Chapter 9 Force and Laws of Motion Class 9 Notes | EduRev

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Class 9 : Chapter 9 Force and Laws of Motion Class 9 Notes | EduRev

 Page 1


Chapter 9 Force and Laws of Motion 
Force – A push or a pull is that can change the motion of an object. 
Balanced forces – Forces applied from opposite direction that are equal in magnitude and do not change the 
state of motion of an object. 
Unbalanced forces - Forces applied from opposite direction having different magnitude. The object moves will 
move in the direction where more force is applied. Such forces are called unbalanced forces. 
 
Laws of motion 
1. First law of Motion – An object at rest remains at rest and an object in motion remains in motion 
without the application of an external force. 
Explanation – A moving object such as a moving ball will always keep on moving unless an external 
force (such as frictional force or muscular force) is applied. 
 
Inertia – The ability of an object to remain at rest or in motion without the application of an 
external force. 
Inertia of an object can be measured by its mass. Heavier objects have higher inertia as they can 
resist any change in their state of motion on applying larger amount of force. 
Mass – The quantity of matter present inside an object is called its mass. 
 
2. Second law of Motion – The rate of change of momentum of an object is directly proportional to the 
applied unbalanced force on an object in the direction of force. 
 
Momentum – It is defined as the product of mass and velocity. 
 Momentum (p) = mass (m) * velocity (v) 
S I units of momentum = kg*m/s 
 
Significance of momentum – Both mass and velocity are important components of motion. For 
e.g. A truck moving at very slow speed of 10kmph can kill a person while a cycle at the same 
speed hardly cause any injury.  
A small bullet at very high speed can again be dangerous but the same bullet thrown with a hand 
will cause minor injuries to a person. 
 
Mathematics of Second law of motion 
We know that 
Momentum = mv 
If the initial velocity is ‘u’ and the final velocity is ‘v’ OR After an external force ‘F’ was applied on an object 
moving with initial velocity ‘u’ its velocity changed to final velocity ‘v’ 
Initial momentum (p1) = mu 
Final momentum (p2) = mv 
Change in momentum = p2 – p1 
Page 2


Chapter 9 Force and Laws of Motion 
Force – A push or a pull is that can change the motion of an object. 
Balanced forces – Forces applied from opposite direction that are equal in magnitude and do not change the 
state of motion of an object. 
Unbalanced forces - Forces applied from opposite direction having different magnitude. The object moves will 
move in the direction where more force is applied. Such forces are called unbalanced forces. 
 
Laws of motion 
1. First law of Motion – An object at rest remains at rest and an object in motion remains in motion 
without the application of an external force. 
Explanation – A moving object such as a moving ball will always keep on moving unless an external 
force (such as frictional force or muscular force) is applied. 
 
Inertia – The ability of an object to remain at rest or in motion without the application of an 
external force. 
Inertia of an object can be measured by its mass. Heavier objects have higher inertia as they can 
resist any change in their state of motion on applying larger amount of force. 
Mass – The quantity of matter present inside an object is called its mass. 
 
2. Second law of Motion – The rate of change of momentum of an object is directly proportional to the 
applied unbalanced force on an object in the direction of force. 
 
Momentum – It is defined as the product of mass and velocity. 
 Momentum (p) = mass (m) * velocity (v) 
S I units of momentum = kg*m/s 
 
Significance of momentum – Both mass and velocity are important components of motion. For 
e.g. A truck moving at very slow speed of 10kmph can kill a person while a cycle at the same 
speed hardly cause any injury.  
A small bullet at very high speed can again be dangerous but the same bullet thrown with a hand 
will cause minor injuries to a person. 
 
Mathematics of Second law of motion 
We know that 
Momentum = mv 
If the initial velocity is ‘u’ and the final velocity is ‘v’ OR After an external force ‘F’ was applied on an object 
moving with initial velocity ‘u’ its velocity changed to final velocity ‘v’ 
Initial momentum (p1) = mu 
Final momentum (p2) = mv 
Change in momentum = p2 – p1 
   = mv – mu  
Rate of change in momentum = p2 – p1 
           t 
    = mv – mu 
             t 
  
According to the second law of motion – rate of change of momentum is directly proportional to the force 
applied 
F a p2 – p1 
                       t 
F a mv – mu 
                 t 
F a mv – mu 
                 t 
To change the sign of ‘proportionality’ to ‘equals to’ a constant ‘k’ is added whose value is found out 
experimentally.  
            F = k (mv – mu) 
     t 
            F = k*m (v – u) 
     t 
   And,  v – u = a (acceleration) 
        t 
     So, F = k*m*a  
The value of ‘k’ was found out to be 1 when calculated experimentally. 
Hence,     
            F = 1*ma 
            F = ma 
SI units of force = kg*ms
-2
 or Newton (N) 
1N force is defined as the amount of force applied on 1kg of object to accelerate it by 1ms
-2
 
Second law of motion also proves the first law: 
F = m (v-u)/t 
If v = u  
F = 0 
That is if there is no change in velocity of an object the net force applied is zero so it also proves the first law of 
motion. 
 
