08 - forces iii.pptx

7/29/2019 08 - Forces III.pptx

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Applying Newtons Laws:

Frictionless Surfaces

Mikaela Irene Fudolig

Physics 71


2/31

Review

1. Newtons 1st and 2nd Law

netF ma


0

0

netF

a


0net

net

F

Fa

m

1st Law 2nd Law


3/31

Review

2. Newtons 3rd Law

For any two objects A and B:

A on B B on AF F

A on B B on Aand comprise an

ACTION-REACTION PAIR.

F F


4/31

Review

2. Newtons 3rd Law

The action force and the reaction force act on

DIFFERENT bodies.


5/31

Review

3. Free-body diagrams

Identify the object.

Identify the forces acting on the object. Weight

Normal force

Tension

Friction


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Review


Identify the direction in which those

forces act.

Weight points downwards.

Normal force points perpendicular to the

surface, pushing the object.

Tension pulls the object.

Friction is against the direction of motion.


7/31

Review


Draw the coordinate axes.

Identify clearly where +y and +x directions

are.

Apply Newtons 2nd Law PER AXIS.

x xF may y

F ma


8/31

Review


For inclined planes, TILT the axes such

that one axis isparallelto the surface,

and the other axis isperpendicularto

the surface.


9/31

Example 1a: The Weighing Scale

A weighing scale does NOT measure the

WEIGHT.

It measures the NORMAL FORCE it

exerts on an object.

Alternatively, it measures the reaction force to

the normal force it exerts.


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Example 1a: The Weighing Scale

You are standing on a weighing scale

inside an elevator at rest. If your weight is

500N, what is the reading on the weighing

scale?


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Example 1b: The Weighing Scale


inside an elevator going up at a constant

speed of 2m/s. If your weight is 500N,

what is the reading on the weighing scale?


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Example 1c: The Weighing Scale


inside an elevator going up and

accelerating at 1m/s2. If your weight is

500N, what is the reading on the weighingscale?


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Example 1d: The Weighing Scale


inside an elevator going down and

accelerating at 1m/s2. If your weight is



14/31

Example 1e: The Weighing Scale


inside an elevator going down and

decelerating at 1m/s2. If your weight is



15/31

Example 2: A girl pulling a box


16/31

Example 2: A girl pulling a box

If the girl pulls on the rope connected to

the 20-kg box with a force of 150N at 25

from the horizontal, find

the acceleration of the box (magnitude and

direction)

the normal force exerted by the surface on the

box.Note: Neglect friction.


17/31

Example 3: Box on an inclined plane

A 1-kg box slides down with aninitial speed of 10cm/s on a

frictionless ramp 5m long inclined

at 30 from the horizontal.What is the normal force on the box?

What is the acceleration of the box?

What is the velocity of the box upon

reaching the end of the ramp if it

starts from the top of the ramp?


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Exercise 4: Box on an inclined plane

A 1-kg box is on a frictionless rampinclined at 30 from the horizontal.

You push it up the ramp with a

force of 1N.What is the acceleration of the box?

If it initially starts from rest:

in what direction will it move?

how far has it traveled after 1s?


19/31

Things to know about STRINGS

Assume strings are massless and taut.

Strings ALWAYS PULL on objects

The magnitude of the tension in the stringis the same everywhere on the string.


20/31

Things to know about STRINGS

N

mg

T

TB on G = TG on B T

(magnitude)

N

mg

ThandT


21/31

Example 5

If the object (mass m)below is at rest,

what is the tension in the string?


22/31

Exercise 6

If the object (mass m)below is at rest,

what is the tension in the string?


23/31

Conceptual Exercise 7

A boy rides an

improvised swing as

shown. If the boys

weight is 400N andthe rope used has a

breaking point of

300N, why doesnt

the rope break?


24/31

Exercise 8

A picture frame

weighing 8N is

supported by two

different wires withtension T1 and T2

respectively. Find

each tension.

60 30

1T

2

T


25/31

Two or More Objects

If object A and object B are connected:

the magnitudes of their accelerations are the

SAMEthe magnitudes of their velocities (a.k.a.,

speed) are the SAME


26/31

Two or More Objects

Draw free-body diagrams foreachbody.Dont forget to include the action-reaction

forces!Foreach object, write a separate

equation foreach component ofNewtons 2nd Law.

Identify the unknown and givenquantities.

Solve!


27/31

Example 9

If the motor,

which is at rest,

has a mass m,and the O-ring

has negligible

mass:Find T1, T2, and

T3.


28/31

Example 10

You push on a box of mass m1 with a force F.

The box is in direct contact with a second box of

mass m2.

What is the acceleration of the boxes?What is the magnitude of the force exerted by one box

on the other?

F


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Exercise 11

Two boxes of the same mass m=1kg are

connected in the manner shown below. If

you pull the gray box with a force of 5N,

what is the acceleration ofthe gray box?

the blue box?


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Example 12

In what direction

will the masses

(initially at rest)

move if:=15

m1=2kg

m2=1kg


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Example 13: Atwood Machine

Find the acceleration

of each mass

(magnitude and

direction).

08 - forces iii.pptx

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