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Newton's Second Law of Motion


According to this law, the change in momentum of a body per unit time (i.e. rate of change of momentum) is directly proportional to the unbalanced force acting on the body and the change in momentum takes place in the direction of the unbalanced force on the body.

i.e. F ∝ dp/dt

where, dp = change in momentum and dt = time taken for this change in momentum.

Derivation of the Second Law of Motion


Consider a body of mass m moving with initial velocity u. Let a force F acts on the body for time t.

Newton`s Second Law of Motion and Concept of Linear Momentum and Impulse | Science Class 9Initial momentum of the body, Pi = mu
Final momentum of the body, Pf = mv
Now, change in momentum of the body = Pf - Pi = mv - mu = m(v - u)
Time taken to change this momentum = (t - 0) = t

∴ Rate of change of momentum =  Newton`s Second Law of Motion and Concept of Linear Momentum and Impulse | Science Class 9

According to the definition of Newton's second law of motion

Force applied µ rate of change of momentum

or   Newton`s Second Law of Motion and Concept of Linear Momentum and Impulse | Science Class 9 ... (i)

Since v = u + at   Newton`s Second Law of Motion and Concept of Linear Momentum and Impulse | Science Class 9

Therefore, equation (i) can be written as

∴  F = ∝ ma

or F = k ma ... (ii)

where k is constant of proportionality,

If F = 1 unit

m = 1 unit

and a = 1 unit

Then from equation (ii) 1 = k or k = 1
Put this value of k = 1 in equation (ii), we get

F = ma

Thus, force acting on the body is directly proportional to (i) it's mass (m) and (ii) its acceleration (a). Equation (i) gives the mathematical form of Newton's second law of motion 

Newton's Second Law of Motion in Vector Form


We know, force (Newton`s Second Law of Motion and Concept of Linear Momentum and Impulse | Science Class 9) and acceleration (Newton`s Second Law of Motion and Concept of Linear Momentum and Impulse | Science Class 9) are vector quantities, whereas mass (m) is a scalar quantity. Therefore, Newton's second law of motion can be written in vector form as

Newton`s Second Law of Motion and Concept of Linear Momentum and Impulse | Science Class 9

This relation shows that the direction of force applied to the body is same as that of the acceleration produced in the body.

Newton's First Law of Motion is a Special Case of Newton's Second Law of Motion


We know,   Newton`s Second Law of Motion and Concept of Linear Momentum and Impulse | Science Class 9 

(Newton's Second law of motion)

If velocity of a body is constant i.e. u = u, then F = m × 0 = 0
Thus, no force is required to move a body with constant velocity or uniform velocity. If u = 0, then u is also zero, it means the body will remain at rest if no external force is applied on the body. This is the first law of motion.

Units of Force


We know, F = ma
If m = 1 unit and a = 1 unit
Then F = 1 unit

Thus, force is said to be unit force if it products unit acceleration in a body of unit mass.

SI unit of force:

We know, F = ma
SI unit of mass,m = 1 kg
SI unit of acceleration, a = 1 ms-2
∴  SI unit of force = 1 kg × 1 ms-2 = 1 kg ms-2.
1 kg ms-2 is known as 1 newton (N).
∴ 1 newton (N) = 1 kg ms2

Definition of Newton (N): The force is said to be 1 newton if it products 1 ms2 acceleration in a body of 1 kg mass.

Newton`s Second Law of Motion and Concept of Linear Momentum and Impulse | Science Class 9C.G.S. unit of force:

We know, F = ma
In CGS system, unit of mass, m = 1 g
unit of acceleration, a = 1 cm s-2
∴ 1 unit of force = 1 g × 1 cm s2 = 1 g cm s-2
1 g cm s-2 is known as 1 dyne
1 dyne = 1 g cm s-2

Definition of dyne: The force is said to be 1 dyne if it produces 1 cm s-2 acceleration in a body of 1 g mass.

Relation between newton and dyne
We know, 1 newton (N) = 1 kg ms... (i)
Now 1 kg = 1000 g = 103 g
1 m = 100 cm = 102 cm
Put these values in equation (i), we get
∴ 1 newton = 103 g × 102 cm s-2 = 105 g cm s-2
But 1 g cm s- 2 = 1 dyne
∴ 1 newton = 105 dyne

Newton's Second Law in Terms of Linear Momentum


Definition: The rate of change of momentum of a body with respect to time is directly proportional to the external force acting on the body and takes place in the direction of the force.

Suppose a body of mass m is acted upon by an unbalanced external force F which creates an acceleration a in the body. Let the initial velocity of the body be u. Let the force continues to act for a time interval t and the final velocity of the body be v. Then we can write

Newton`s Second Law of Motion and Concept of Linear Momentum and Impulse | Science Class 9

Here, mv = Final momentum of the body (P)
mu = Initial momentum of the body (P)
Then mv - mu = (Pf - Pi ) represents the change in momentum.
The change in momentum written as Δp and is called delta P.

