mass, weight, and normal force · force 2nd law. unbalanced forces cause objects to accelerate. the...
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Tuesday weight and friction.notebook
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October 18, 2016
Oct 165:33 PM
Warm Up: Hand in Newton's third law Handout in your class basket.
In your notebook write down final Summary of Newton's Laws:
1st Law. Objects resist changes in motion. This tendency is called inertia. Inertia is not a force
2nd Law. Unbalanced forces cause objects to accelerate. The acceleration is proportional to force and inversely proprtional to mass.
3rd law. For every force, there is an equal and opposite force. The forces are applied to 2 different objects.
Oct 165:40 PM
Take out your plickers card so we can take a quiz.
https://plickers.com/liveview
Jun 292:17 PM
Return toTable ofContents
Mass, Weight, and Normal Force
https://www.njctl.org/video/?v=EiJvrDFswA
Tuesday weight and friction.notebook
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October 18, 2016
Sep 242:31 PM
Mass is the measure of the inertia of an object, the resistance of an object to accelerate. In the SI system, mass is measured in kilograms.
Mass is not weight!Mass is a property of an object.
It doesn't depend on where the object is located.
Weight is the force exerted on that object by gravity. If you go to the moon, whose gravitational acceleration is about 1/6 g, you will weigh much less. Your mass, however, will be the same.
The case of mass versus weight
Click on this link to see a Veritasium video about mass and weight!
Sep 242:33 PM
Weight is the force exerted on an object by gravity. Close to the surface of Earth, where the gravitational force is nearly constant, weight can be calculated with:
Weight – the Force of Gravity
Near the surface of Earth, g is 9.8 m/s2 downwards.
FG = mg
or
W = mg
Jul 1910:52 AM
1 Determine the Force of Gravity (weight) on a 6.0 kg bowling ball.
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Answer Given:
m=6kgW=?
W = mgW= (6 kg) (9.8 m/s2) = 58.8 N
https://www.njctl.org/video/?v=EiJvrDFswA
Tuesday weight and friction.notebook
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October 18, 2016
Jul 1910:52 AM
2 Determine the weight of a small car with a mass of 900 kg.
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Answer
given: m=900 kgW=?
W = mg
W = (900 kg) (9.8 m/s2) = 8820N
https://www.njctl.org/video/?v=90QebbPtyAA
Jul 1910:52 AM
3 Using a spring scale, you find that the weight of a friction block in the lab is around 24 N. What is the mass of the block in kilograms?
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Answer
Given:W=24 Nm=?
W = mgm = W/gm = (24 N)/(9.8 m/s2) = 2.45 kg
https://www.njctl.org/video/?v=qnu5gNseagY
Oct 115:43 PM
4 An object located near the surface of Earth has a weight of a 245 N. What is the mass of the object?
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Answer
Given:W=245 Nm=?
W = mgm = W/g
m = (245 N)/(9.8 m/s2) = 25 N
https://www.njctl.org/video/?v=rP83F6CcdA
Tuesday weight and friction.notebook
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October 18, 2016
Jul 1910:59 AM
5 Which of the following properties of an object is likely to change on another planet?
A MassB WeightC ColorD Volume (size and shape)
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Answer
B
https://www.njctl.org/video/?v=vTR_E6QTkGM
Sep 242:55 PM
6 The acceleration due to gravity is lower on the Moon than on Earth. Which of the following is true about the mass and weight of an astronaut on the Moon's surface, compared to Earth?
A Mass is less, weight is sameB Mass is same, weight is lessC Both mass and weight are lessD Both mass and weight are the same
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Answer B
https://www.njctl.org/video/?v=8HpLY1vQKrA
Sep 242:33 PM
FG
An object at rest must have no net force on it.
If it is sitting on a table, the force of gravity is still there...
Weight – the Force of Gravity
but additionally, what other force is there?
https://www.njctl.org/video/?v=rMNsjZF0g8
Tuesday weight and friction.notebook
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October 18, 2016
Sep 242:33 PM
FG
FN
The force exerted perpendicular to a surface is called the normal force.
The Normal Force
What is the other force?
The normal force is exactly as large as needed to balance the force from the object. (if the required force gets too big, something breaks!)
