Typically an RN like Jolene will walk several miles over the course of a 12 hour shift on the MED floor. Her () can be calculated as the distance covered divided by the time she worked. If she walks three miles, then her average speed would be:
Jolene’s is very different from her speed at any one moment in time, which could be anything from zero to about 4.5 mph (she tries to avoid running in the hospital). Jolene’s instantaneous speed and direction of motion change often as she starts, stops and turns corners. The process of generating, maintaining, and changing motion is known as .
Newton’s Third Law of Motion
tells us that Jolene must experience a in order to initiate a change in motion, also known as a change in velocity. We know that tells us how to calculate the Jolene needs in order to achieve a particular amount of velocity change each second (id=”4053″]acceleration[/pb_glossary]). However, Jolene can’t apply a net force to herself, so how exactly does Jolene control how much net force she experiences? provides the answer. 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 (even if that object is air or hot exhaust gas from a rocket engine).
Third Law Pair Forces
The equal and opposite forces referenced in Newton’s Third Law are known as (or third law pairs).
- 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!)
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.]
Everyday Example: Headrest
The headrest in your car is not actually designed as a place to rest your head. Its real purpose is to prevent injury. If someone rear-ends your car it will accelerate forward. As a result your body is accelerated forward by and from the seat. If the head rest were not there, your head would momentarily remain in place due to as your body moved forward. The lag in head position gives the impression that the head snapped back, but really the body moved forward and left the head behind. Your head does remain attached to your accelerating body though, so the tissues in your neck must provide the large force required to accelerate the head along with the body. According to Newton’s Third Law, the tissues of the neck will feel an equal and opposite force to that large force they apply to the head. That large force may damage the tissue (cause a stress larger than the yield stress of the tissue).
The headrest provides a normal force on your head so that it accelerates along with the body, keeping your head above your shoulders and your neck in a safe position. You can see the importance of the headrest in these crash-test videos:
The headrest doesn’t necessarily reduce the felt by the head as much as provide the force needed to accelerate the head along with the body, so that the neck doesn’t have to, thus reducing the third law pair forces between the head and neck.
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 on falling 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.
average rate at which distance was traversed, equal to total distance traveled within a time interval, divided by the time interval
existing or measured at a particular instant
movement or the ability to move from one place to another
an object's motion will not change unless it experiences a net force
the total amount of remaining unbalanced force on an object
the acceleration experienced by an object is equal to the net force on the object divided my the object's mass
for every force applied by an object on a second object, a force equal in size, but opposite in direction, will be applied to the first object by the second object
the state being in equilibrium (no unbalanced forces or torques) and also having no motion
the outward force supplied by an object in response to being compressed from opposite directions, typically in reference to solid objects.
the tenancy of an object to resist changes in motion
a quantity of speed with a defined direction, the change in speed per unit time, the slope of the position vs. time graph
a pair of equal and opposite forces applied between two different objects as described by Newton's Third Law of Motion
a force acting opposite to the relative motion of any object moving with respect to a surrounding fluid
the force that is provided by an object in response to being pulled tight by forces acting from opposite ends, typically in reference to a rope, cable or wire
a force that resists the sliding motion between two surfaces
a force that acts on surfaces in opposition to sliding motion between the surfaces
a force that resists the tenancy of surfaces to slide across one another due to a force(s) being applied to one or both of the surfaces
a graphical illustration used to visualize the forces applied to an object
the change in velocity per unit time, the slope of a velocity vs. time graph