Content Benchmark P.8.B.1
Students know the effects of balanced and unbalanced forces on an object's motion. E/S
When asked if an object is balanced, a person may check to see whether the object is still or wobbling back and forth. Students often realize that the occurrences discussed in physical science are more detailed explanations of daily experiences. To help students fully grasp the effects of balanced and unbalanced forces on an object’s motion, the content background for this benchmark will include, motion, graphing motion, speed, velocity, acceleration, Newton’s three laws of motion, and friction.
Motion is defined as an act or instance of moving. It can be described as any change in position. If you raise your arms in the air, you can say that motion has occurred. Everything in the world is moving. Even things that seem still are in motion because the atoms are vibrating. All things on Earth including our atmosphere are rotating with the Earth.
You don’t have to actually see motion take place to prove that an object has moved. To know whether the position of something has changed, you need a reference point. A reference point is a specific location that can be used to compare if something is moving (i.e., if an object changes its position relative to the reference point, then we consider that the object has moved). Speed is the measure of motion. You can find it by dividing the distance covered by the time it takes to travel that distance. Velocity is the speed of an object moving in a particular direction.
For more information on speed and velocity, go to
http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/1DKin/U1L1d.html
Graphing Motion
A distance-time graph makes it possible to “see” the motion of an object over a period of time. Referring to Figure 1, the green line covers the greatest distance in the least amount of time and therefore has the greatest speed. The green line also shows a constant speed because the speed does not vary with time (i.e., the line is straight with a constant slope).
Figure 1. Distance-Time graph for three different objects.
(From http://www.bbc.co.uk/schools/gcsebitesize/physics/forces/speedvelocityaccelerationfhrev2.shtml)
The red line shows how an object has speed changing with time. First the object has a constant speed (shown by the straight line with the positive slope). Then the object stops moving because the time continues to increase, but the distance stays the same (i.e., the slope of the line is zero). The object then returns to its start position with a constant speed (straight line and negative slope) because the distance goes back down to where it began. With students, you could say that someone went somewhere stayed for a while and went back home.
The blue line shows an object where the velocity is continually changing or accelerating. The object is starting off slow because it is covering a small distance in a greater amount of time and then the distance increases substantially within a shorter time period. In this case, the object is speeding up. In physics, acceleration is any change in velocity and occurs when an object speeds up (positive acceleration), slows down (negative acceleration, or more commonly, deceleration), or changes direction.
For background notes supporting interpretation of motion graphs visit
http://cmp.ameslab.gov/physics106/lecture5.pdf
The following graph shows a car traveling at a constant speed because every second its position increases 10 meters. You can say the car is traveling 10 m/s.
Figure 2. Data and graph illustrating constant speed.
(From http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/1DKin/U1L3a.html)
The following graph shows a car accelerating because each time interval the car covers a different amount of distance. Again, acceleration is any change in velocity, whether it goes faster, slower, or changes direction.
Figure 3. Data and graph illustrating acceleration.
(From http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/1DKin/U1L3a.html)
For more information about motion graphs, go to
http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/1DKin/U1L3a.html
Balanced and Unbalanced Forces
A force is defined as a push or a pull and more than one force can and usually does act on an object at the same time. Every force has a certain strength or magnitude. Forces also have a direction. The net force on an object is the sum (in both magnitude and direction) of all the forces acting on it. If the net force is zero, the forces are balanced. Balanced forces produce no changes in motion of an object. In the picture below, there are two forces acting upon the person. The gravitational force exerts a downward force. The floor exerts an upward force. Because these two forces are of equal magnitude and in opposite directions, they balance each other. The person is at equilibrium because there is no unbalanced force acting upon the person, and thus, the person maintains his/her state of motion (in the case the motion is zero or at rest).
Figure 4. Object at rest with no net force.
