Download SummaryofNotesonForces - stpats-sph3u-sem1-2013

SPH 3U - Forces
Forces, Gravity & Newton’s Laws
Introduction To Forces
Force: A push or a pull that causes things to speed up, slow down, or can distort
matter by compressing, stretching or twisting.
1 N is roughly the force felt by a 0.1 kg
SI Unit: Newtons (N) = kg•m/s2
(100 g) apple being held in an open
hand near the surface of the earth
Examples of Forces:
Gravity (Fg), Normal (FN), Tension (FT), Applied (FA), Friction (Ff)
Forces & Gravity
Acceleration Due to Gravity
 Objects accelerate when dropped because gravity is exerting a force on
the falling objects
 The acceleration due to gravity ( g ) at the surface of the earth is 9.8m/s2
 Any two objects (regardless of mass) will fall at the same rate and they will
hit the ground at the same time (if we neglect air friction)
Force of Gravity
The force due to gravity is the force of attraction between all objects of the
universe. On the earth’s surface, the force of gravity is directed towards the
earth’s centre and the magnitude is given by the formula:


Fg = mg
Quantity
force of gravity
Symbol
acceleration due to
gravity
mass of object

g
m/s2
m
kg

Fg
SI Unit
N
Difference Between Mass and Weight
A common error is to use mass and weight interchangeably; this should not be
done in physics since they have a number of different properties:
Quantity
Type
Description
Variable
Units
Mass
scalar
amount of matter
“m”
“kg”
Weight
vector
force of gravity
on an object
“Fg”
“N”
How is it
measured?
by comparison on
a balance
using a spring
scale
Constant?
same
everywhere
varies with
location
Newton’s Universal Law of Gravitation
Gravity not only changes in different places on the earth’s surface, but, also with
changing distances from the center of the earth according to the equation:
Fg =
Quantity
gravitational force of attraction
between two objects
gravitational constant
mass of object 1
mass of object 2
distance between centre of two
objects
Gm1m2
r2
Symbol

Fg
SI Unit
G
m1
m2
Nm2/kg2
kg
kg
r
m
N
Newton’s Laws of Motion
1. Law of Inertia
An object at rest or in uniform motion will remain at rest or in uniform motion
unless acted on by an external force.


2. F = ma 
The net force ( F ) required to accelerate an object of mass (m) by an amount
( a ) is the product of the mass and acceleration.


F = ma
Quantity
force
mass
acceleration
Symbol

F
m

a
SI Unit
N
kg
m/s2
Note: There is often more than one force acting on an object. In order to use
Newton’s 2nd Law, you need to find the net force.
3. Action-Reaction Forces
For every action force on an object (B), due to another object (A), there is a
reactions force equal in magnitude but opposite in direction on object A due to
object B.


FA on B = - FB on A
Application of Newton’s 3rd Law
The jet engine works using this principle: The walls of the jet’s combustion
chamber exert a backward force on the high-pressure gas, causing it to stream
out. In turn, the gas exerts a force on the walls of the combustion chamber,
pushing it and the rocket forward.
2
Free-Body Diagrams
A free body diagram (FBD) is a drawing in which only the object being analyzed
is drawn, with arrows showing all the forces acting on the object.



+y
Length of vectors is proportional to
magnitude of force
Each force vector labelled
Positive directions (x & y) are indicated

FN

Ff
+x

FA

Fg
Example: A box is pushed along the floor.
Friction
Friction: a force that resists motion and acts in a direction opposite to the direction
of motion, and parallel to the surfaces that are in contact. The magnitude is given by
the formula:


F f = FN
Quantity
force of friction
friction coefficient
normal force
Symbol

Ff


FN
SI Unit
N
(no units)
N
Note: Coefficient of friction is a property of the material; every material has both a coefficient of
static friction and a coefficient of kinetic friction.
Types of Friction


Static Friction
Friction preventing a stationary object from
moving
The maximum static friction is called
starting friction (to get objects to move)
Kinetic Friction


3
Friction of moving objects acting in the
opposite direction of motion
Usually less than static friction