Download Chapter 4 forces - student practice notes

Name
Class
Date given
Date due
Chapter 4: Forces
Student Expectations/Self-assessment
I can:

Define inertia (Newton’s First Law) and relate it to mass
o Ex. As the mass of an object increases, what is the effect on the object’s inertia?
o Initial understanding
Final understanding
A B C D F
A B C D F

describe the affect of net force on motion (distinguish between acceleration and equilibrium)
o Ex. Describe the acceleration of an object moving North at 15m/s when forces on the object are in
equilibrium.
o Initial understanding
Final understanding
A B C D F
A B C D F

distinguish between contact and field forces
o Ex. One of these things is not like the other. One of these things just doesn’t belong: gravity, electromagnetic,
contact, nuclear.
o Initial understanding
Final understanding
A B C D F
A B C D F

identify types of forces including but not limited to normal force, tension, weight, friction, balanced & unbalanced
forces, net force
o Ex. ID all the forces acting on a pendulum swinging on a string.
o Initial understanding
Final understanding
A B C D F
A B C D F

distinguish between static and kinetic friction
o Ex. Which type of friction must be overcome for an object to start moving? Which type of friction must an
object overcome to continue moving?
o Initial understanding
Final understanding
A B C D F
A B C D F

distinguish between mass and weight
o Ex. If your mass is 70 kg, what is your weight? Which one, mass or weight, is measured in Newtons?
A B C D F
A B C D F

distinguish between balanced and unbalanced forces
o Ex. How can two 3 Newton forces not be equal?
o Initial understanding
A B C D F
Final understanding
A B C D F
develop and describe free-body (force) diagrams
o Ex. Draw a free body diagram for a sled sliding down an icy slope.
o Initial understanding
A B C D F
Final understanding
A B C D F


Use Newton’s Second Law to determine and calculate the net external force on an object
o Ex. A 2.1kg bat has a force of 2.5N applied to it by a baseball. What is the acceleration of the ball?
A B C D F
A B C D F
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
Apply Newton’s Third Law to identify force pairs
o Ex. What is the action/reaction pair of the above problem?
o Initial understanding
A B C D F
Final understanding
A B C D F

distinguish between vertical and horizontal forces
o Ex. A car is driving 30 degrees N of W. A bird is flying at 30 degrees above the horizon. Which object is
moving horizontally?
o Initial understanding
Final understanding
A B C D F
A B C D F

use kinematic equations, Newton’s 2nd Law and force diagrams to solve physical problems including horizontal
forces(with or without friction), vertical (i.e. rockets, elevators, etc.) problems, equilibrium
o Ex. A 24kg crate initially at rest on a horizontal floor requires a 75 N horizontal force to set it in motion. Find
the coefficient of static friction between the crate and the floor.
o Initial understanding
Final understanding
A B C D F
A B C D F