(From http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l1d.html)
Objects moving at a constant velocity are also balanced and have a net force of zero. A plane at cruise altitude has four primary forces (weight, lift, thrust and drag) that when balanced, the plan travels at a high yet constant speed.
Figure 5. Object in motion with constant speed and no net force.
(From http://www.grc.nasa.gov/WWW/K-12/airplane/forces.html)
When the net force is greater than zero, the forces acting on the object are not balanced. Unbalanced forces cause an object at rest to move and an object in motion to change speed and/or direction. The animation below demonstrates an unbalanced force.
Figure 6. Objects in motion with net force.
(From http://www.schools.utah.gov/curr/science/sciber00/8th/forces/sciber/forces.htm)
For more information and sample problems on balanced and unbalanced forces, go to
http://www.schools.utah.gov/curr/science/sciber00/8th/forces/sciber/forces.htm
For a table with types of forces and descriptions for each one, go to
http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l2b.html
Friction
Friction is a force that opposes motion between two surfaces that are in contact with each other. The amount of friction depends on two things: the type of surface and how hard they press against each other. When we consider “how hard” two surfaces are pressing against each other, we are really talking about the force of contact between the surfaces.
It can be beneficial to increase or decrease friction. Students lubricate the keys on their brass instruments to decrease friction and avoid sticking. On the other hand, people can increase friction to avoid slipping.
Figure 7. Friction in footwear.
(From http://revelsports.com/yaktrax.asp)
Three types of friction are sliding friction, rolling friction and fluid friction. Sliding friction is the friction between two solid surfaces as shown below. If it weren’t for friction, students would slip right off their chairs and pencils would slide right out of their hands.
Figure 8. Sliding Friction.
(From http://www.school-for-champions.com/science/friction.htm)
Rolling friction is much less than sliding friction. It is the resistance that occurs when an object rolls. People use wheel barrels and dollies to help move large or heavy objects.
Figure 9. Rolling Friction.
(From http://www.school-for-champions.com/science/friction.htm)
Fluid friction occurs when a solid object is in contact with a fluid, such as a liquid or gas. When a force is applied to either the object or the fluid, there is a friction force that resists the motion. Examples where fluid friction occurs are water flowing through a hose, an airplane flying through the atmosphere, and oil lubricating moving parts. Air resistance is a type of fluid friction. As the force of gravity pulls an object down, fluid friction from the air pushes back up on the object. The diagram below illustrates the amount of force acting on a skydiver throughout their fall. If in free fall long enough, the person will reach terminal velocity when the force of air resistance is balanced with the force of gravity. To increase fluid friction, a person uses a parachute to increase surface area. This allows the person to “catch air”.
Figure 10. Fluid Friction.
(From http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/newtlaws/u2l3e.html)
For more information on friction and the types of friction, go to
http://www.school-for-champions.com/science/friction.htm
Newton’s Three Laws of Motion
Newton’s Three Laws of Motion are valuable for teaching and tying together force and motion vocabulary and concepts. Vocabulary will be highlighted within this section to show where these relationships occur.
Newton’s First Law of Motion
Newton’s First Law of Motion states that an object at rest remains at rest, and an object in motion continues in motion at a constant velocity in a straight line, unless acted upon by an external force or unbalanced force. The First Law of Motion is also known as the Law of Inertia. Inertia is the tendency in all objects to resist any change in motion. A person can pull a tablecloth out from under dishes because the dishes have inertia. The more mass an object has the more inertia it has. An object in motion has inertia because it won’t stop unless an unbalanced force acts on it. For example, if a ball were rolled down a street it would keep going in a straight line forever until a force changes its speed or direction (in this case, friction forces would eventually slow the ball to a stop). Acceleration is any change in speed or direction, so you can say an object will remain at a constant speed and straight line unless a force causes it to accelerate. And if an object has more mass, it is much harder to accelerate when an unbalanced force is applied. Going at the same speed, it is harder to stop a large truck than a motorcycle.
