Chapter 3 Dynamics

1
Chapter 4
Mass, Weight and Density
2
Forces
force
A force is a push or a pull. It is a vector and
has both magnitude and direction.
3
Forces
force
A force is a push or a pull. It is a vector and
has both magnitude and direction.
Newtons first law of motion
An object at rest will remain at rest and an
object in motion will continue in motion at
constant speed in a straight line in the absence
of a net force acting on it (when the net force
acting on it is zero).
4
Mass
State of Inertia

• When a body of matter is stationary, it needs a
  force to make it start moving
• The bigger the mass, the bigger the force needed
• Masses have inertia, a reluctance to start moving

5
Mass
State of Inertia

• When a body of matter is moving, it needs a
  force to stop it
• The bigger the mass, the bigger the force needed
• Masses have inertia, a reluctance to stop moving

Question Do you know why you need to put on a
seat belt in a car?
6
(Straits Times 8 May 2002)(5)  Drivers who do
not belt up will be fined S120 and given three
demerit points from today. Passengers who fail to
do so will be fined S120. The tough stand comes
after ten years of relying on education, rather
than enforcement, to get people to comply with
the law. The Traffic police will be on the
lookout for such offenders. Traffic Police chief,
Assistant Commissioner Rajoo Gopal said
yesterday, "There will be no more warnings and no
escapes."
FINE
7
Mass
State of Inertia

• When a body of matter is stationary, it needs a
  force to make it start moving
• The bigger the mass, the bigger the force needed
• Masses have inertia, a reluctance to start moving

The mass (m) of a body of matter is a
quantitative measure of its inertia, i.e., its
resistance to a change in the state of rest or
motion of the body, when a force is applied.
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Mass
Amount of Substance
Conventionally,
Mass (m) of a body of matter is a measure of its
amount of substance in the body
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Mass
Amount of Substance
- SI Unit kg - Scalar quantity
A sliding balance
An electronic balance
10
Gravitational Field Field Strength
Gravitational Attraction

• - Any two bodies in the universe attract each
  other with a force.
• This spectacle is called the
• gravitational attraction.
• This force of attraction is known as the
• gravitational force or force due to gravity.

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Gravitational Field Field Strength
F Force between two particles m1, m2
masses of the two particles r separation
between two masses G Universal Constant of
gravitational 6.67 x 10-11 Nm2kg-2
Note Equation is NOT required in Pre-IB
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Gravitational Field Field Strength
Newtons Law of Gravitational
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Gravitational Field Field Strength
Newtons Law of Gravitational
Radius of Earth 6.4 x 106 m Mass of Earth 6.0
x 1024 kg Radius of Moon 1.74 x 106 m Mass of
Moon 7.4 x 1022 kg
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Gravitational Field Field Strength
Newtons Law of Gravitational
Note Earth is not spherical but ellipsoidal
(equatorial radius exceed polar radius by 21 km).
Centrifugal effects not felt at pole due to
rotation at pole axis. Hence,
gravitational force is max at poles compare to
equator
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Gravitational Field Field Strength
Gravitational Field
A gravitational field is a region in which a mass
experiences a force due to gravitational
attraction

• Represented by imaginary line of forces
• For Spherical body, lines are directed radially
  inward towards the centre of body
• Lines closer, strong field
• Lines widely separated, weak field
• Near surface of earth, lines of force almost
  parallel, hence constant field

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Gravitational Field Field Strength
Gravitational Field Strength
Gravitational field strength at a point in free
space surrounding any mass is the force per unit
mass acting on any object placed there.
g Gravitational Field Strength
Or numerically Acceleration due to
gravity in the case of planet Unit
N Kg-1 or ms-2
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Weight
Weight of a body is the gravitational force
exerted on it by Earth.
W Weight of body, N m mass of body,
kg g gravitational field strength,
Nkg-1 10 Nkg-1 near the surface of the earth
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Weight
Compression balance
Spring balance
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Comparison Between Mass Weight
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Weight
Question
A satellite is launched from Earth into orbit.
What happens to the mass and weight of the
satellite? Mass Weight A Decreases Decrea
ses B Decreases Stays constant C Stays
constant Decreases D Stays constant Stays
constant
21
Weight
Question to ponder
What is the cause of low tide and high tide on
Earth?
22
free fall
acceleration due to gravity
All objects fall freely towards the centre of the
earth and have the same acceleration
(acceleration of free fall).

• all objects fall freely at g ? 10 m s-2 when near
  the earth and air resistance is negligible
• speed of a free-falling body increases by 10 m
  s-1 every second or when a body is thrown up, its
  speed decreases by 10 m s-1 every second

Although the acceleration due to gravity is
considered constant, it tends to vary slightly
over the earth since the earth is not a perfect
sphere.
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free fall
acceleration due to gravity

• - Free-falling objects do not encounter air
  resistance.
• - All free-falling objects (on Earth)
  accelerate downwards at a rate of
  approximately 10 ms-2 (to be exact, 9.81
  ms-2).

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Free Fall (no air resistance)
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Free Fall (no air resistance)
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Falling with air resistance
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Density
Density (?) is defined as the mass (m) of a
substance per unit volume (V)
? unit is kgm-3 m mass of object V volume
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Density
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measurement of volume
irregular volume
Volume of an irregular-shaped object can be found
with

• measuring cylinder alone

for an object that floats on water
for an object that sinks in water
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measurement of volume
irregular volume
Volume of an irregular-shaped object can also be
found with

• displacement can and measuring cylinder

for an object that sinks in water
for an object that floats on water
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Floating Sinking

• When an object is placed in a liquid of lower
  density, the object sinks
• - If it is placed in a liquid of a greater
  density, it floats

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Floating Sinking
Hydrometer