5. ch5 - force ii tuth

8/3/2019 5. Ch5 - Force II Tuth

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Chapter 5Using Newtons Laws


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This Semester Mechanics

Kinematics description of motion (Ch. 2-3)

Dynamics forces and acceleration (Ch 4-5)

Energy (Ch 6-7)

Systems of Particles (Ch 9)

Oscillations (Ch 13)

Gravity (Ch 8)

Rotation (Ch 10-11)


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Question

An object is moving with a constant velocity.Which statement MUST be true?

A) The net force on the object is zero.B) A net force is acting on the object, in the

direction of motion.

C) No forces are acting on the object.D) An even number of forces are acting on the

object


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Question

If you were to move into outer space far from anystellar objects

A) your mass would change, but your weightwould not change.

B) both your weight and mass would change.

C) your weight would change, but your mass

would not change.D) neither your weight nor your mass would

change.


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Question

An object is moving to the right in a straight line.The net force acting on the object is also directedto the right, but the magnitude of the force isdecreasing with time. The object will:

A) stop and then begin moving to the left.B) continue to move to the right, with its speed

decreasing with time.C) continue to move to the right with a constant

speed.D) continue to move to the right, with its speed

increasing with time.


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Question

A person gives a shopping cart an initial push to get itmoving, then lets go. The cart travels forward along thefloor, gradually slowing down as it moves. Which of thefollowing is true while the cart is slowing down?

A) Both a forward and a backward force are acting on thecart, but the backward force is larger.

B) Only a backward force is acting, no forward force.

C) Only a forward force is acting which diminishes with

time.

D) Both a forward and a backward force are acting on thecart, but the forward force is larger.


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Question

Which is the correct FBD for Mass #2 (NoFriction)? m=2 kg

M=1 kg

30 degrees


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How does Friction behave?

Direction of force of friction always opposesmotion

Friction proportional to the Normal Force


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Simulation

http://phet.colorado.edu/simulations/sims.php?sim=Forces_in_1_Dimension
http://phet.colorado.edu/simulations/sims.php?sim=Forces_in_1_Dimensionhttp://phet.colorado.edu/simulations/sims.php?sim=Forces_in_1_Dimensionhttp://phet.colorado.edu/simulations/sims.php?sim=Forces_in_1_Dimensionhttp://phet.colorado.edu/simulations/sims.php?sim=Forces_in_1_Dimension


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Friction

Static Friction

Kinetic Friction

In general,

What are the units of m

- coefficient of static friction

s s N

s

f Fm

m

- coefficient of kinetic friction

k k N

k

f Fm

m

S km m


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Friction

The coefficient m is determined by the twosurfaces in contact

The force of friction does not depend on the

area of contact, just the force pushing the twosurfaces together.

These are empirical laws

Empirical=Relying on or derived fromobservation or experiment


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Question

Is it easier to push or pull a sled if there isfriction?

A) Easier to pushB) Easier to pull

C) Same either way


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Question

Is it easier to push or pull a sled if there isfriction?


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Cars

s k

s k

so

F F

m m

What type of friction is there between the tiresof a car and the road, static or kinetic?

Why are antilock brakes a good idea in car? You can stop faster! (Also keep better control)


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Example

A force is exerted on a block as shown below.The coefficient of static friction is 0.58. Will theblock move?

10 kg 20 N


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Example continued

Now apply a force of 70 N. What will theacceleration of the block be? Assume

10 kg 70 N

.58S K

m m


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Problem

A driver locks his brakes and the car skids to astop. The mk= 0.70, and the cars initial speedwas 30 m/s. What is the stopping distance?


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Problem

A man pulls a sled and child with a combined massof 20.0 kg. If he exerts a force of 50.0 N at an angleof 30.0 degrees, and mk = 0.20, what is the

acceleration of the sled?


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Problem

A block is given an initial velocity up an inclineof -30 m/s, and mk = 0.3. How far up the inclinewill the block travel?

V=-30 m/s30o


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Uniform Circular Motion

Magnitude of the velocityis constant. Only thedirection is changing.

TOP VIEW


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What is the direction of theacceleration?

1v

2v

2v

1v

2 1v vAcceleration isdirected to thecenter of thecircle


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What is the magnitude of theinstantaneous acceleration?

2v

1v

2 1v v v

l arc length

l v

r v

v l v lvv r r

2

0 0

lim limt t

v v l va

t r t r


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Uniform Circular Motion

2

accelerationv

a

r

Period (T, measured in seconds)

Frequency (f, measured in revolutions/s, or hertz)

1T

f

2

rvelocity v

T


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Example

A rock on a string is swung around with aperiod of .50 s. The radius of the circle is 1.0meters. What is the magnitude and direction of

the acceleration of the rock?


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Dynamics of Uniform Circular Motion

The direction of the radial orcentripetal acceleration istowards the center of thecircle.

So, by Newtons second law,the sum of the forces actingon the object must be in adirection towards the centerof the circle and be given by:

2

net

vF ma m

r


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But isnt there a force out?

Called the centrifugal (center-fleeing) force

Doesnt really exist! It is really just because of

inertia and Newtons 3rd Law


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How fast can a car take a turn withoutsliding out?

Assume thecoefficient of staticfriction is 0.7, and the

radius of the turn is15 m.


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A ball on a string

A ball with mass 0.50 kg swings on a 0.75 m longrope with an angle of 50 degrees. What is thetension in the string and the speed of the ball?


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A ball on a string


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How does the Gravitron work?


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What is the minimum coefficient of static frictionso that a person will not fall? Assume theradius of the ride is 2.0 m, and when the floor

drops out, the ride is rotating at 1.1 revolutionsper second.

Gravitron Continued

5. ch5 - force ii tuth

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