Newton`s Second Law of Motion and Concept of Linear Momentum and Impulse | Science Class 9

Some Applications of Newton`s Second Law of Motion and Concept of Linear Momentum and Impulse | Science Class 9


Example 1. A cricket player lowers his hands while catching a ball.
By doing so the player increases t, the time interval for the change in momentum. Because of this the force experienced by his hands is less.

A fielder pulls his hands gradually with the moving ball while catching it.A fielder pulls his hands gradually with the moving ball while catching it.Example 2. A karate player breaks the pile of the tiles or bricks with a single blow.

Example 3. Our hand hurts more when we hit the wall than when we hit a sponge seat of a car.

Solved Problems


Ex. When a force of 40 N is applied on a body it moves with an acceleration of 5 ms-2. Calculate the mass of the body.
Sol. Let m be the mass of the body.
Given : F = 40 N, a = 5 ms-2
From the relation F = m a we have 40 = m × 5
or  m = 40/5 = 8 kg

Ex. It is required to increase the velocity of a scooter of mass 80 kg from 5 to 25 ms-1 in 2 seconds. Calculate the force required.

Sol. Given : m = 80 kg, u = 5 ms-1 v = 25 ms-1 and t = 2s

Now acceleration  Newton`s Second Law of Motion and Concept of Linear Momentum and Impulse | Science Class 9

Force = mass × acceleration of F = ma
Therefore, F = 80 × 10 = 800 N

Ex. Two blocks made of different metals identical in shape and sizes are acted upon by equal forces which cause them to slide on a horizontal surface. The acceleration of the second block is found to be 5 times that of the first. What is the ratio of the mass of the second to the first?

Sol. Let, mass of the first block = m1
Acceleration of the first block = a1
Mass of the second block = m2
Acceleration of the second block = a2
Also, a2 = 5 a1
If the force acting on each block be F. Then, F = m1 a1 and F = m2 a2 = m2 × 5 a1
or m1a1 = m2a2 = m2 × 5a1
or m1 = 5m2
This gives, Newton`s Second Law of Motion and Concept of Linear Momentum and Impulse | Science Class 9  .Thus, the mass of the second block is one-fifth that of the first block.

Ex. An object undergoes an acceleration of 8 ms-2 starting from rest. Find the distance travelled in 1 second.
Sol. We are given,
Acceleration, a = 8 ms-2
Initial velocity, u = 0
Time interval, t = 1 s
Distance travelled, S = ?
Using the equation of motion, Newton`s Second Law of Motion and Concept of Linear Momentum and Impulse | Science Class 9, one gets
Newton`s Second Law of Motion and Concept of Linear Momentum and Impulse | Science Class 9
The object travels a distance of 4m.

Higher Order Thinking Skills


Impulse


The forces which act on bodies for short time are called impulsive forces. For example:
(i) In hitting a ball with a bat,
(ii) in driving a nail into a wooden block by a hammer,
Newton`s Second Law of Motion and Concept of Linear Momentum and Impulse | Science Class 9(iii) in firing a gun, etc.

An impulsive force does not remain constant but changes first from zero to maximum and then from maximum to zero. Thus it is not possible to measure easily the value of impulsive force because it changes with time. In such cases, we measure the total effect of the force, called impulse.

Hence impulse of a force is a measure of total effect of the force. It is given by the product of average force and the time for which the force acts on the body; i.e.,
Impulse = Change in linear momentum
Impulse is a vector quantity
SI unit of impulse is N-s and kg ms-1
CGS unit = dyne-sec and g cm s-1.

Applications of the Concept of Impulse 


1. When a person falls from a certain height on a cemented floor, the floor does not yield. The total change in linear momentum is produced in a smaller interval of time. Therefore, as explained above, the floor exerts a much larger force. Due to it, a person receives more injury.

2. On the other hand, when a person falls on a heap of sand, the sand yields. The same change in linear momentum is produced in much longer time. The average force exerted on the person by the heap of sand is, therefore, much smaller and hence the person is not hurt.

3. China wares and glasswares are wrapped in paper or straw pieces before packing. In the event of fall, impact will take a longer time to reach the glass/chinawares through paper/straw. As a result, the average force exerted on the china or glassware is small and chances of their breaking reduce.

4. The vehicles like scooter, car, bus, truck etc. are provided with shockers. When they move over an uneven road, impulsive forces are exerted by the road. The function of shockers is to increase the time of impulse. This would reduce the force/jerk experienced by the rider of the vehicle.

5. Bogies of a train are provided with the buffers. They avoid severe jerks during shunting of the train.

Due to the presence of buffers, time of impact increases. Therefore, force during jerks decreases. Hence the chances of damage decreases.

6. It is difficult to catch a cricket ball than to catch a tennis ball. The cricket ball is heavier has much larger momentum and therefore, exerts a much larger force on the hands during catch, in comparison to the force exerted by tennis ball.

