PHYSICS 20
Unit 2: Dynamics
Unit Themes and Emphases
Change and SystemsFocusing Questions
- How do changes in position, velocity, and acceleration allow us to predict the paths of moving objects and systems?
- How do the principles of kinematics influence the development of new mechanical technologies?
Unit B: Dynamics
Themes: Change and Systems
Overview:
In this unit, students investigate causes of change in the position and velocity of objects and systems in a study of dynamics and gravitation. The concept of fields is introduced in the explanation of gravitational effects.This unit builds on:
- • Grade 7 Science, Unit D: Structures and Forces
- • Grade 8 Science, Unit D: Mechanical Systems
- • Science 10, Unit B: Energy Flow in Technological Systems
- • Physics 20, Unit A: Kinematics
Unit B will require approximately 25% of the time allotted for Physics 20.
Focusing Questions:
- How does the understanding of forces help humans improve or change their environment?
- How do the principles of dynamics influence the development of new mechanical technologies?
- What role do gravitational effects play in the universe?
General Outcomes:
There are two major outcomes in this unit. Students will:- B1. explain the effects of balanced and unbalanced forces on velocity
- B2. explain that gravitational effects extend throughout the universe.
Key Concepts: The following concepts are developed in this unit and may also be addressed in other units or in other courses. The intended level and scope of treatment is defined by the outcomes.
- Newton’s laws of motion
- inertia
- vector addition
- static and kinetic friction
- gravitational force
- Newton’s law of universal gravitation
- gravitational field
General Outcome: B1. explain the effects of balanced and unbalanced forces on velocity.
Specific Outcomes for Knowledge
Students will:
20–B1.1k explain that a nonzero net force causes a change in velocity
20–B1.2k apply Newton’s first law of motion to explain, qualitatively, an object’s state of rest or uniform motion
20–B1.3k apply Newton’s second law of motion to explain, qualitatively, the relationships among net force, mass and acceleration
20–B1.4k apply Newton’s third law of motion to explain, qualitatively, the interaction between two objects, recognizing that the two forces, equal in magnitude and opposite in direction, do not act on the same object
20–B1.5k explain, qualitatively and quantitatively, static and kinetic forces of friction acting on an object
20–B1.6k calculate the resultant force, or its constituents, acting on an object by adding vector components graphically and algebraically
20–B1.7k apply Newton’s laws of motion to solve, algebraically, linear motion problems in horizontal, vertical and inclined planes near the surface of Earth, ignoring air resistance.
20–B1.2k apply Newton’s first law of motion to explain, qualitatively, an object’s state of rest or uniform motion
20–B1.3k apply Newton’s second law of motion to explain, qualitatively, the relationships among net force, mass and acceleration
20–B1.4k apply Newton’s third law of motion to explain, qualitatively, the interaction between two objects, recognizing that the two forces, equal in magnitude and opposite in direction, do not act on the same object
20–B1.5k explain, qualitatively and quantitatively, static and kinetic forces of friction acting on an object
20–B1.6k calculate the resultant force, or its constituents, acting on an object by adding vector components graphically and algebraically
20–B1.7k apply Newton’s laws of motion to solve, algebraically, linear motion problems in horizontal, vertical and inclined planes near the surface of Earth, ignoring air resistance.
Specific Outcomes for Science, Technology and Society (STS) (Social and Environmental Contexts Emphasis)
Students will:
20–B1.1sts explain that the goal of technology is to provide solutions to practical problems, that technological development includes testing and evaluating designs and prototypes on the basis of established criteria, and that the products of technology cannot solve all problems (ST1, ST5d, ST6) [ICT F2–4.4]
• assess the design and use of injury-prevention devices in cars and sports in terms of Newton’s laws of motion
• explain how buffalo jumps represented a technological solution to food supply problems and describe the advantages and limitations of such a technique
20–B1.2sts explain that science and technology are developed to meet societal needs and that society provides direction for scientific and technological development (SEC1, SEC4) [ICT F2–4.8] • discuss the use of seat belts in school buses
20–B1.3sts explain that scientific knowledge and theories develop through hypotheses, the collection of evidence, investigation and the ability to provide explanations (NS2)
• analyze the trajectory of lunar dust particles as illustrated in a video.
Note: Some of the outcomes are supported by examples. The examples are written in italics and do not form part of the required program but are provided as an illustration of how the outcomes might be developed.
• assess the design and use of injury-prevention devices in cars and sports in terms of Newton’s laws of motion
• explain how buffalo jumps represented a technological solution to food supply problems and describe the advantages and limitations of such a technique
20–B1.2sts explain that science and technology are developed to meet societal needs and that society provides direction for scientific and technological development (SEC1, SEC4) [ICT F2–4.8] • discuss the use of seat belts in school buses
20–B1.3sts explain that scientific knowledge and theories develop through hypotheses, the collection of evidence, investigation and the ability to provide explanations (NS2)
• analyze the trajectory of lunar dust particles as illustrated in a video.
Note: Some of the outcomes are supported by examples. The examples are written in italics and do not form part of the required program but are provided as an illustration of how the outcomes might be developed.
