Linear Momentum

You may have heard of Isaac Netwon’s First Law: Objects in motion tend to stay in motion, and objects at rest tend to stay at rest. In this demo Jared uses carts on an air track to explore this concept of momentum.

When Jared pushes a single cart, the cart moves with speed in the direction it was pushed. It continues moving at the same speed until it reaches the wall. At the wall, it bounces off a spring, and moves in the opposite direction with the same speed.

What will happen if Jared collides two carts into one another, and they stick together? Let’s find out!

In this collision, Jared collides two carts and they stick together. Jared explains that the momentum of an object depends on its mass (or amount of stuff) and velocity (speed and direction of motion). For a whole system, momentum is conserved. This means it can be transferred between objects within a system, but not created or destroyed. In this example, the first cart initially starts with all the momentum. Once the two carts stick together, they have double the mass and so they move with half the original speed.

What will change if the carts bounce off one another instead of sticking together?

In this second collision, the carts bounce off one another, and the first cart gives its momentum to the second cart. The first cart stops moving and the second cart starts moving, but the overall momentum is conserved.

In this fourth video, Jared introduces a big cart, which has twice as much mass as the original cart. When he collides the big cart into the little cart, the little cart shoots off with a higher velocity due to its smaller mass.

Now Jared has 3 objects of three different masses. When he collides the littlest mass into the others, we see that the biggest mass moves the slowest.

In explosions, some internal force separates two or more objects. In this case, Jared uses springs to bounce two carts off one another. They exert equal and opposite forces on one another, and because they have the same mass, they move apart with the same speed. In order for momentum to be conserved in explosions, the center of mass must stay stationary.

In this Seventh video, two carts are attached by a spring, and a red tag denotes the center of mass. When Jared starts the two carts moving, the red tag moves with a constant speed, and the two springs expand and contract as the carts move.

Next, Jared rolls a ball down a ramp on a cart. As the ball rolls down the ramp, it generates momentum to the right. In order for the center of mass to remain stationary, the cart moves to the left!

Finally, Jared uses carts with magnets attached. Magnets can transmit momentum between the carts without the carts even touching! (We’ll delve more into the properties of magnets in the Magnetism demo.)