COURSE: PHYSICS Grade 11
University Preparation
By: Sonu Bhutani
CONCEPT PRESENTATION
TOPIC: FORCES ACTING ON OBJECTS
A SUMMARY
Background Information:
The concept of forces is fundamental for solving statics and dynamics problems. If we
know the forces that are acting on an object we can determine how it moves. Conversely,
if we know how an object moves we can calculate the forces acting on it. Forces only
exist as a result of an interaction.
Contact forces are those types of forces that result when two interacting objects are
perceived to be physically contacting each other. Examples- Frictional forces, normal
forces, air resistance, applied force etc. Action-at-a-distance forces are those types of
forces that result when two objects are not in physical contact with each other yet are able
to exert a push or pull despite their physical separation for example gravitational forces,
electrical forces. If two individual forces are of equal and opposite direction, then the
forces are said balanced. An object is said to be acted upon by an unbalanced force that is
not being balanced by a force of equal magnitude and in opposite direction.
Free body diagrams are used to show the relative magnitude and direction of all forces
acting upon by an object in a given situation. The only rule for drawing free body
diagram is to depict all the forces that exist for that object in the given situation. The free
body diagram allows you to set up the problem properly so that you can correctly solve
for unknowns such as forces, and acceleration. It allows you to get a “feel” for what’s
going on before applying the equations of motion.
Specific expectations covered:
C2.1 use appropriate terminology related to forces, including, but not limited to: mass,
time, speed, velocity, acceleration, friction, gravity, normal force, and free-body
diagrams.
C2.2 conduct an inquiry that applies Newton’s laws to analyze, in qualitative and
quantitative terms, the forces acting on an object, and use free-body diagrams to
determine the net force and the acceleration of the object.
C2.5 plan and conduct an inquiry to analyze the effect of forces acting on objects in one
dimension, using vector diagrams, free-body diagrams, and Newton’s laws.
Instructional plan outline:
Lesson sequence
Meaning of force and type
of forces-Review
Pre-content: Balanced and
unbalanced forces
Free body diagrams
Calculation of net force
using free body diagrams
Effect of forces acting on
an object
Instructional strategy
Brainstorming
Assessment/Evaluation
Brainstorm response
Video representation
Questioning
Demonstration
Video representation
Group activity-Cooperative
learning method
Individual activityComputer animation
Problem solving method.
Worksheet given
Students’ response
Inquiry based learning
Reflection on demonstration
questions
Involvement in group
activity with rating scale.
Worksheet response with
rubric
Activity Task sheet with
rubric
Concept Development Process (Details)
Lesson 1
Review - Force and type of forces
Teaching ideas:
Brainstorming-Teacher may divide students in teams and ask few questions by means of
a quiz to review concept of force and its types –
Examples:
1. What effects force can produce?
2. A body is moving with a constant velocity. Is there any force acting on it?
3. Teacher draws a table – Contact forces and Action-at-a-distance forces and asks
a. What type of force is Electrical force?
b. What type of force is applied force? Etc.
Teacher fills the table according to students’ response and review giving brief description
of each type of force.
Evaluation procedure:
Teacher evaluates students on the basis of brainstormed response.
Lesson 2
Pre content – Balanced and unbalanced forces
Teaching ideas:
Teacher asks following questions to studentsIf a book is lying on the table, is there any force acting on it?
If a ball is rolling on the floor with constant velocity, is there any force acting on it?
Why does it stop after some time?
Video is shown to the students, below is the link:
http://www.teachertube.com/viewVideo.php?video_id=58727
1. During video representation, teacher asks students to answer the questions that
come up during the representation. Teacher provides feedback to students’
response and review the concept of balanced and unbalanced forces with more
examples.
2. Teacher asks students to state situation where any unbalanced force is acting.
Evaluation procedure:
Students’ response on questioning during video
Students, response in stating situation where unbalanced force is acting.
Lesson 3
Drawing free body diagrams
Learning objective: The student will be able to:
 Find all the forces acting on an object
 Draw free body diagrams
Teaching ideas:
1. Demonstration – Teacher shows the following pictures to the students
Teacher asks students to state all the forces acting on the computer lying on table.
Teacher draws a free body diagram according to students’ response and explains them,
how to depict forces acting on an object in a free body diagram in case forces are
balanced.
Teacher asks students to state all the forces acting on the fish. Teacher again draws free
body diagram according to students’ response and explain them how to draw length of
arrows in free body diagram in case of unbalanced forces.
