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Change in momentum

Let us consider a tennis ball (mass = 0,1 kg) that is dropped at an initial velocity of 5 m · s - 1 and bounces back at a final velocity of 3 m · s - 1 . As the ball approaches the floor it has a momentum that we call the momentum before the collision. When it moves away from the floor it has a different momentum called the momentum after the collision. The bounce on the floor can be thought of as a collision taking place where the floor exerts a force on the tennis ball to change its momentum.

The momentum before the bounce can be calculated as follows:

Because momentum and velocity are vectors, we have to choose a direction as positive. For this example we choose the initial direction of motion as positive, in other words, downwards is positive.

p i = m · v i = ( 0 , 1 kg ) ( + 5 m · s - 1 ) = 0 , 5 kg · m · s - 1 downwards

When the tennis ball bounces back it changes direction. The final velocity will thus have a negative value. The momentum after the bounce can be calculated as follows:

p f = m · v f = ( 0 , 1 kg ) ( - 3 m · s - 1 ) = - 0 , 3 kg · m · s - 1 = 0 , 3 kg · m · s - 1 upwards

Now let us look at what happens to the momentum of the tennis ball. The momentum changes during this bounce. We can calculate the change in momentum as follows:

Again we have to choose a direction as positive and we will stick to our initial choice as downwards is positive. This means that the final momentum will have a negative number.

Δ p = p f - p i = m · v f - m · v i = ( - 0 , 3 kg ) - ( 0 , 5 m · s - 1 ) = - 0 , 8 kg · m · s - 1 = 0 , 8 kg · m · s - 1 upwards

You will notice that this number is bigger than the previous momenta calculated. This is should be the case as the ball needed to be stopped and then given momentum to bounce back.

A rubber ball of mass 0,8 kg is dropped and strikes the floor with an initial velocity of 6 m · s - 1 . It bounces back with a final velocity of 4 m · s - 1 . Calculate the change in the momentum of the rubber ball caused by the floor.

  1. The question explicitly gives

    • the ball's mass (m = 0,8 kg),
    • the ball's initial velocity (v i = 6 m · s - 1 ), and
    • the ball's final velocity (v f = 4 m · s - 1 )

    all in the correct units.

    We are asked to calculate the change in momentum of the ball,

    Δ p = m v f - m v i

    We have everything we need to find Δ p . Since the initial momentum is directed downwards and the final momentum is in the upward direction, we can use the algebraic method of subtraction discussed in the vectors chapter.

  2. Let us choose down as the positive direction.

  3. Δ p = m v f - m v i = ( 0 , 8 kg ) ( - 4 m · s - 1 ) - ( 0 , 8 kg ) ( + 6 m · s - 1 ) = ( - 3 , 2 kg · m · s - 1 ) - ( 4 , 8 kg · m · s - 1 ) = - 8 = 8 kg · m · s - 1 upwards
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Exercise

  1. Which expression accurately describes the change of momentum of an object?
    1. F m
    2. F t
    3. F · m
    4. F · t
  2. A child drops a ball of mass 100 g. The ball strikes the ground with a velocity of 5 m · s - 1 and rebounds with a velocity of 4 m · s - 1 . Calculate the change of momentum of the ball.
  3. A 700 kg truck is travelling north at a velocity of 40 km · hr - 1 when it is approached by a 500 kg car travelling south at a velocity of 100 km · hr - 1 . Calculate the total momentum of the system.

Newton's second law revisited

You have learned about Newton's Second Law of motion earlier in this chapter. Newton's Second Law describes the relationship between the motion of an object and the net force on the object. We said that the motion of an object, and therefore its momentum, can only change when a resultant force is acting on it. We can therefore say that because a net force causes an object to move, it also causes its momentum to change. We can now define Newton's Second Law of motion in terms of momentum.

Newton's Second Law of Motion (N2)

The net or resultant force acting on an object is equal to the rate of change of momentum.

Mathematically, Newton's Second Law can be stated as:

F n e t = Δ p Δ t

Questions & Answers

what is a vector
Pule Reply
how to get a resultant force?
Pule
what are vectors in 2 dimension all about?
Ammi Reply
the magnitude and direction of the resultant force
Syanda Reply
agree with you
Eskay
hello
Pule
hey
Cronelia
hello
Sangay
what is scalar
Smangele Reply
quantities that have a magnitude,but no direction e.g speed, distance
Mbongeni
Agreed
Tino
Oh yah
Syanda
what are properties of vectors
Musa Reply
wht is the total resistance in the circuit?
Zinhle Reply
which circuit?
andy
yaah parallel circuits can't be treated the same as series in terms of resistance so u must specify which circuit u are dealing with
Musa
what is happening between the two
Nhlanhla Reply
what is vector
banele Reply
what is a vector
banele
physical quantity that has both magnitude and direction
Nhlanhla
a quantity having direction as well as magnitude especially as determining the position of one point in space relative to another
siyabonga
so the apple story is true afterall
Musa Reply
what is a nuclei
Acacia Reply
it's the core off an atom.
Emmanuel
it inner most of an atom
andy
Define the term electromagnetism
Bokiyana Reply
Definition of ohm's law
Bokiyana
The current flowing through a conductor is directly proportional to the potential difference across the end of the conductor.
Lettie
how do i calculate ohms law
Tiisetso Reply
How can i resolve vectors into components
Nhlalala Reply
Fx=fcos Fy=fsin
Vutomi
how many Atoms Do exist?
Jhon Reply
bullet fired from the gun
Prem Reply

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Source:  OpenStax, Siyavula textbooks: grade 11 physical science. OpenStax CNX. Jul 29, 2011 Download for free at http://cnx.org/content/col11241/1.2
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