The words "normal" and "perpendicular" are synonyms.
Oct 115:41 PM
7 A 14 N brick is sitting on a table. What is the normal force supplied by the table?
A 14 N upwards
B 28 N upwards
C 14 N downwards
D 28 N downwards
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Answer A
https://www.njctl.org/video/?v=hnu4FsDE3IQ
Oct 115:43 PM
8 What normal force is supplied by a desk to a 2.0 kg box sitting on it?
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Answer
ΣF = ma = 0ΣF = FN mg = 0FN = mg = (2 kg)(9.8 m/s2) FN = 19.6 N
0
Given:
m=2 kg
FN=?
https://www.njctl.org/video/?v=p4UCHVTKyJg
Tuesday weight and friction.notebook
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October 18, 2016
Jun 292:17 PM
Return toTable ofContents
Friction
https://www.njctl.org/video/?v=6qMm8y2DJU
Dem
oLab
Sep 242:35 PM
There are many different types of forces that occur in nature, but perhaps none is more familiar to us than the force of friction (Ffr).
Friction A Resistive Force
Friction is a resistive force that opposes the motion of an object.
What does sandpaper have to do with friction?
Sep 242:35 PM
Friction is the reason objects stop rolling or sliding along a surface. It is the reason it is difficult to start pushing a heavy box along the floor.
Friction A Resistive Force
There are many different types of friction:
Friction between solid objects and air is often called air resistance.
Friction between two fluids is called viscosity.
Tuesday weight and friction.notebook
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October 18, 2016
Sep 242:35 PM
Where does friction come from?
On a microscopic scale, most surfaces are rough. This leads to complex interactions between them that we don't need to consider yet, but the force can be modeled in a simple way.
Kinetic Friction Force
v
Sep 242:35 PM
Kinetic Friction Force
Friction that acts on an object that is already in motion is called kinetic friction.
For kinetic – or sliding – friction, we write:
Kinetic friction is the product of two things: μk is called the coefficient of kinetic friction, and is different for every pair of surfaces. FN is simply the Normal Force, which, on flat surfaces, is equal to the weight of the object.
Ffr = μkFN
Sep 242:35 PM
Kinetic Friction ForceA larger coefficient of friction means a greater frictional force. Notice the friction that occurs between different materials
in the table below:
Surface Coefficient of Kinetic Friction
Wood on Wood 0.2
Ice on Ice 0.03
metal on metal (lubricated) 0.07
Steel on steel (unlubricated) 0.6
Rubber on dry concrete 0.7
Rubber on wet concrete 0.6
Rubber on other solid surface 0.5 0.9
Teflon on Teflon 0.05
Human Joints in limbs 0.01
Tuesday weight and friction.notebook
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October 18, 2016
Oct 1510:28 AM
FN
mgFappFfr
a
Kinetic Friction ForceA man accelerates a crate along a rough surface.
Draw the crate's free body diagram:
Determine ΣF in the x and y directionsmove for answer
move for answer move for answer
Oct 116:22 PM
9 A brick is sliding to the right on a horizontal surface. What are the directions of the two surface forces: the friction force and the normal force?
A right, down
B right, up
C left, down
D left, up
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Answer
D
https://www.njctl.org/video/?v=HDqDH1Rkx9Q
Oct 116:22 PM
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Answer
Ffr = μkFNFN = mgFfr = μkmgFfr = (0.25)(4 kg)(9.8 m/s2) = 9.8 N
Given:m=4 kgμk=0.25Ffr=?
10 A 4.0kg brick is sliding on a surface. The coefficient of kinetic friction between the surfaces is 0.25. What it the size of the force of friction?
https://www.njctl.org/video/?v=_im4E6iCcA
Tuesday weight and friction.notebook
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October 18, 2016
Oct 116:22 PM
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Answer
Ffr = μkFNFN = mgFfr = μkmgFfr = (0.4)(50 kg)(9.8 m/s2) = 196 N
Given:m=50 kgμk=0.4Ffr=?