2. Video Representation – Video is shown to the students in which various situations are
shown and explained how to draw free body diagrams. Below .is the link:
http://youtu.be/cErlwSUXwsg
3. Group Activity – To draw free body diagram
About activity: Students are divided into groups, teacher prepares a box in which cards
are kept containing different situations, each group is supposed to pick a card and draw a
free body diagram. Each group is supposed to submit the task sheet at the end of the
activity and present and explain free body diagram in front of the class.
4. Individual Activity: Through an animation, students will practice their skill at
constructing free body diagrams for given physical situations. Students can use web help
button. Below is the link.
www.physicsclassroom.com/shwave/fbd.cfm
Evaluation procedures:
Group involvement of the students will be judged. Group activity task sheet and
presentation is evaluated using a rubric.
Lesson 4
Calculation of net force and acceleration
Learning objectives: Students will be able to:
 Calculate net force using free body diagrams.
 Calculate acceleration produced in the body.
Teaching ideas
Teacher asks students:
 If two forces are acting in the same direction, how will you get resultant
force?
 If two forces are acting in opposite direction, how will you obtain resultant
force?
 What will be effect of resultant force?
 On what factors this effect (change in velocity) depends?
Teacher reviews Newton’s Second Law and defines force quantitatively as product of
mass and acceleration.
Problem solving method:
Teacher describes following points before solving a problem
1. Determine what type of free body diagram best depicts the situation.
2. Determine if the force of friction will be considered in the problem.
3. Draw a free body diagram of the situation.
4. Label all of the forces acting on the object.
5. Show all of the known forces in the free body diagram.
6. List all of the equations that apply to the problem.
7. Determine what variables are known and needed.
8. Substitute the given quantities into the equations using proper units.
9. Solve the equation, carrying units throughout the problem.
10. Check your answer to make sure it makes sense.
When solving force problems, all of the forces acting on an object must be calculated.
When determining the net force acting on an object, the horizontal and vertical forces
are calculated independently. If an object is not accelerating horizontally or
vertically, the net force in that direction is zero and it does not affect the object’s
acceleration. The net force acting on an object is found by adding all of the forces
acting on the object either horizontally or vertically.
Teacher explains systematic steps to solve following problems in the class
o Draw a free body diagram of a 10 kg rock sitting on the ground. Calculate
the net force acting on the rock.
o Draw a free body diagram of a 10 kg rock as it is lifted straight up with a
constant force of 148 N. Calculate the net force acting on the rock and its
acceleration.
o Two people are pushing a stalled car. The mass of the car is 1850 kg.
One person applies a force of 275 N to the car, while the other person
applies a force of 395 N. Both people are pushing in the same direction.
If there are 560 N of friction opposing the car’s motion, what is the
acceleration of the car?
Evaluation Procedure:
Following worksheet is given:
Worksheet
o What is the acceleration of the baby as she falls with her diaper-parachute
open?
o An 8000 kg Navy jet is accelerating upward at 4 m/s2. Calculate the
upward force provided by the jet’s engines to achieve this acceleration.
o Determine the normal force of a 328 kg tree in the forest.
o A 4500 kg helicopter is accelerating upward at 2 m/s2. Calculate the
upward force provided by the propellers to achieve this acceleration.
o A 100 kg trunk is lifted straight up with a constant force of 1080 N.
Calculate the net force acting on the trunk and its acceleration.
o A 157 kg barrel needs to be moved across a warehouse. A force of 395 N
is applied to the barrel to slide it across the tile floor. The coefficient of
friction between the tile and barrel is 0.19. Calculate the net force acting
on the barrel and its acceleration.
Lesson 5
Effect of forces acting on an object
Learning objectives:


Predict qualitatively how an external force will affect the speed and direction of
an object’s motion.
Explain the effect of force with help of free body diagram.
Teaching ideas:
Activity: Inquiry based learning
Teacher shows following picture to students and asks
Think about this situations
If you were pushing these elephants to move…
1. Which will need more force to move? Why?
2. Which direction would they be travelling? Why?
3. Which elephant will move faster? Why?
Iffat Sardharwalla
March 2010
Teacher tells students to perform a simulation lab which will help them to study
qualitatively, how an external force will affect the speed and direction of an object’s
motion.
Below is the link of computer simulation force and motion activity 1
http://phet.colorado.edu/en/simulation/forces-and-motion
About Activity – Students are given instruction about computer simulation and expected
learning outcomes. Students will be given following task sheet.
1. Use Forces and Motion simulation to create Joe’s situation from the Check Your
Understanding page.
a. Talk about how your force drawings compare to the free body diagram window for
each scene and adjust your sketches with a new color if necessary.
b. Look at your reasoning 1b. Have your thoughts changed now that you run the
experiment? Explain your answer.
2. Use the simulation to verify or correct your drawings and reasoning’s for Annette’s
book. Make changes in a new color.