Specific Outcomes for Skills (Science and Technology Emphasis)
Initiating and Planning
Students will:
20–B1.1s formulate questions about observed relationships and plan investigations of questions, ideas, problems and issues
• identify questions to investigate arising from practical problems;
e.g., What are the relationships among acceleration, mass and force acting on a moving object? (IP–ST1).
Performing and Recording • identify questions to investigate arising from practical problems;
e.g., What are the relationships among acceleration, mass and force acting on a moving object? (IP–ST1).
Students will:
20–B1.2s conduct investigations into relationships among observable variables and use a broad range of tools and techniques to gather and record data and information
• conduct experiments to determine relationships among force, mass and acceleration, using available technologies; e.g., using interval timers or motion sensors to gather data (PR–ST3) [ICT C6–4.4]
• research the use of kinematics and dynamics principles in everyday life; e.g., research traffic accident investigation methods, using the Internet and/or interviews (PR–ST1).
• research the use of kinematics and dynamics principles in everyday life; e.g., research traffic accident investigation methods, using the Internet and/or interviews (PR–ST1).
Analyzing and Interpreting
Students will:
20–B1.3s analyze data and apply mathematical and conceptual models to develop and assess possible solutions
• analyze a graph of empirical data to infer the mathematical relationships among force, mass and acceleration (AI–NS6) [ICT C6–4.1]
• use free-body diagrams to describe the forces acting on an object (AI–NS1).
Communication and Teamwork • analyze a graph of empirical data to infer the mathematical relationships among force, mass and acceleration (AI–NS6) [ICT C6–4.1]
• use free-body diagrams to describe the forces acting on an object (AI–NS1).
Students will:
20–B1.4s work collaboratively in addressing problems and apply the skills and conventions of science in communicating information and ideas and in assessing results
• select and use appropriate numeric, symbolic, graphical or linguistic modes of representation to communicate findings and conclusions (CT–ST2).
Note: Some of the outcomes are supported by examples. The examples are written in italics and do not form part of the required program but are provided as an illustration of how the outcomes might be developed.
Note: Some of the outcomes are supported by examples. The examples are written in italics and do not form part of the required program but are provided as an illustration of how the outcomes might be developed.
Links to Mathematics:
The following mathematics topics are related to the content of Unit B but are not considered prerequisites.Concept Mathematics Course, Strand and Specific Outcome
Data Collection and Analysis
Grade 9 Mathematics, Statistics and Probability (Data Analysis), Specific Outcome 3
Measurement and Unit ConversionsMathematics 10C, Measurement, Specific Outcomes 1 and 2;
Mathematics 10-3, Measurement, Specific Outcome 1;
Mathematics 20-3, Algebra, Specific Outcome 3
Trigonometry
Mathematics 10C, Measurement, Specific Outcome 4;
Mathematics 10-3, Geometry, Specific Outcomes 2 and 4
Rate and Proportions
Mathematics 20-2, Measurement, Specific Outcome 1
Graph Analysis
Mathematics10C, Relations and Functions, Specific Outcomes 1, 4 and 7;
Mathematics 20-3, Statistics, Specific Outcome 1 Solving Equations
Grade 9 Mathematics, Number, Specific Outcome 6;
Mathematics 20-1, Algebra and Number, Specific Outcome 6;
Mathematics 30-2, Relations and Functions, Specific Outcome 3
Scale Diagrams
Mathematics 20-2, Measurement,Specific Outcome 2;
Mathematics 20-3, Geometry, Specific Outcome 2
Slope
Mathematics10C, Relations and Functions, Specific Outcomes 3 and 5;
Mathematics 20-3, Algebra, Specific Outcome 2
Area Calculations
Mathematics 10-3, Measurement, Specific Outcome 4
Powers
Mathematics10C, Algebra and Number, Specific Outcome 3
Note: The use of systems of equations, the quadratic formula and trigonometric ratios for angles greater than 90º is not required in this unit.
Chapter 3: Forces can change velocity
Key Concepts
- Vector addition
- Newton’s laws of motion
- Static and kinetic friction
Knowledge
- Explain that a non-zero net force causes a change in velocity
- Calculate the net force
- Apply Newton’s three laws to solve motion problems
- Explain static and kinetic friction.
STS
- Explain that the goal of technology is to provide solutions to practical problems
- Explain that science and technology develop to meet societal needs
- Explain that science develops through experimentation.
3.1 The Nature of Force
Dynamics – branch of mechanics dealing with the cause of motionForce – a quantity measuring a push or pull on an object
Free-body diagram – vector diagram of an object in isolation showing all the forces acting on it A free-body diagram helps you write the net force acting on an object.
Net force is the vector sum of two or more forces acting simultaneously on an object. The net force causes the acceleration
Ex. A person pulls on an object with 100 N of force east, and the force of friction is 30 N west.
The sum of the forces is:
The net force causes the acceleration
Ex. Two forces are acting on an object. The first force is 40 N east and the second force is 30 N north.
a. Draw a free body diagram.
b. What is the net force acting on the object?
Example 3.4 p 134
Normal force – force on an object that is perpendicular to a common contact surface (ground holding an object up)
Review Questions
What is a force?What is shown on a free body diagram?
