Force and Motion

FORCE & MOTION

The Relationship Between Force and Motion

A force is a push or pull on matter. Forces are the cause of motion.  Forces have magnitude (size) and direction.  The direction of the force is important.  An object that is not subjected to a force will move at a constant speed and in a straight line.  Some forces reinforce or cancel each other out.  Forces that are different in size and direction will cause changes in the speed or direction of an object’s motion.  A common unit used to measure force is known as the Newton (N).  Friction is a force that slows down motion.  Larger forces create more friction. 

Forces can be represented by arrows as shown in the illustrations below.  Arrows show direction.  The length of an arrow and/or numbers that accompany the arrows show magnitude.  The width of an arrow should not be used to indicate the magnitude of a force. To calculate the net force in simple models or problems.

			net force

     +                            =                                    Two forces combine to produce a

                                                                       larger force. This is called having                                                                      an overall “net force”.

					net force

 

                

                 +                    =                        Two forces subtract from each

                                                                   other, and it causes movement in

                                                                   the direction of the larger force.

                                                                  

			net force

                                    =             0              Balanced forces = net force of zero

                +            

                 

When the net force an object is greater than zero, the forces are said to be unbalanced.  Unbalanced forces can cause an object to start to move, to stop moving or even change directions.  When two forces are acting in the same direction, they add together to become a larger force.  If one force acting on an object is larger than the other force acting on the object in the opposite direction, the forces are subtracted and it is said that there is a net force in the direction of the greater force.  Two equal forces pushing against one another in opposite directions will cancel each other out and no movement will occur. 

When net forces equal zero, or there is no movement in a system, a state of equilibrium has been reached.  This concept will reappear in earth systems when addressing catastrophic events, in ecology when studying succession, and in human body systems when addressing homeostasis

	These skaters are in a state of equilibrium, and movement is not occurring.

 

Motion is described as the state in which one object’s distance from a point of reference is changing.  Some motions are simple and some are complex.  When a student leaves school at the end of the day, there is motion all around. For example the student might notice that birds are flying, clouds are moving, the wind is blowing, and a car goes by.  When a student is sitting perfectly still, there is still motion occurring.  The student is breathing and blood is circulating throughout the body.  Since motion is relative, reference points are needed to describe the motion.  The location of an object is how far the object is from the reference point (also known as the position of the object), and as an object moves, its location to a reference point is changing also.  One way to show that an object has moved is to use a distance-time graph.  Students first represented motion graphically in 6^th grade (TEKS 6.6B). 

Newton’s First Law of Motion

Newton’s First Law states that an object in motion tends to stay in motion and an object at rest will stay at rest unless acted upon by an unbalanced force.  In discussing Newton’s First Law, students should understand it is also called the Law of Inertia.  Inertia is the tendency of an object to resist any change in motion.  This law explains why people riding in cars need to wear seat belts. A moving car has inertia and so do the people riding in it.  If the driver suddenly applies the brakes, the riders continue moving forward at the speed the car was going, unless the rider is wearing a seat belt.   The seat belt applies the unbalanced force to prevent the rider from going forward.  In another example, if an object is tossed from a space station located in the vacuum of outer space, it will move forever by virtue of its own inertia because there is no outside unbalanced force to stop it.

Newton’s First Law of Motion also describes how the net forces acting on an object is zero if the object is at rest or not moving. Equal forces acting on an object in opposite directions are called balanced forces.  A diver standing on a diving platform is affected by different forces.  Gravity pulls the diver down towards the pool, but the diving platform pushes him up.  These two forces balance each other and as a result, the diver stays put on the platform.  If the diver steps off the end of the platform, however, the force of the platform no longer balances the force of gravity.  The unbalanced force causes the diver to accelerate into the water.

Forces in Living Things

The 7^th grade student is expected to relate forces to basic processes in living organisms, including the flow of blood and the emergence of seedlings.  Although students will study human body systems in more detail in a separate unit, this is a good time to introduce processes in living systems relating to forces and motion.  The heart supplies the force that causes blood to flow through blood vessels. The heart, however, is only one example of a body part in which forces are at work to carry on life processes.  Other examples include the following:

§  The teeth apply force in breaking food into smaller pieces

§  Muscles in the esophagus exert a force in pushing food down the digestive tract through peristalsis

§  Muscles contract and relax, creating a enough force to move bones to walk, run, jump or engage in other life processes

Seeds sprout because the force of water pressure (also called turgor pressure) inflates the emerging shoot.  The force of the shoot straightening pulls the seed leaves above ground. 

Students should demonstrate or model the concept of forces at work in a living organism.  A small pump, plastic tubing and colored water can be utilized to show blood flow.  Students could also germinate seeds, making observations and drawing conclusions as the process occurs.

Potential and Kinetic Energy

Usually when a student thinks of energy, the student thinks about action or some kind of movement.  Kinetic energy is the energy of motion.  The more mass an object has, the more kinetic energy it can produce.  Stored energy due to position is called potential energy.  The 7^th grade student needs to illustrate examples of potential and kinetic energy in everyday life.  An example of both types of energy is a stretched rubber band.  While the band is stretched, it has potential energy.  When the rubber band is released and it flies across a room, it has kinetic energy.  An apple has potential energy in the form of chemical energy.  Once eaten, the body converts it into kinetic energy.  A ball at the top of a hill has potential energy.  The ball rolling down the hill has kinetic energy.  Roller coasters operate through a series of conversions from potential to kinetic energy and back again:

The concepts of potential and kinetic energy to earth science, such as through study of geologic faults and the movement of water.  An example is when rock along a fault, a break in the earth’s crust, is locked into place or at rest.  At this time there no movement is taking place and the fault has potential energy.  If rock on either side of the fault suddenly shifts, potential energy is released and changed into kinetic energy.  This kinetic energy produced by movement along a fault causes earthquakes, tremors, tsunamis, landslides and other natural disasters.   Another example of this kind of relationship can be found in a water fall.  Water at the top of the fall has potential energy, but once water moves down the fall, it has kinetic energy.

An object with a small mass has less inertia than an object with a large mass.  A object with less speed also has less inertia than an object moving with a greater speed.  Students should understand these two factors, mass and speed, influence the amount of kinetic energy in a moving object.  Objects with more mass and/or greater speed have more kinetic energy.  If a baseball and a softball are pitched at the same speed, the softball will have more kinetic energy because it has more mass. A fast pitch softball has more kinetic energy than a slow pitch softball. 

The slow pitch ball has less kinetic energy.