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Forces: The Basics Part II

Ms. Mikaela Irene Fudolig

Physics 71

2nd sem 09-10

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Last Meeting

Newtons 1st Law of Motion

Newtons 2nd Law of Motion

netF ma

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For Today

More exercises on Newtons 1st and 2nd

Law

Newtons 3rd Law

Free-body Diagrams

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Summary

1st and 2nd Law combined

0netF F

0a

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Summary

1st and 2nd Law combined

netF F ma

VECTORS

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Newtons 2nd Law

In component form:

x xF may y

F ma

z zF ma

SIGNS

MATTER!

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IMPORTANT!

Consider all objects as PARTICLES for

Chapters 4-5.

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Conceptual Exercise 11a

You apply a force of magnitude F on a

box. The box moves with an acceleration

a.

If I apply the same force to a mass with box 2m,

what is the boxs acceleration in terms ofa?

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Conceptual Exercise 11b

You apply a force of magnitude F on a

box. The box moves with an acceleration

a.

If I apply a force 2F to the same box, what is

the acceleration of the box in terms ofa?

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Conceptual Exercise 12: Pandanggo

sa Apple

An apple, weighing 1N, is placed securely

on top of your head while you stand still.

What is the net force acting on the apple?

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Conceptual Exercise 13

A box is moving with constant speed in a

straight line.

What is the force acting on the box?

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Conceptual Exercise 14

A particle is moving with constant speed in

a circle. Is there a force acting on the

particle?

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Conceptual Exercise 15

A particle is moving to the right in a

straight line at constant velocity. At

some time t=t0, you simultaneouslyapply two forces on the particle: one to

the right, with magnitude F, and the

other to the left, with magnitudeF

.What happens to the motion of the particle?

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Conceptual Exercise 16

From t=0 to t=15s, you see two cars

moving in a straight line, one moving at a

constant speed of 15m/s, and the other

moving at a constant speed of 10m/s.

Which car experiences a greater external force

(consider only t=0 to t=15s)?

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Recall: EXTERNAL forces

Newtons 1st and 2nd law deals only with

EXTERNAL forces.

For INTERNAL forces, we have Newtons

3rd law.

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Newtons 3rd Law

If object A exerts a force on object B,

object B also exerts a force on A.

A on B B on AF F

SAME MAGNITUDE

OPPOSITE DIRECTION

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Conceptual Exercise 17: Newtons 3rd

Law

N

w=mg

N

w=mg

Forces actingon the box?

Forces actingon the table?

Forces acting

on the ground?

on box, by table

on box, by earth

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Newtons 3rd Law

The action force and the reaction force

act on two DIFFERENT bodies!

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Conceptual Exercise 18: Newtons 3rd

Law

massm

massm

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Conceptual Exercise 18: Newtons 3rd

Law

N

mg

ThandTB on G

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Conceptual Exercise 18: Newtons 3rd

Law

N

mg

TG on B

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Conceptual Exercise 18: Newtons 3rd

Law

N

mg

TG on B

TB on G = TG on B T(magnitude)

N

mg

ThandTB on G

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Conceptual Exercise 18: Newtons 3rd

Law

N

mg

T

TB on G = TG on B T(magnitude)

N

mg

ThandT