2. Gravity
• A Universal Force
a)Until 17th century, - a heavy object would fall
faster than a light object as Aristotle once
taught.
b)After research by Galileo Galilei - if air
resistance can be ignored, all bodies fall with
an equal acceleration.
3. 1. Newton- studied the force of gravity. He got the inspiration when
apple fell from a tree, hitting his head.
2. In Newton’s opinion, the falling apple was acted on by a force of
attraction (i.e. the force of attraction due to gravity from the centre
of the earth)
3. Any object which falls only under the influence of the force of
attraction due to gravity and without any influence of other forces is
said to experience free fall.
4. All object which experience free fall will fall with the same
acceleration
(i.e. the acceleration due to gravity)
4. Gravitational Field
1. The force of gravity acts through space and can
cause a body which is not in contact with any
surface to fall freely to the ground – the earth is
surrounded by a gravitational field which exerts
a force on any body in the field.
2. The strength of a gravitational field – the
gravitational force acting per unit mass on an
object in the field. It is denoted by the symbol ‘g’
3. On the earth, the gravitational field strength, g is
9.8 N kg-1
4. Gravitational field strength:
g = Gravitational force, F
Mass of body, M
5. The non-uniform gravitational field of the
Earth, g is represented by radial lines
directed towards the centre of the earth.
The field is strongest where the lines
are close.
6. Example
Question :
• A body of mass 2 kg has a weight of 20 N.
Find the value of the gravitational field
strength
8. Gravitational acceleration
Mass , m Weight , w
The amount of The force of gravity
matter in the object. definition on the object.
Varies with the
The mass of an magnitude of
object is constant Changing of value gravitational field
everywhere. strength, g of the
location.
Scalar quantity Physical quantity Vector quantity
Base quantity Type of quantity Derived quantity
Kilogram, kg SI unit Newton, N
9. FORMULA
Force, F = ( mass, m ) ( acceleration, a )
Weight, w = ( mass, m ) ( gravitational
acceleration, g )
10. Gravitational field strengh
Gravitational field = the region in which an object
experiences a force due to gravitational attraction.
Gravitational field strength = the ratio of the weight to
the mass of the object / weight per mass / W
m
Rearrange the formula : W = ( m ) ( Gravitational field
strength )
When compared to : W = ( m ) ( g )
so : g = Gravitational
acceleration
= Gravitational field
11. FREE FALLING OBJECT
☻ Is an object falling under
the force of gravity only
☻ Does not encounter other
force (example : air
resistance or friction that
would oppose its motion
12. The difference between a fall in air and a free fall in
a vacuum of a coin and a feather
14. When a girl stands
When a girl stands
on the platform of a
on the platform of a
weighing scale, there are two
weighing scale, there are two
forces acting on her ::
forces acting on her
b) The upward normal reaction
a) The girl’s weight, w(=mg) force, R exerted on her feet
Acting downwards By the platform of the scale
The reading of the scale gives the value
of the normal reaction Force, R
15. Different situation in the
lift
Note : when the lift is accelerating upwards
or downwards, the reading on the scale
gives the ‘apparent’ weight, which is equal
to the normal reaction force on the feet of
the girl
20. 1 A box of mass 1.6 kg is suspended from a spring balance
hanging from the ceiling of a lift
What is the reading on the spring balace if :
(a) The lift is stationary
(b) The lift moves upwards at an acceleration of 2 ms-2?
(c) The lift moes downwards at an acceleration of 3ms-2
[take g = 10 ms-2]
21. SOLUTION
(a) If the lift is stationery :
T = mg
= 1.6 x 10
= 16 N
(b) When the lift is accelerating upwards :
T1 - mg = ma
T1 = (1.6 x 10) + (1.6 x 2)
= 19.2 N
(c) When the lift is accelerating downwards :
mg – T2 = ma
T2 = (1.6 x 10) – (1.6 x 3)
= 11.2 N
22. 2 The weight of a student on earth’s is
450N. What is this weight on the moon if
earth’s gravitational strength is six times
the gravitational strength of the moon ?
23. SOLUTION
(Symbol with subscript 1 represent the situation on earth while
symbols with subscript 2 represent the situation on the moon)
w1 = 450 N
g1 = 6g
w1 = mg
w2 = mg
w1 mg1 g1
w2 = mg2 = g2
w2 = w 1 g 2
g1
= 450 x g2
6g2
= 75 N
24. 3 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:
(a) the maximum height reached
(b) the time taken before it reaches the
ground
25. SOLUTION
a)The velocity of the ball is zero when it reaches its
maximum height. The acceleration of the ball when it is
thrown upwards is -g as it experiences a deceleration
u = 20 ms-1 v = 0 a = -10 ms-2
v2 = u2 + 2as
0 = 202 + 2(-10)s
s = 400 m
20
= 20 m
(maximum height reached = 20 m)
26. (b) The displacement of the ball on reaching the ground is zero
s = ut + 1 at2
2
0 = 20 t + 1 (-10)t2
2
5 t - 20 t = 0
2
5 t (t-4) = 0
t = 0 or 4
( time taken before it reaches the ground = 4 s)
