Download Name - Humble ISD

Name
Class
Date given
Date due
Chapter 4: Forces
Student Expectations/Self assessment
Read each “I can” statement and the example question that could be used to test your understanding. Before
instruction, if you are sure you can correctly answer the example question, mark the . If you have no idea, mark the
. Repeat the procedure at the end of the unit and before the test to check your understanding.
PHYS 3: use critical thinking and scientific problem solving to make informed
decisions.
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



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



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



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



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



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?



distinguish between balanced and unbalanced forces
o Ex. How can two 3 Newton forces not be equal?
o Initial understanding

Final understanding

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

Final understanding



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?
o Initial understanding
Final understanding



Apply Newton’s Third Law to identify force pairs
o Ex. What is the action/reaction pair of the above problem?
o Initial understanding

Final understanding


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



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



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

Chapter 4 force assignments.
Pull car lab
HW
p133 # 1-4
Day at the Races competition
Static and Kinetic Friction lab
Galileo vs. Newton report
Forces test
Final understanding

Please check grade book for due dates.
HW p128 #1-6
HW
p138 # 1-5
HW
p140 # 1-5
HW
p145 # 1-3
p147 # 1-4
Tension in a string lab
Reminder: Your textbook can be viewed online at http://my.hrw.com. The user name is jwilliams6213. The password is
j2z4p. After logging in, go to the top left menu to choose a chapter or page.
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
= final, i = initial
Δ = delta = “change in”
t = time, SI unit is seconds (s), also shows up as minutes (min) or hours (hr)
x = horizontal displacement = xf - xi. SI unit is meters (m), also shows up as kilometers (km) or
miles (mi)
y = vertical displacement = yf - yi. SI unit is meters (m), also shows up as kilometers (km) or
miles (mi)
v = velocity (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
m/s.
a = acceleration = vf-vi/ tf-ti = Δv/ Δt. Usually constant. Notice the units m/s/s or m/s2.
g = acceleration due to gravity = - 9.81m/s2. Always.
New stuff:
Pull car lab
New variables! Yeah!
SohCahToa is back! Yeah!
Inertia – tendency of an object to continue doing what it is already doing and its resistance to
doing something different.
Mass - a measure of an object’s inertia, SI unit is Kg.
Acceleration is still m/s2
Force – a push or a pull, will produce a change in motion if unbalanced. SI units are Kg • m/s2.
Measured in Newtons (N). 1 N – 0.225 lb. Calculate your weight in Newtons.
Examples of an object changing direction: car wreck, ball bouncing against a wall, baseball and
bat, ball thrown up changes to down
If mass is Kg and acceleration is m/s2 then the formula for force must be F = ma.
Softball/wiffleball drop – person with eyes closed describe difference in feel when someone drops a softball in their hands
vs a wiffleball
Back to inertia. The weight of something is its mass times gravity (acceleration due to gravity).
A ‘heavy’ object is harder to start moving than a ‘light’ one.
Different types of forces:
Contact force – involves physical contact; mechanical force
Field force – no physical contact; gravity, electrical, magnetic, nuclear, electromagnetic. Ex.
balloon attracting hair.
Static between balloon and hair
Field – region of influence for the force
Vector – has magnitude and direction with a reference point. We will assume all forces act on
the center of an object in force diagrams.
Force diagram – all forces shown as vectors
Free body diagram – only shows forces acting ON a body, not those done BY the body.
Action/reaction pairs – for every force there is an equal and opposite counteracting force. Walle and the fire extinguisher. You pushing on the floor and the floor pushing back. Very important
to learn to ID these!
Medicine ball/ rolling chair toss – what happens when person in chair catches ball. What happens when she throws it?
