chapt 4 2.pdf

Copyright 2010 Pearson Education, Inc.Copyright 2010 Pearson Education, Inc.

Megat Mohd Izhar Sapeli(B.Sc Physics (Hons), M.Sc Microelectronics)

Bilik Pensyarah 21Level 7, FSK 1,5

UiTM Puncak Alam

[email protected]+60332584961

PHY094FOUNDATION PHYSICS 1

NEWTONS LAWS OF MOTIONcont

Copyright 2010 Pearson Education, Inc.

Units of Chapter 5

5.1 Force and Mass

5.2 Newtons First Law of Motion

5.3 Newtons Second Law of Motion

5.4 Newtons Third Law of Motion

5.5 The Vector Nature of Forces: Forces in

Two Dimensions

5.6 Weight

5.7 Normal Forces

11.1 Torque


5-6 Weight

The weight of an object on the Earths surface

is the gravitational force exerted on it by the

Earth.

F ma

W mg

W mg

W mg


Apparent weight:

When elevator moving downward comes to rest,

we feel heavier.

When elevator moving upward comes to rest,

we feel lighter.

( )

y

a

a

a

a

F ma

W W ma

W ma W

W ma mg

W m a g


5-7 Normal Forces

The normal force is

the force exerted by

a surface on an

object.

Normal force always

perpendicular to the

surface.

The normal force may be

equal to, greater than, or

less than the weight.


12

45

20

?

m kg

F N

N

sin 20 0

sin 20

(12)(9.81) 45sin 20

103

y

y y y

F ma

W F N ma

mg F N

N mg F

N

N N


The normal force is always perpendicular to

the surface.


sin sin

cos cos

x

y

W W mg

W W mg


11-1 Torque

From experience, we know that the same force

will be much more effective at rotating an

object such as a nut or a door if our hand is not

too close to the axis.


We define a quantity called torque:

The SI unit for torque is N.m

The torque increases as the force increases,

and also as the distance increases.

Consider only F tangent to r

rF


Only the tangential component of force causes

a torque:


This leads to a more general definition of torque:

r F

( sin )r F


If the torque causes a counterclockwise angular

acceleration, it is positive; if it causes a

clockwise angular acceleration, it is negative.


Summary of Chapter 5

Force: a push or pull

Mass: measures the difficulty in

accelerating an object

Newtons first law: if the net force on an

object is zero, its velocity is constant

Inertial frame of reference: one in which the

first law holds

Newtons second law:

Free-body diagram: a sketch showing all the

forces on an object

F ma



Newtons third law: If object 1 exerts a force

on object 2, then object 2 exerts a force on

object 1.

Contact forces: an action-reaction pair of

forces produced by two objects in physical

contact

Forces are vectors

Newtons laws can be applied to each

component of the forces independently

Weight: gravitational force exerted by the Earth

on an object

F

F



On the surface of the Earth, W = mg

Apparent weight: force felt from contact with a

floor or scale

Normal force: force exerted perpendicular to a

surface by that surface

Normal force may be equal to, lesser than, or

greater than the objects weight

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