This teaching Material is for Grade - 9 Science. Here we are trying to cover Force from Unit 2. We provide teaching material at no cost. Please don't forget to recommend/reference our teaching Material after use.

1. Science
Shree Shitaladevi Community Secondary School
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Grade - 9
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2. UNIT: 2 Force
• Force is an external agent , which changes or tends to change the
state ,shape ,acceleration or direction of motion of an object.
• SI unit of force is Newton (N) while its CGS unit is dyne
(1N=105dyne )
• 1 Newton Force: The force required to produce the acceleration
of 1 m/s2 on a body of 1 kg mass is called one Newton force.
i.e 1N= 1 Kg x 1mls2
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3. Force
Scalar Quantities: The physical quantities, which have a
magnitude but no direction are called scalar quantity.
Mass , length, time, density, speed, work, work etc. are a few
examples of scalars.
• Vector Quantities: The physical quantities, which have both
magnitude as well as direction are called vectors quantities.
Velocity, displacement, acceleration, force, momentum etc.
are examples of vector quantities.
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4. Uniform Velocity & Non-uniform Velocity
• A body is said to have uniform velocity if it covers equal distance
in equal intervals of time in a particular direction.
• A body is said to have non-uniform velocity (i.e. variable velocity)
if it covers unequal distance in equal interval of time in a
particular direction.
• Acceleration: The rate of change of velocity with respect to time is
called acceleration.
Its SI unit is m/s2 & it is a vector quantity. Its value may be
positive or negative. 8/18/2020www.yepnepal.com
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5. Retardation & Equations of motion
• Acceleration = Change in velocity / time taken
Acceleration = Final velocity – Initial velocity / time taken
a= v-u / t
• Retardation: The negative acceleration is called retardation. It has the same
unit like acceleration.
• Equations of motion:
1. v= u + at ………. (i)
2. s = u + v/2 x t …..(ii)
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6. Equations of motion
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3. v2 = u2 + 2as ……… (iii)
4. s= ut + ½ at2 ……….(iv)
Case 1: For a freely falling body under gravity from a certain height
(h), in such case: a = g & s = h.
Case 2: When a body is thrown vertically upwards, in such case: a =
-g & s=h.
Case 3: When body is just released from rest, u=0 , a= g & s=h.

7. Equations of motion
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8. Inertia
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9. Inertia
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10. Momentum
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11. Newton’s First Law of Motion
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12. Newton’s Second Law of Motion
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15. Any Questions?
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16. Exercise based on Rest,Motion,Scalar & Vector
quantities
1. What is the acceleration of body moving with a uniform
velocity? Why?
2. The velocity of an accelerated body may be zero. Explain with
reason.
3. Total displacement can be zero but not the total distance of a
moving body, why?
4. A train is travelling with a velocity of 72 km/hr and after 10
seconds its velocity changes to 120 km/hr. What is its
acceleration? (Ans: 1.33 m/s2 )
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17. Exercise based on Inertia & Momentum
1. A moving fan continues to move after switch off. Why?
2. A moving truck takes a much longer time to stop than that taken
by a car when brakes are applied to both of them at once, why?
3. An athlete runs some distance before taking a long jump, why?
4. A moving ball on the play ground is stopped even if no player
touch it. Does it against the Newton’s first law of motion? Why?
5. A cricket ball causes severe hurt than a shuttlecock when one is
hit by them. Why?
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18. Exercise based on Equation of Motion & Newton’s
Laws of Motion
1.When a moving bus suddenly stops, the passengers fall forward,
why?
2. It is difficult to catch a cricket ball as compared to a tennis ball
moving with the same velocity. Why?
3. A person falling on a cemented floor is injured more than
falling on a heap of sand. Why?
4. While swimming, the swimmer pushes the water backward,
why?
5. While catching a cricket ball a cricket player lowers his hand
backwards, why?
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19. Numerical Problems
1. If a car starts from rest is accelerated at the rate of 0.2 m/s2 , find its
final velocity at the end of 1 minute. What will be the distance
covered during this time? (Ans:
V= 12m/s, S=360 m)
2. When brakes are applied in the train moving with a velocity of 90
km/hr, its acceleration is reduced to -0.5 m/s2 . What much distance
does it cover before coming to rest? (Ans: S= 625m)
3. A car of mass 2000kg is moving with the velocity of 72km/hr. If it is
stopped after 2 seconds by applying the brakes. Calculate: i)
Retardation of the car (ii) Distance covered at that interval of time.
(iii) Force required to stop the car. (Ans: -10m/s2 , 20m, 2x104 N)8/18/2020www.yepnepal.com
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20. Numerical Problems
4. A car starts from rest with a uniform acceleration of 6m/s2 . What will
be its velocity when it has travelled a distance of 300 meters ? How many
time did it take to cover the distance? ( Ans: v= 60m/s , t= 10s )
5. A stone is dropped freely from the top of the tower. It reaches the
ground in 4 seconds. Calculate the height of the tower.(g=10 m/s2 ) (Ans:
h= 80m)
6. A stone is thrown vertically upwards with an initial velocity of 72
km/hr. If air resistance is neglected, find the maximum height reached by
the stone and the time taken for it to return the ground.(g=10m/s2 ) ( Ans:
h=20m t=4 seconds)
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