The picture below is a demonstration of inertia. If a person is not wearing a seatbelt when the car is abruptly stopped, the person will continue forward at the same speed the car was originally going. People wear seatbelts to attach them to the car. Students are surprised to hear that when a car is going 75 mph, everything in the car is also going 75 mph and if you’re not attached to the car and it stops, you will continue to travel at 75 mph. All students have experienced inertia in a car. When brakes are slammed, everyone in the car leans forward. When cars turn, our bodies lean because inertia is the tendency to keep going straight.
Figure 11. Demonstration of inertia.
(From http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l1a.html)
For examples of inertia and a home experiment, go to
http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l1a.html
Newton’s Second Law of Motion
Newton’s Second Law of Motion says that net, unbalanced forces acting on an object cause the object to accelerate in the direction of the net force. If a boy is riding a bike and the wind blows against him, then the boy will slow down and therefore negatively accelerate or decelerate. The acceleration is determined by the size of the force and the mass of the object. If the wind is not a strong enough force to slow the boy down then there will be no negative acceleration. If the mass of the boy is small, and the force of the wind is great, then there may be a large negative acceleration. Again, it takes more force to cause a massive object to accelerate.
The relationship between acceleration, mass, and force can be written mathematically, as follows: Force = mass x acceleration, or F = ma. This picture illustrates the relationship between acceleration, mass, and force.
Figure 12. Newton’s 2nd Law.
(From http://www.astronomynotes.com/gravappl/s2.htm#A1.2)
For more information on Newton’s 2nd law of motion and sample problems, go to
http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l3a.html
Newton’s Third Law of Motion
Newton’s Third Law of Motion states that when one object exerts a force on a second object, the second one exerts a force on the first that is equal in size and opposite in direction. This can also be stated as every action has an equal and opposite reaction. When a person jumps on a trampoline, they exert a downward force on the trampoline and the trampoline exerts an equal force upward, sending the person into the air. A person swimming exerts a force on the water and the water pushes back on the swimmer. Rockets illustrate Newton’s Third Law because the burning fuel inside produces hot gases that push against the inside of the rocket and escape out the bottom. The downward push of the gases results in a “paired” force, which is opposite in direction, and therefore, pushes the rocket upward.
Figure 13. Action-Reaction pairs.
(From http://www.schools.utah.gov/curr/Science/sciber00/8th/forces/sciber/newton3.htm)
Content Benchmark P.8.B.1
Students know the effects of balanced and unbalanced forces on an object's motion. E/S
Common misconceptions associated with this benchmark
1. Students inaccurately assume that the lines on a distance-time graph represent a hill on which an object is traveling.
Instead of referring to the variables to read a graph, students will look at the picture only and make assumptions. When students look at the blue line, many incorrectly think that the object has slowed down because the line is getting steeper and steeper. However, the blue line actually shows an object that is accelerating, with its velocity increasing steadily (e.g., an object in freefall). Students may incorrectly think that the green line represents an object that couldn’t go very fast because the hill was too steep, and the red line represents an object that didn’t have a hard time at all because for a while, the hill was flat.
When graphing the position of an object or distance traveled as a function of time, the slope at any point on the curve gives the velocity.
Figure 1. Distance-Time graph for three different objects.
(From http://www.bbc.co.uk/schools/gcsebitesize/physics/forces/speedvelocityaccelerationfhrev2.shtml)
For more information about graphing misconceptions, go to http://physics.montana.edu/physed/misconceptions/graphs/graphs.html
2. Students incorrectly believe that acceleration is only increasing speed.
Acceleration is often used to describe an object that is going faster, but acceleration is not just increasing speed or going fast. Any change in velocity is acceleration. Velocity is defined as the speed and direction of a moving body. Therefore, acceleration can be an increase in speed, decrease in speed, or change in direction. A Ferris wheel going a constant speed is still accelerating because it is continuously changing direction.
For more information on this misconception, go to