Transcript force and motion

2.6 UNDERSTANDING GRAVITY
 Is a region around the earth where an object
experiences a force acting on it towards the centre of
the earth.
Gravitational
field
earth
 It is the force of gravity that pulls all objects towards
the centre of the earth.
earth
 Gravitational force acting on a unit mass in the field.
 Formula: g = F
Gravitational force
m
mass
 Also can be define as the ratio of the weight to the mass of the object
 Formula : g = W
weight
m
 SI unit Nkg-1
 When an object is at rest, g (= 9.8 Nkg-1) is the earth’s gravitational
strength acting on it.
 Will decrease when the distance of the object from the centre of earth
increase.
 gmoon = 1/6 gearth
• Acceleration of an object due to the pull of the
gravitational force.
• The acceleration due to gravity for a freely falling body
has a value of about 9.8 ms-2. (+g = object moving
downward, -g = object moving upward )
• Free fall:
– Refer to an object which is falls due to gravitational force
–
–
–
–
only.
Not effected by mass and shape of the object.
Only occur in vacuum.
Assumed that air resistance is zero.
g = 9.8 ms-2
 Weight is the force of gravity acting on the object.
 Symbol : W
 Vector quantity
 SI unit Newton, N
 Formula :
 F = ma since F = W, a = g
 W = mg
acceleration due to gravity
mass of object
Comparison between weight and mass
Weight, W
Differences
Mass , m
Measurement of
gravitational force on
the body
Definition
Measurement of the
quantity of matter in
the body
Vector quantity
Physical quantity
Scalar quantity
Derived quantity
Types of quantity
Base quantity
SI unit
Kilogram , kg
Newton , N
According to the
gravitational field
strength
Changing of value
Constant everywhere
regardless of the
place and location
Example
What is the weight of an object of mass 5.0 kg
1. On the surface of the earth
2. On the gravitational of the moon
( gravitational field strength = 10 Nkg-1)

Solution :
1. We = mg
= 5.0 kg x 10 Nkg-1
= 50 N
2. Wm = mg
= 5.0 kg x 1/6 ( 10 Nkg-1)
= 8.33N
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2.
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1)
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a)
b)
Wei Bo is a basketball player. His vertical leap is 0.75m. What is his
take-off speed?
Answer : 3.87ms-1
A coconut 1.5 seconds to fall to the ground. What is
Its speed when it strike the ground?
The height of the coconut tree?
Answer = 15 ms-1
Answer = 11.3 m
A ball is thrown vertically upwards with an initial velocity of 20 ms-1.
If g = 10 ms-2 and air resistance can be neglected, find
Maximum height reached
The time taken before it reaches the ground
Answer = 2om
Answer = 4s