What is Momentum? - Definition, Equation, Units & Principle

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  • 0:00 Gaining Momentum
  • 0:50 Linear Momentum
  • 3:00 Conservation of…
  • 3:40 Elastic Collision & Example
  • 9:20 Inelastic Collisions
  • 11:50 Lesson Summary
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Lesson Transcript
Instructor: Damien Howard

Damien has a master's degree in physics and has taught physics lab to college students.

People talk about momentum all the time; now you can discover what that word really means. Then learn about the principle of conservation of momentum and what it has to do with objects colliding.

Gaining Momentum

Momentum is a word you're probably very familiar with. Often, you'll hear that something is gaining or gathering momentum. It might be an actual moving object or it might be used more metaphorically like with a sports team. But have you ever taken a moment to think what, exactly, momentum means?

Momentum is the quantity of motion of a moving body. In a basic sense, the more momentum a moving object has, the harder it is to stop. This is why you see the term used metaphorically like in the example of the sports team. It means the team is on a roll (generally, a winning streak) and is becoming a stronger team for it. The other teams will have a harder time stopping the team gaining momentum.

Linear Momentum

catching a baseball

We know that momentum is the quantity of motion of a moving body, but what, exactly, does that mean? Let's think about a baseball being thrown in a straight line through the air in order to try and understand this. When you catch a baseball, you feel the momentum of the ball being imparted to you. The ball will probably push your hand back towards you when you catch it. The more momentum the ball has, the more it will push back your hand as it transfers its momentum to you.

Imagine two baseballs are being thrown at you. One is traveling at 50 mph and the other at 150 mph. Even if you somehow catch that 150 mph ball, it might knock you off your feet. It will take more effort on your part to stop the 150 mph ball than the 50 mph one. So it stands to reason that velocity is a very important aspect of momentum. However, that's not all there is to momentum.

Now imagine two balls being thrown at you at 50 mph. One is a baseball and the other is a bowling ball. You probably aren't going to want to try to stop the bowling ball. It's going to keep traveling even after it hits you. Both balls are traveling at the same velocity, so what makes the bowling ball so much harder to stop? It's because it is heavier. It has more mass. So the other important aspect of momentum is mass.

In physics, we define momentum mathematically as the multiplication of mass and velocity as seen in this equation: p = m * v

  • p = momentum
  • m = mass
  • v = velocity

As far as momentum's units go, we have no special symbol used just for momentum. Instead, it is simply the combination of mass's standard unit of kilograms (kg) and velocity's standard unit of meters per second (m/s). Momentum is measured in standard units of kilograms times meters per second (kg m/s).

The Conservation of Momentum Principle

The principle of conservation of momentum states that in an isolated system, two objects that collide have the same combined momentum before and after the collision. That is, momentum is not destroyed in the collision, but transferred between the two objects. In an isolated system, momentum is always conserved in a collision. In the example of you catching a baseball, the momentum from the ball is transferred into your hand.

How momentum is transferred depends on the type of collision. There are three types of collisions: elastic, perfectly inelastic, and partially inelastic.

Elastic Collision

pool balls colliding

In an elastic collision, two objects collide and bounce off of each other. A good example of this type of collision would be a game of pool. You hit the cue ball with the cue, and it bounces off another ball. Hopefully, that other ball goes in the table pocket, but what's most important for us is that the two balls don't stick together at all. They bounce off of each other.

We can also express the conservation of momentum in an elastic collision mathematically. We know the momentum is conserved, so the total momentum before the collision must equal the total momentum after the collision. We'll call the momentum before the collision the initial momentum (i), and the momentum after the collision the final momentum (f). Total initial momentum equals total final momentum:

p{i}total = p{f}total

Now let's separate the total before and after momentum into the momentum for ball 1 and ball 2.

p{1i} + p{2i} = p{1f} + p{2f}

Next, we'll add in the equation for linear momentum we learned earlier (p = m * v). This will give us our equation for the conservation of momentum in an elastic collision:

m{1} * v{1i} + m{2} * v{2i} = m{1} * v{1f} + m{2} * v{2f}

You'll notice we didn't specify an initial and final mass like we did with momentum and velocity. This is because the mass of the two objects remains the same before and after the collision. Even though we used two pool balls in our example, you can't always guarantee that the two objects' masses will be the same, so we do keep the masses defined separately as m1 and m2 in the equation.

Bowling Problem

bowling ball traveling down lane

Now let's try an example with this equation and unlike the pool balls, we'll use two objects with different masses. Imagine you've gone bowling and a child before you rolled the ball so slowly it stopped mid-lane. No one is around to help you and you don't want to walk out on the lane yourself. You decide to roll your bowling ball into the one stopped on the lane to get it moving.

Your throw your 5.4 kg ball at a velocity of 20 m/s towards the child's ball weighing 2.7 kg. You hit the ball dead on and both balls continue to move in a straight line after the collision. The child's ball is sitting still before the collision so you know it was at an initial velocity of 0 m/s and after the collision, it moves at a velocity of 14 m/s. What velocity is your ball moving at after the collision?

Let's declare your bowling ball to be ball number 1 and the child's to be ball number 2. Note that this is an arbitrary decision since the math works out the same either way it is declared for a 1-dimensional elastic collision. We can now take the values we know and put them into the equation for conservation of momentum in an elastic collision.

m{1} * v{1i} + m{2} * v{2i} = m{1} * v{1f} + m{2} * v{2f}

(5.4 kg) * (20 m/s) + (2.7 kg) * (0 m/s) = (5.4 kg) * v{1f} + (2.7 kg) * (14 m/s)

(108 kg m/s) + (0 kg m/s) = (5.4 kg) * v{1f} + (38 kg m/s)

(108 kg m/s) = (5.4 kg) * v{1f} + (38 kg m/s)

(108 kg m/s) - (38 kg m/s) = (5.4 kg) * v{1f}

(70 kg m/s) = (5.4 kg) * v{1f}

(70 kg m/s) / (5.4 kg) = v{1f}

13 m/s = v{1f}

So we get the final velocity of your ball to be 13 m/s, which is just a little slower than the lighter ball it hit.

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