on the Earth from you and the force of gravity on you from the Earth (weight), so in fact falling is a form of locomotion. That means that throughout the previous unit we were already studying locomotion, although falling is sort of an uncontrolled, or passive form of locomotion. The next few chapters will help us examine active forms of locomotion like walking, jumping and driving.


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