Please explain the chapter 1 of 10th gravitation q.no 5.g
Answers
Explanation:
1
1.
Gravitation
direction at the position of the apple and thus, the
Can you recall?
Can you recall?
.2
Introduction to scientist Great Scientists: Sir Issac Newton (1642-1727) was one of
the greatest scientists of recent times. He was born in England.
He gave his laws of motion, equations of motion and theory of
gravity in his book Principia. Before this book was written,
Kepler had given three laws describing planetary motions.
However, the reason why planets move in the way described by
Kepler’s laws was not known. Newton, with his theory of
gravity, mathematically derived Kepler’s laws.
Try this
Tie a stone to one end of a string. Take the other end in your
hand and rotate the string so that the stone moves along a circle
as shown in figure 1.2 a. Are you applying any force on the
stone? In which direction is this force acting? How will you
stop this force from acting? What will be the effect on the stone?
As long as we are holding the string, we are pulling the
stone towards us i.e. towards the centre of the circle and are
applying a force towards it. The force stops acting if we release
the string. In this case, the stone will fly off along a straight line
which is the tangent to the circle at the position of the stone
when the string is released, because that is the direction of its
velocity at that instant of time (Figure 1.2 b). You may recall
that we have performed a similar activity previously in which a
5 rupee coin kept on a rotating circular disk flies off the disk
along the tangent to the disk.Thus, a force acts on any object
moving along a circle and it is directed towards the centre of the
circle. This is called the Centripetal force. `Centripetal’ means
centre seeking, i.e. the object tries to go towards the centre of the
circle because of this force.
1.2 A stone tied to a string,
moving along a circular
path and its velocity in
tangential direction
In addition to this, Newton did ground breaking work in several areas including
light, heat, sound and mathematics. He invented a new branch of mathematics. This is
called calculus and has wide ranging applications in physics and mathematics. He was
the first scientist to construct a reflecting telescope.
You know that the moon, which is the natural satellite of the earth, goes round it in a
definite orbit. The direction of motion of the moon as well as its speed constantly changes
during this motion. Do you think some force is constantly acting on the moon? What must
be the direction of this force? How would its motion have been if no such force acted on
it? Do the other planets in the solar system revolve around the Sun in a similar fashion? Is
similar force acting on them? What must be its direction?
From the above activity, example and questions it is clear that for the moon to go
around the earth, there must be a force which is exerted on the moon and this force must
be exerted by the earth which attracts the moon towards itself. Similarly, the Sun must be
attracting the planets, including the earth, towards itself.
Circular motion and Centripetal force
b.
a.3
A
B
D C
S
E
F 1
An ellipse is the curve obtained when
a cone is cut by an inclined plane. It has
two focal points. The sum of the distances
to the two focal points from every point
on the curve is constant. F1
and F2
are two
focal points of the ellipse shown in figure
1.3. If A, B and C are three points on the
ellipse then,
AF1
+ AF2
= BF1
+ BF2
= CF1
+ CF2
Kepler’s Laws
observations. He had no explanation as to why planets obey these laws. We
will see below how these laws helped Newton in the formulation of his theory of gravitation.
Do you know ?
T2
r3 = constant = K ............. (1)
B4
From equation (2), it can be seen that
the value of G is the gravitational force
acting between two unit masses kept at a
unit distance away from each other. Thus,
in SI units, the value of G is equal to the
gravitational force between two masses of
1 kg kept 1 m apart.
Show that in SI units, the unit of G
is Newton m2
kg-2. The value of G was
first experimentally measured by Henry
Cavendish. In SI units its value is
6.673 x 10-11 N m2
kg-2.
Use your brain power If the area ESF in figure 1.4 is equal to area ASB, what
will you infer about EF?
An introduction to scientists
Johannes Kepler (1571-1630) was a German astronomer
and mathematician. He started working as a helper to the famous
astronomer Tycho Brahe in Prague in 1600. After the sudden
death of Brahe in 1601, Kepler was appointed as the Royal
mathematician in his place. Kepler used the observations of
planetary positions made by Brahe to discover the laws of
planetary motion. He wrote several books. His the two centres of mass.
d
Answer:
______________ used the observations of planetary positions made by Brahe to discover
the laws of planetary motion.
Explanation: