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What Is Momentum?

Newton's Three Laws Of Motion

  1. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it. In other words, an object at rest stays at rest until acted upon by a force, an object in motion stays in motion until acted upon by a force
  2. The greater the force, the greater the motion. The greater the mass, the greater the motion. The relationship between an object's mass m, its acceleration a, and the applied force F is F = ma.
  3. For every action, there is an equal but opposite reaction.

The world is wide, and I will not waste my life in friction when it could be turned into momentum. — Frances E. Willard

What Is Momentum?

Fast moving objects — such as a baseball, a bullet fired from a gun, or a moving car— are very hard to stop.

  • And according to Newton's First Law, due to an inherent laziness or inertia, objects have a tendency to want to maintain their current velocity.
  • Thus, momentum can be described as mass in motion. All matter that is moving has momentum.
An illustration of Newton's third law in which two skaters push against each other. The skater on the left exerts a force F on the skater on the right, and the skater on the right exerts a force −F on the skater on the right. Although the forces are

An illustration of Newton's third law in which two skaters push against each other. The skater on the left exerts a force F on the skater on the right, and the skater on the right exerts a force −F on the skater on the right. Although the forces are

Momentum is a vector quantity meaning that not only does it have magnitude but it also has direction.

  • Momentum is defined as the tendency of an object to keep moving in the same direction. If an object has lots of momentum, it is difficult to change its direction.
  • By applying enough force to an object that can move or is already moving, a change in momentum occurs.
  • According to Newton's Second Law, applying force to an object will cause it to accelerate or increase its velocity.
  • Momentum depends not only on an object's mass and speed, it also depends upon the direction it is travelling.
  • Thus, an object's momentum can change if:
  1. The object changes speed, moving either faster or slower;
  2. The object changes direction.

An object will remain in motion unless it is interfered with. Friction or a collision from another object are two factors which change momentum. Momentum is conserved as long as an outside force does not interfere with the object.

Newton's Third law applies to collisions between objects. When a collision occurs between two objects, the objects experience force equal in magnitude but opposite in direction.

Elastic and Inelastic Collisions

Elastic Collisions

In a perfectly elastic collsion, there is no loss of kinetic energy, for example, to heat, friction or sound. The total kinetic energy of the two objects before the collision is equal to the total kinetic energy of the two objects after they collide. Elastic collisions typically occur between rigid, non-sticky objects such as pool balls or the steel balls of a pendulum.

The Law of Conservation of Momentum which states that both momentum and kinetic energy are conserved, applies to elastic collisions when there is no external force present.

Inelastic Collisions

In an inelastic collision, the total kinetic energy of the two objects before the collsion is less than the total kinetic energy of the two objects after the collision. At the same time, momentum is conserved. This is true because some of the kinetic energy is transformed into other forms of energy such as heat and sound. Inelastic collisions typically occur between non-rigid, sticky objects such as clay.


Equation For Momentum

what-is-momentum

Applications Of Momentum

  • Rockets using expulsion of gases through the rear of the rocket produce forward momentum allowing the rocket to travel upward.
  • Momentum is seen in the game of billiards. Ball A is struck by a pool cue and hits ball B. The momentum is transferred from ball A to ball B. The momentum of ball A decreases as its velocity slows, allowing the momentum of ball B to increase as it gains velocity. The total momentum of both billiard balls stays constant.
  • A ballistic pendulum is a device for measuring the momentum of a bullet fired from a gun. Although now obsolete it was used for many years and led to advances in the science of ballistics. This pendulum is still found in physics classrooms today because it can easily demonstrate the principles of momentum and energy.
  • A cuckoo clock uses weights and a pendulum system to keep time. Weights slowly pull down the chains which move the gears inside the clock. These gears are kept in time by the moving pendulum. WInding regularly, usually once a day, keeps the clock working.

Test Your Skill At Calculating Momentum

For each question, choose the best answer. The answer key is below.

  1. Determine the momentum of a 65 kg half-back moving westward at 10 m/s.
    • 540 kg*m/s
    • 600 kg*m/s west
    • 65 kg*m/s east
  2. Determine the momentum of a 1100 kg car moving at 22 m/s north.
    • 24200 kg*m/s north
    • 22000 kg*m/s south
    • 1100 kg*m/s
  3. Determine the momentum of a 15 kg dog walking south at 3m/s.
    • 15 kg*m/s
    • 45 kg*m/s south.
    • 40 kg*m/s north east

Answer Key

  1. 600 kg*m/s west
  2. 24200 kg*m/s north
  3. 45 kg*m/s south.

Resources Used

Andrew Rader Studios. Physics 4 Kids. Momentum Basics. 2012

Science Clarified. Real Life Chemistry vol3- Physics vol 1. Momentum.

The Physics Classroom. The Impulse-Momentum Change Theorum 2012.

WIkipedia. Ballistic Pendulum. June 1, 2012

Comments

Teresa Coppens (author) from Ontario, Canada on October 11, 2014:

Glad you enjoyed it.

muhammad abdullah javed on September 24, 2014:

Teresa Coppens, nice hub, good info about momentum and its applications. Thank you.

Teresa Coppens (author) from Ontario, Canada on October 30, 2012:

I am so glad you enjoyed the hub! I appreciate the compliment on my work.

MG Singh from UAE on October 30, 2012:

Well written my friend. Such articles are a rarity on hubpages.

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