7. In a head on collision between two vehicles, change in linear momentum is equal to sum of the linear momentum of the two vehicles. As time of impact is small, an extremely large force developed which causes damage to both vehicles.

8. An athlete is advised to come to stop slowly, after finishing a fast race, so that time of stop increases and hence force experienced by him decreases.

The document Newton's Second Law of Motion and Concept of Linear Momentum and Impulse | Science Class 9 is a part of the Class 9 Course Science Class 9.
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FAQs on Newton's Second Law of Motion and Concept of Linear Momentum and Impulse - Science Class 9

1. What is Newton's Second Law of Motion and how is it related to the concept of linear momentum and impulse?
Ans. Newton's Second Law of Motion states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. It can be mathematically expressed as F = ma, where F is the net force, m is the mass of the object, and a is the acceleration produced. The concept of linear momentum is closely related to Newton's Second Law. Linear momentum is the product of an object's mass and velocity, and it is a measure of the object's motion. According to Newton's Second Law, the rate of change of momentum of an object is equal to the net force acting on it. This relationship can be written as F = Δp/Δt, where F is the net force, Δp is the change in momentum, and Δt is the change in time. Impulse, on the other hand, is the change in momentum of an object and is equal to the force applied multiplied by the time interval over which it acts. It can be calculated using the formula J = FΔt. Therefore, Newton's Second Law is directly related to the concepts of linear momentum and impulse as it explains how forces cause changes in an object's momentum over time.
2. How does Newton's Second Law explain the relationship between force, mass, and acceleration?
Ans. Newton's Second Law of Motion explains the relationship between force, mass, and acceleration. According to the law, the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. This relationship can be mathematically expressed as F = ma, where F is the net force, m is the mass of the object, and a is the acceleration produced. In simpler terms, this means that the greater the force applied to an object, the greater its acceleration will be. Similarly, if the mass of the object is increased, its acceleration will decrease for the same amount of force applied. Conversely, if the mass is decreased, the acceleration will increase for the same force applied. This relationship can be better understood by considering the example of pushing two objects with different masses. If the same force is applied to both objects, the one with a smaller mass will experience a greater acceleration compared to the object with a larger mass. This is because the smaller mass offers less resistance to the applied force.
3. How is linear momentum defined and how does it relate to Newton's Second Law of Motion?
Ans. Linear momentum is defined as the product of an object's mass and velocity. It is a vector quantity, meaning it has both magnitude and direction. Mathematically, linear momentum (p) can be expressed as p = mv, where m is the mass of the object and v is its velocity. The concept of linear momentum is closely related to Newton's Second Law of Motion. According to the law, the rate of change of momentum of an object is equal to the net force acting on it. This can be written as F = Δp/Δt, where F is the net force, Δp is the change in momentum, and Δt is the change in time. This relationship implies that an object will experience a change in momentum when a net force is applied to it. The greater the force or the longer the force is applied, the greater the change in momentum will be. Conversely, if no net force acts on an object, its momentum will remain constant. Therefore, Newton's Second Law of Motion provides a quantitative explanation of how forces cause changes in an object's momentum over time.
4. What is impulse and how is it related to Newton's Second Law of Motion?
Ans. Impulse is the change in momentum of an object and is equal to the force applied multiplied by the time interval over which it acts. It can be mathematically expressed as J = FΔt, where J is the impulse, F is the force applied, and Δt is the time interval. The concept of impulse is closely related to Newton's Second Law of Motion. According to the law, the rate of change of momentum of an object is equal to the net force acting on it. This relationship can be written as F = Δp/Δt, where F is the net force, Δp is the change in momentum, and Δt is the change in time. By rearranging this equation, we can derive the relationship between impulse and force as J = Δp, where J is the impulse and Δp is the change in momentum. This implies that the impulse experienced by an object is equal to the change in its momentum. Therefore, Newton's Second Law of Motion explains how forces cause changes in an object's momentum over time, and impulse quantifies the magnitude of that change in momentum.
5. How can Newton's Second Law be applied to real-life situations?
Ans. Newton's Second Law of Motion can be applied to various real-life situations. Here are a few examples: 1. Car Acceleration: When a car accelerates, the engine applies a force to the wheels. According to Newton's Second Law, this force causes a change in momentum, resulting in the car's acceleration. 2. Sports: In sports like baseball, the force exerted by the player on the ball determines its acceleration and trajectory. Similarly, in sports like swimming, the force applied by the swimmer's arms and legs determines their acceleration in the water. 3. Braking in a Car: When a car applies brakes, the friction between the brake pads and the wheels generates a force that opposes the car's motion. Newton's Second Law explains that this force causes a change in momentum, resulting in the car's deceleration. 4. Rocket Propulsion: Newton's Second Law is crucial in understanding rocket propulsion. The force exerted by the expelled gases backward propels the rocket forward, following the law's principle of force and acceleration. By understanding and applying Newton's Second Law of Motion, we can analyze and predict the behavior of objects in various physical situations.
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