Draw diagram of stick figure pushing on a wall showing all force. 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 a simple version of the forces acting on the kid and by the kid.
3. 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?
HW p128 #1 – 6 Use a large dot to represent the object. Notice the difference
between numbers 4 and 5.
Weights sitting on desk and suspended. What forces are acting on these weights?
Sir Isaac Newton - 1642-1727, developed calculus to help explain physics, still use Laws of
Newtonian Physics to explain forces.
Newton’s First Law of Motion – an object at rest will stay at rest and an object in motion will
continue (same speed, same straight direction) unless acted on by an unbalanced outside force.
Also called the Law of Inertia.
External force - a single force acting on an object.
Net force – sum of all force vectors added together, ∑ is the Greek symbol for “sum”.
External and net force do not equal if there is more than one force.
Draw a free body diagram of a ball moving through the air.
Consider what the ball will do if any of the forces change magnitude.
When opposite forces are in equilibrium, what is the net force?
What is the ball doing when opposite forces are equal?
How to solve a net force problems –
1. Draw diagram
2. Apply coordinate system that fits best. ie, look for right angles in drawing and draw the x
and y axis along those lines.
3. ID all know and unknown variables.
4. Use SohCahToa to resolve x and y values for any vectors at an angle to the chosen x,y
coordinate system.
5. Add resulting components to solve for final x and y values.
6. Use SohCahToa to get final resultant vector.
p132
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 moving vertically?
HW p133 # 1-4
point
Answers MUST have magnitude, direction and reference
Newton’s Second Law of Motion – ∑F = ma; sum of all forces acting ON an object is m•a
Forces can still be broken into component vectors
∑Fx = max
∑Fy = may
If net external force = 0 then acceleration = 0 so velocity stays the same.
Force ON an object, not BY an object
HW p138 # 1-5
Newton’s Third Law of Motion – the force from one object on a second object equals the
force of the second object back on the first; action – reaction pairs.
Watch http://high_speed_video.colostate.edu/ and ID the action/reaction pairs.
Day at the Races competition
When a bat hits a ball with F, the ball pushes back with an = force. When a bullet is shot
forward, the force back against the gun is =. Why does the baseball and the bullet fly off while
the bat and the gun pretty much stays put? Back to 2nd law. F=ma.
According to the 3rd law, that force on both the bat and the ball are equal. According to the 2nd
law, for the force to stay equal, when mass goes up, acceleration has to go down and vice versa.
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 # 1-5
Weight is a measure of gravity’s force on a mass. F = ma.
Your weight is your mass times gravity and is measured in Newtons (N). Where would you
weight the most, Jupiter, Mars or Earth? Where would you weigh the least?
Weight and mass and NOT about how much space is taken up. That is volume and it is a
function of the material’s density.
Normal force = weight; perpendicular to the surface the object is on, regardless of the angle of
the surface. So, not always perpendicular to the ground. Resolve components with SohCahToa.
Friction – opposes any force applied to move an object, to resists its inertia.
 Fs – static friction – the resistance of an object to start moving.
o The rougher the surface, or the heavier the object, the higher the Fs is.
o It is always opposite the applied force.
o Fs, max is the most friction an object can withstand before it starts moving
 Fapplied finally overcomes Fs
 Fk – kinetic friction – the tendency of a moving object to stick to the surface its in contact
with.
o Only after an object starts moving
o Always less than Fs
 Net F = F – Fk
 The higher the mass, the higher the friction
 Rough surfaces have higher Fs and Fk
 Measured in mu (µ)
µk = Fk/Fn
µs – Fsmx/Fn
Ff = µFn
Static and Kinetic Friction lab
Review:
What is friction?
What direction does the force of friction go?
HW p145 # 1-3
p147 # 1-4
Tension – a force applied through a string or rope. The mass of the rope is not considered. The
forces on either end of a string are equal to each other.
Compare ranking tasks
Blocks Attached to Fixed Objects—Rope Tension
E
Two Different Blocks and a Pulley—Tension
E F C D B A
Ropes Pulling Boxes—Rope Tension
Tension in a string lab
What can you conclude about tension in a rope?
D
ACDFG
BE
ACF
B
H
p27
p29
p24