3. Explain how you could use the simulation to study Emily and Fran’s situations even
though there are no people in the simulation. Test your ideas and make corrections to
your page in a new color.
4. Start with a short investigation using the file cabinet. In an organized fashion, record
observations about how pushing on the cabinet changes it’s speed and direction of
motion. Include the free body diagrams.
For example, you might test the following: Does a push from the right always make the
file cabinet go right? What roll does friction play? How can you use the free body
diagram to help you make predictions?
5.Using your observations, summarize how you could predict what happens to the speed
and direction of a file cabinet when a force is applied.
6.Test how well your understanding applies in specific situations. For each, make a
prediction, and then test your ideas using the simulation. Make a table to record your
prediction, observations, and draw the free body diagram. Include comments about
whether the test supports or refutes your summary in question 5.
a.
b.
c.
d.
How much force does it take to make the cabinet move from rest with friction on?
What’s different with the friction off?
What happens if you change the cabinet to a book and also to a refrigerator?
If the cabinet is moving when the force is applied, what do you need to consider?
Are there different things to consider if you switch the cabinet to a dog or crate?
e. Think of other experiments that would help you verify your ideas. Describe your
experiments and continue to fill in your table.
7.Write a final summary of how you can predict, with the help of free body diagrams,
what effects an external force will have on the speed and direction of an object's
motion.
Evaluation Procedure:
Checking the Task sheets submitted by the Students at the end of the activity with help of
a rubric.
Assessment and Evaluation
Brainstorming Quiz – Force and type of forces (K/U)
Video Questioning reflection (C)
Demonstration reflection (C)
Group activity rating scale ( K/U and C)
Lab simulation task sheet (I & C)
Culminating task rubric for lab activity task sheet and worksheet (K/U, I and C)
Unit test (K/U, A and C)
Advanced Preparation
1. Prepare quiz questions
2. Teacher must perform lab simulation himself/herself before assigning it to
students.
3. Prepare cards for group activity.
4. Prepare rubric for computer simulation task sheet and work sheet.
5. Go through videos before presenting in front of the class.
Potential difficulties faced by students and possible solutions
1. Students may have difficulty in understanding how different forces can be
depicted by a free body diagram. For this purpose, teacher first demonstrates an
example in front of the class and then video is shown to students to develop a
thorough understanding how free body diagrams are drawn.
2. Students may face problems while they are drawing free body diagrams, since
students are struggling with the new concept, a group activity is designed for
students so that they can share their ideas and think creatively which may help
them to deeply understand the concept.
3. Students may find problem in calculating the net force with the help of free body
diagrams. For this purpose, problems have been arranged from simple to complex.
Through problem solving method, teacher shows and explains systematic steps to
calculate net force and acceleration.
4. Students may have a difficult time in explaining the affect of force with the help
of free body diagram. For this purpose inquiry based learning is incorporated,
students are supposed to perform a computer simulation and submit their
reflections on a task sheet provided to them.
Annotated references
Textbook: Mc Graw Hill Ryerson – High school Physics Grade 11
The following physics websites are particularly useful for quick tutorials, review and
simulations
• http://www.physicsclassroom.com/Class/
The Physics Classroom Tutorial is an online physics tutorial with multimedia and
shockwave studios.
• http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
Hyper Physics is an exploration environment for concepts in physics which employs
concept maps and other linking strategies to facilitate smooth navigation.
• http://phet.colorado.edu/
This site contains interesting computer simulations helping students to develop deeper
understanding about concept.
• http://www.mrfizix.com/home/applicationnewtonlaws.htm#_top
You can find answers to your most complex physics questions, and even find ideas for
lab experiments and projects.
The following websites are for enrichment - the joy of physics and science
http://ocw.mit.edu/courses/physics/8-01-physics-i-classical-mechanics-fall-1999/videolectureshttp://www.pbs.org/wgbh/nova/elegant/
Practical applications and societal implications
1. Airbags are used in automobiles because they are able to minimize the effect of a
force on an object involved in a collision.
2. Mountain climbers use nylon ropes to increase the stopping time and decrease the
stopping force.
3. In tennis, baseball, racket ball etc. giving the ball a high velocity often leads to
greater success.
4. Knowledge of force is taken into account by a driver when car goes around a
corner.
5. Principles of launching rockets and satellites require knowledge of forces.
6. The cranes are used to build high rise buildings of a large city. The ability to lift,
balance and move object from feather light sheet to massive steel girders requires
knowledge of force.
7. The introductions of technologies that absorb or dissipate force, such as crumple
zones, seat belts and air bags have reduced automobile injuries.
8. Designing of golf club requires knowledge of forces.