Why is the net force important?
What is the normal force?
3.2 Newton’s First Law
Newton’s first law states that an object will continue being at rest or moving at constant speed in a straight line unless acted upon by a non-zero net force.Newton’s first law: ΔF = 0 when a = 0 or Δv = 0 →→→
Inertia – property of an object that resists acceleration
Principle of uniform motion When the forces on an object are balanced then the object travels in uniform motion (no acceleration).
Principle of uniformly accelerated motion When the forces on an object are not balanced, net force, the object will accelerate uniformly.
What is Newton’s First Law? Objects in motion stay in motion, objects at rest stay at rest until a net force acts on the object. Apply Newton’s first law to a car. The car won’t move until the engine pushes it. Car will roll until the brakes are applied.
Review Questions
What is Newton’s First Law?Apply Newton’s first law to a car.
3.3 Newton’s Second Law
Newton’s second law states that when a non-zero net force acts on an object, the object accelerates in the direction of the net force. The magnitude of the acceleration is directly proportional to the magnitude of the net force and inversely proportional to the mass of the object.Newton’s second law:
F=ma
| a = | Fnet |
→
→
|
| m |
F - Force, N = kg·m/s2
m - mass, kg
a - acceleration, m/s2
Inertial mass – mass measurement based on the ratio of a known net force on an object to the acceleration of the object
Ex. A Porsche 997 Turbo can accelerate from 0 to 100 kph in 3.2 seconds and masses 1443 kg. How much force is required to accelerate it?
Review Questions
What is Newton’s second law?What are the units for force?
3.4 Newton’s Third Law
Newton’s third law states that if object A exerts a force on object B, then B exerts a force on A that is equal in magnitude and opposite in direction.Newton’s third law: FA on B = FB on A→ →
Action force – force initiated by object A on object B
Reaction force – force exerted by object B on object A
Action-Reaction Pairs
Ex. Tigger jumps up accelerating upwards at 10 m/s2. If Tigger masses 8.0 kg, what is the force of the ground holding up Tigger?
Review Questions
What is Newton’s Third Law?If every force has an equal opposite force why do things move?
3.5 Friction Affects Motion
Forces on an inclineFg|| = mg×sinθ →
Fg⊥ = mg×cosθ normally Fg = -FN →→→
Fg|| = mg×sinθ→
Fg|| - force of gravity parallel to the surface, N
Fg⊥ = mg×cosθ
Fg⊥ - force of gravity perpendicular to the surface, N Ex. A 75 kg person is standing on an icy frictionless slope at 30o. What is the person’s acceleration down the hill?
Friction – force that opposes either the motion of an object or the direction the object would be moving in if there were no friction
Static friction – force exerted on an object at rest that prevents the object from sliding on another object
Kinetic friction – force exerted on an object in motion that opposes the motion of the object as it slides on another
The magnitude of the force of friction acting on an object is directly proportional to the normal force on the object.
| |Ff| = μ |FN| |
→
→
|
μ - coefficient of friction, no units
FN - force normal, force of the ground pushing up on the object, N
INSERT FORCE OF FRICTION VIDEO
The coefficients of friction are proportionality constants that relate the magnitude of the force of friction to the magnitude of the normal force.
Temperature, moisture, and the smoothness or roughness of the contact surfaces, and the materials forming the contact surface are some factors that affect the value of the coefficients of friction.
Coefficient for static friction – proportionality constant relating Ff static maximum and FN
Coefficient for kinetic friction - proportionality constant relating Ff kinetic and FN
FOCES PUSHING ON AN OBJECT VIDEO
Ex. A Cooper mini with a mass of 617 kg skids to a halt. Use table 3.4 p. 183 to calculate the kinetic friction between the rubber tires and the dry asphalt.
Ex. The same mini Cooper is traveling at 20 m/s and then skids to a halt.
A. What is the car’s acceleration?
B. How far does the car skid?
Ex. The same Cooper mini skids to a halt in the rain. What is the ratio of the kinetic friction from dry to wet conditions?
Ex. A 30 kg child on waxed hickory skis on dry snow is standing on a 40o hill.
a. Draw a free body diagram.
b. What is the force of gravity perpendicular?
c. What is the force normal?
d. What is the force of static friction?
e. What is the force of gravity parallel?
f. What is the child’s initial acceleration?
Ex. A skateboarder (m=50 kg) is rolling up a 30o hill at 4.5 m/s. How far will the skateboarder go up the hill if μ=0.25.
Review Questions
What is friction?What is the difference between static and kinetic friction?
What is larger static or kinetic friction?
What is the force of friction proportional to?
What is the equation for the force of friction?
What are the two components of gravity for objects on a slope?
What is the equation for the force of gravity parallel to the surface?
What is the equation for the force of gravity perpendicular to the surface?
Program of Studies
Alberta Physics 20
Program of Studies
Alberta Physics 20
Program of Studies
Contact me:
BCHS Barrhead AB
Steven Montgomery
steven.montgomery@pembinahills.caBCHS Barrhead AB
Textbook
Physics 20 & 20 Textbook (2009)
Physics 20 & 20 Textbook (2009)