Recognize contributions of Galileo and Newton to the studies of physics
o Ex. What did Galileo conclude about gravity that Newton did not?
o Initial understanding
A B C D F
Chapter 4 force assignments.
HW p128 #1-6
HW p133 # 1-4; 138 # 1-3, 5
Day at the Races competition
HW p145 # 1-3; p147 # 1-4
Forces test
Final understanding
A B C D F
Please check grade book for due dates.
Galileo vs. Newton report
Luke, use the Force lab
HW p140 # 1, 3-5
Static and Kinetic Friction lab
Review:
Always check your units. They MUST match each other. For example, you can’t have speed in meters per
second and multiply by hours. Use dimensional analysis to make units match.
f
= ____________, i = _______________
Δ = called ____________; means _________________
t = ___________, SI unit is ______________, also shows up as minutes (min) or hours (hr)
x = ______________________________________ = xf - xi. SI unit is __________, also shows up as
kilometers (km) or miles (mi)
y = ________________________________, ____ = yf - yi. SI unit is __________, also shows up as kilometers
(km) or miles (mi)
v = _________________ (this is a lower case v!) = motion in a given direction. ANY change in speed OR
direction (like turning) is a change in velocity. The slope on a distance/time graph. SI units are
_______________________.
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a = _________________ = vf-vi/ tf-ti = Δv/ Δt. Usually constant. Notice the units ___________. The slope on a
velocity time/graph. SI units are ________________________.
g = called _____________________________________ ; value is ______________________
New stuff:
Practice:
Draw a force diagram of stick figure pushing on a wall showing all forces. Then draw a free body diagram for
the person and the wall separately.
Draw a free body diagram:
1. Consider the actual situation of me pulling a screaming kid down the hall.
2. Draw and label each vector indicating a force ON the kid, one at a time, all acting on the center of the
kid. Remember to make the length of the arrow represent the magnitude of the force.
Review:
What is a force?
What is the formula for force?
What are the units for force?
What’s the difference between a force diagram and a free body diagram?
What is the difference between mass and weight?
What is Fg?
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What is Fn?
What is F(applied)?
What is an action/reaction pair?
See http://www.magicofphysics.com/flashes/detail/56/Forces_Lab.html for examples of action/reaction pairs
put to use.
HW
p128 #1 – 6 Use a large dot to represent the object. Notice the difference between numbers 4 and 5.
Galileo vs. Newton report – 1 to 1.5 pg, dbl spaced, 12pt font, MLA format, at least 2 citations, in text citing,
summative grade. Summarize the contributions of both Galileo and Newton to the understanding of moving
bodies and gravity. Were their findings independent or did one build on the other? What parts of their
findings or what systems they created do we still use today? How did society receive each of them at the
time? I will check for plagiarism.
How to solve a net force problem (without friction) –
1. Draw free-body diagram
2. Apply coordinate system that fits best. ie, rotate your x-y plane so that the most vectors are along the x
and y axis.
3. Use SohCahToa to resolve x and y values for any vectors still at an angle. ie: find components, aka legs,
to hypotenuse
4. Add all components along x axis to find ƩFx and add all components along the y axis to find ƩFy in the
y direction.
5. Redraw diagram
6. Use SohCahToa and p-theorem to get final resultant vector (net force). ie: find the hypotenuse
Practice:
3. Back to pulling the kid down the hall…If I am pulling with 80N at an angle of 25° to the floor, what are
the horizontal and vertical components of the applied force?
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4. During Field Day, four families play fourway tug of war. Tritons pull with 100N North. Aztecs pull with
120N South. Tigers pull with 125N East. Blue Angels use their mind power to pull with 132N West.
What is the resultant x axis force? What is the resultant y axis force? What is the sum of the forces on
the center knot and which direction will the center knot move?
5. A 2.3kg piñata is hanging from a tree when little Susie whacks it with a force of 95N at an angle of 15°
above the horizon. What is the net force on the piñata?
review:
What is Newton’s 1st Law of motion?
What are the forces involved in a static object?
What are the forces involved in a suspended object?
What are the forces involved in an object on a surface?
What are the forces involved in an object moving vertically?
HW p133 # 1-4 (Answers MUST have magnitude, direction and reference pt)
Practice:
6. After Susie’s hit sends the piñata flying, what acceleration will it move at?
HW p138 # 1-3, 5
Day at the Races competition
Review:
What are Newton’s 2nd and 3rd Laws of Motion?
How are mass and acceleration related in the formula F = ma?
How do the 2nd and 3rd laws relate to each other to explain the motion of shooting a three point basket?
HW p140 # 1, 3-5
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Friction – opposes any force applied to move an object, to resists its inertia. Always in the opposite direction of
the ∑ of the applied force along the surface.
 Fa - applied force(s) done TO a body from outside; a push or a pull or a combination
 Fg – gravity, total Fg is always and only pointed straight down; = ma = mg.
o If surface is at an angle, rotate the x-y plane (coordinate system) so that the x axis lines up with the
surface
 Fgy - use SohCahToa to resolve component of Fg that is perpendicular to the surface (this is
what is pulling the object into the surface); theta is the same as the angle the surface is at
 Fgx - use SohCahToa to resolve the component of Fg that is parallel to the surface (this is
what is pulling the object down a surface at an incline); theta is the same as the angle the
surface is at
 Fn - Normal force; perpendicular to the surface the object is on, regardless of the angle of the surface; is =
to Fgy (the proportion of Fg that is perpendicular to the surface) so not equal to all of Fg if the surface is at
an angle
 Fs – static friction – the resistance of an object to start moving
o The rougher either surface is, or the higher Fn is, the higher the Fs will be
o It is always opposite and equal to the applied force (the applied force includes Fgx when the
surface is at an angle) when the object is not moving
o µs – coefficient of static friction is a measure of how ‘sticky’ a surface is before the object is
moving; depends on both surfaces; no units
o Fs, max is the most friction an object can withstand before it starts moving
 The point at which Fapplied finally overcomes Fs
 Fk – kinetic friction – the tendency of a moving object to stick to the surface it’s in contact with. Continued
resistance to sliding.
o Only after an object starts moving
o Force needed to keep an object in motion going instead of slowing down/stopping
o Always less than Fs
o µk coefficient of kinetic friction is a measure of how ‘sticky’ a surface is after the object is moving;
depends on both surfaces; no units
o The rougher either surface is, or the higher Fn is, the higher the Fk will be
 Ff – force of friction in general, either static or kinetic
 Net F = ƩF = Fapplied – Ff
How to solve a net force problem with friction –
1. Draw diagram, be sure Fn is drawn perpendicular to the surface and Fg is drawn straight down.
2. Rotate the x-y plane (coordinate system) so that the x axis lines up with the surface if on a slope.
3. Use SohCahToa to resolve x and y values for any applied forces still at an angle. ie: find components,
aka legs, to hypotenuse(s)
4. Use F=ma to find Fg.
5. Use SohCahToa to resolve Fg into its Fgx and Fgy components if on a slope, aka legs. If only mass of
the object is given, multiply by g to convert to Fg
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6. Redraw diagram
7. Find Fn – it’s = to Fgy minus any forces pulling up along the y axis or plus any forces pushing down
along the y axis
8. Find Ff – it’s equal to µ(Fn) (if you know the object is moving, use Fk. If it’s not use Fsmax)
9. Find ƩF (this is usually Fx because usually we’re talking about an object on a surface)
10. Use F = ma to determine the object’s acceleration. Which F? ƩFx
µk = Fk/Fn
µs = Fsmax/Fn
Ff = µFn
Practice:
8. A 15kg desk takes 280N of force to start it moving but only 220 to keep it moving. Find the coefficient
of static friction and the coefficient of kinetic friction.
9. Julana wants to move a 20kg box. She attaches a rope to the box and pulls with a force of 90N at an
angle of 30° above the surface. Bobby helps by pushing with a force of 80N parallel to the ground. The
coefficient of friction between the box and surface is 0.5. What is the acceleration of the box?
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10. Julana and Bobby continue moving the box up a 12° hill while applying the same forces. What is the
box’s acceleration?
11. A 5kg textbook is resting on a drafting table that is tilted at a 25° angle to the ground. Will the book
slide off the table if the coefficient of friction is 0.1?
HW p145 # 2-3
p147 # 1-4
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