11 A 50 kg crate is being pushed across a warehouse floor. The coefficient of kinetic friction between the crate and the floor is 0.4. What it the size of the force of friction?
https://www.njctl.org/video/?v=6A_FxFj2I0M
Feb 128:53 PM
12 A 50 kg crate is pushed across a warehouse floor with a force of 100 N, accelerating at a rate of 1 m/s2. What is the coefficient of friction between the floor and crate?
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Answer
Given:m=50 kgFapplied= 100 Na=1 m/s2
μk= ?
FN
mgFappFfr
a
https://www.njctl.org/video/?v=euLjmMhCArI
Oct 116:03 PM
Static friction is the frictional force between two surfaces that are not moving along each other.
Static friction keeps objects from moving when a force is first applied.
Static Friction Force
Fapplied
v = 0
**
https://www.njctl.org/video/?v=C336w6F3MTU
Tuesday weight and friction.notebook
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October 18, 2016
Oct 116:03 PM
μs is the coefficient of static friction, and is different for every pair of surfaces.
Static Friction Force
Fapplied
v = 0
**Ffr ≤ μsFN
Oct 116:03 PM
Static Friction Force**
Note the ≤ symbol in this equation.
Imagine pushing on a box until it moves. You can apply a small force... nothing happens. You apply more and more force until the box finally starts moving this is the maximum amount of static friction.
The friction can be LESS than the maximum amount or EQUAL to the maximum amount, but never greater. The force of friction is equal to μsFN at the instant when the object starts to move.
Then what happens?
Ffr ≤ μsFN
Sep 242:36 PM
Friction Force
The static frictional force increases as the applied force increases, always equal to the net applied force.
Until it reaches its maximum, μsFN.
Then the object starts to move, and the kinetic frictional force takes over, μKFN .
10
30
20
50
40
10 20 30 40 50 60 70
Applied force, FApp
f = μS FN μk FN
0
no motion
sliding
Friction force, f
**
Tuesday weight and friction.notebook
11
October 18, 2016
Sep 242:36 PM
Friction Force
The static frictional force increases as the applied force increases, always equal to the net applied force.
Until it reaches its maximum, μsFN.
Then the object starts to move, and the kinetic frictional force takes over, μKFN .
**
10
30
20
50
40
10 20 30 40 50 60 70
Applied force, FApp
f = μS FN μk FN
0
no motion
sliding
Friction force, f
Sep 242:35 PM
Friction Force
Surface Coefficient of Static Friction Coefficient of Kinetic Friction
Wood on wood 0.4 0.2
Ice on ice 0.1 0.03
Metal on metal (lubricated) 0.15 0.07
Steel on steel (unlubricated) 0.7 0.6
Rubber on dry concrete 1.0 0.8
Rubber on wet concrete 0.7 0.5
Rubber on other solid surfaces 14 1
Teflon on Teflon in air 0.04 0.04
Joints in human limbs 0.01 0.01
**The table below shows values for both static and kinetic
coefficients of friction.
Notice that static friction is greater than kinetic friction. Once an object is in motion, it is easier to keep it in motion.
Oct 116:22 PM
13 A 4.0 kg brick is sitting on a table. The coefficient of static friction between the surfaces is 0.45. What is the largest force that can be applied horizontally to the brick before it begins to slide?
**
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Answer
Ffr = μsFN
FN = mgFfr = μsmgFfr = (0.45)(4 kg)(9.8 m/s2) Fapp = 17.64 N
Given:m=4 kg
μs = 0.45Fapp=?
https://www.njctl.org/video/?v=C336w6F3MTU
Tuesday weight and friction.notebook
12
October 18, 2016
Oct 116:22 PM
14 A 4.0kg brick is sitting on a table. The coefficient of static friction between the surfaces is 0.45. If a 10 N horizontal force is applied to the brick, what will be the force of friction?
**
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Answer Ffr = μsFN
FN = mgFfr = μsmgFfr = (0.45)(4 kg)(9.8 m/s2) = 17.64 NFapp<Ffr (a=0)Ffr=10 N
Given:m=4 kgμs = 0.45Fapp=10 Nmax Ffr=?
https://www.njctl.org/video/?v=XPztoTbZfcc
Oct 185:19 AM
https://www.khanacademy.org/science/physics/forcesnewtonslaws/inclinedplanesfriction/v/staticandkineticfrictionexample
Oct 185:18 AM