Page 3


Chapter 9 Force and Laws of Motion 
Force – A push or a pull is that can change the motion of an object. 
Balanced forces – Forces applied from opposite direction that are equal in magnitude and do not change the 
state of motion of an object. 
Unbalanced forces - Forces applied from opposite direction having different magnitude. The object moves will 
move in the direction where more force is applied. Such forces are called unbalanced forces. 
 
Laws of motion 
1. First law of Motion – An object at rest remains at rest and an object in motion remains in motion 
without the application of an external force. 
Explanation – A moving object such as a moving ball will always keep on moving unless an external 
force (such as frictional force or muscular force) is applied. 
 
Inertia – The ability of an object to remain at rest or in motion without the application of an 
external force. 
Inertia of an object can be measured by its mass. Heavier objects have higher inertia as they can 
resist any change in their state of motion on applying larger amount of force. 
Mass – The quantity of matter present inside an object is called its mass. 
 
2. Second law of Motion – The rate of change of momentum of an object is directly proportional to the 
applied unbalanced force on an object in the direction of force. 
 
Momentum – It is defined as the product of mass and velocity. 
 Momentum (p) = mass (m) * velocity (v) 
S I units of momentum = kg*m/s 
 
Significance of momentum – Both mass and velocity are important components of motion. For 
e.g. A truck moving at very slow speed of 10kmph can kill a person while a cycle at the same 
speed hardly cause any injury.  
A small bullet at very high speed can again be dangerous but the same bullet thrown with a hand 
will cause minor injuries to a person. 
 
Mathematics of Second law of motion 
We know that 
Momentum = mv 
If the initial velocity is ‘u’ and the final velocity is ‘v’ OR After an external force ‘F’ was applied on an object 
moving with initial velocity ‘u’ its velocity changed to final velocity ‘v’ 
Initial momentum (p1) = mu 
Final momentum (p2) = mv 
Change in momentum = p2 – p1 
   = mv – mu  
Rate of change in momentum = p2 – p1 
           t 
    = mv – mu 
             t 
  
According to the second law of motion – rate of change of momentum is directly proportional to the force 
applied 
F a p2 – p1 
                       t 
F a mv – mu 
                 t 
F a mv – mu 
                 t 
To change the sign of ‘proportionality’ to ‘equals to’ a constant ‘k’ is added whose value is found out 
experimentally.  
            F = k (mv – mu) 
     t 
            F = k*m (v – u) 
     t 
   And,  v – u = a (acceleration) 
        t 
     So, F = k*m*a  
The value of ‘k’ was found out to be 1 when calculated experimentally. 
Hence,     
            F = 1*ma 
            F = ma 
SI units of force = kg*ms
-2
 or Newton (N) 
1N force is defined as the amount of force applied on 1kg of object to accelerate it by 1ms
-2
 
Second law of motion also proves the first law: 
F = m (v-u)/t 
If v = u  
F = 0 
That is if there is no change in velocity of an object the net force applied is zero so it also proves the first law of 
motion. 
 
 
3. Third law of motion – Every action has equal and opposite reaction 
Examples – While walking we push the ground downward and ground applies an equal but 
opposite force on us. 
While hitting an object, our hand hurts because the object applied equal and opposite force on 
our hand. 
A gun reloads as soon as a shot is fired because the bullet applied an equal but opposite force on 
the trigger that reloads it. 
 
Conservation of momentum 
The momentum of two objects moving with some velocity before collision is equal to their momentum after 
collision.  
Explanation – Consider two objects having mass ‘ma’ and ‘mb’ moving with velocity ‘ua’ and ‘ub’ collide with 
each other for time ‘t’ 
After collision the objects move with velocity ‘va’ and ‘vb’   
According to the third law of motion 
    FAB = - FBA   
?  ma(va - ua)  = - mb(vb – ub) 
          t       t 
‘t’ cancels out ‘t’ as it is present on both LHS and RHS 
 
? mava - maua = - (mbvb – mbub) 
? mava - maua= mbub - mbvb 
? mava + mbvb = maua + mbub 
? Total initial momentum = total final momentum. 
 
 
 
 
* For activities and numerical refer to NCERT book  
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