Please see the link in the description to download a worksheet for this video. Experiments can be dangerous. If you are a child: never do experiments unless your parent, guardian or adult educator says it's alright, and is there with you the whole time. If you've not already done so, we suggest that you watch the video introducing energy forms and conversions before watching this video. This is a close-up animation of a baseball bat hitting a baseball. In science and engineering, whenever two or more objects make a sudden contact, we call that a collision. Most collisions we see are between objects, like between something we're holding and something we're hitting. But collisions can also involve our body parts directly, such as what happens when we kick a ball. In physics, instead of saying the words person, animal or object, we say the word "body," which could mean any of them. Sometimes one of the bodies is still when the collision happens. Many times, both of the bodies are in motion when the collision happens. Collisions may involve bodies that are similar size or the bodies may be very different sizes, such as between this basketball and the ground it's colliding with. Sometimes the shape of one or more bodies is permanently changed in a collision, and may even break into small parts. Understanding collisions is important in sports, engineering and science. In this video, we'll introduce the following concepts related to collisions: common energy forms in collisions; kinetic energy transfer; kinetic energy conversions into other forms of energy; elastic and inelastic collisions; and predicting how objects will move after some collisions. We'll start by discussing common energy forms in collisions. There are six common forms of energy. Of these, we'll focus on four in this video, which are: the energy of motion, the energy of sound, the energy of heat and the energy of light. In physics, we use the term "kinetic energy" instead of saying "motion energy." A body that is stationary has no kinetic energy. The faster a body moves, the more kinetic energy it has. Sound is another form of energy. For example, when the speaker's volume is high enough and at just the right pitch, the sound energy shatters this glass. Energy can also be in the form of heat and light. For example, a campfire has a lot of energy that we feel as heat and we see as light. A key concept about energy is that it can change forms. We call this energy conversion. For example, a saw blade rotates fast so has a lot of kinetic energy. But when it contacts metal, some of that kinetic energy is converted into sound energy. Some of the blade's kinetic energy is converted into heat energy, which we can see as smoke. And some of the blade's kinetic energy is converted into light energy, because the metal fragments are so hot that they glow. Next, we'll introduce energy transfer during collisions. What do you you notice about the speed of the white pool ball before versus after it hits another pool ball? Immediately after the collision, the white ball is going slower, but the ball it hits has gone from not moving to moving fast. This happens because the white ball gave some of its kinetic energy to the ball it hit. When a body gives some of its energy to one or more other bodies and that energy stays in the same form, then we call this an energy transfer. Collisions are a common way that kinetic energy transfers from one body to another. A bowling ball slows down after it has hit bowling pins because the bowling ball has transferred some of its kinetic energy to the bowling pins. Next, let's look at energy conversions during collisions. If we compare the kinetic energy of the white ball immediately before the collision, to the kinetic energy of the white ball plus the red ball immediately after the collision, we find that there's less total kinetic energy after the collision. Before the collision, all the energy is in a white ball and is in the form of kinetic energy. After the collision, the white ball has only a little kinetic energy and the ball that it hit has a medium amount of kinetic energy. It may seem that some energy is gone, but energy is never destroyed. Some of the white balls kinetic energy was converted into sound and heat energy. During the collision, we can hear that sound energy when the balls collide. When we account for the sound and heat conversions, we can see that the total amount of energy before and after the collision is equal. We use the term "conservation of energy" to describe that the amount of energy stays the same within a closed systems in the case of collisions. Energy is conserved in all types of collisions. Collisions frequently convert kinetic energy into heat energy in several ways. One way is friction between the two bodies when one slides past the other, like these athletes sliding over the ground. When this happens on a rug we call it a "rug burn" but friction between any surface can create a burn. Another way that collisions convert kinetic energy into heat energy is by changing the shape of one or more bodies during the collision. To illustrate how changing a body's shape can increase its temperature, we'll change the shape of this blue tack and in just a few seconds it gets hotter than this man's fingertips. Next, we'll introduce elastic and inelastic collision types. After the bodies collide, if they bounce apart then it's an elastic collision. If they stay together, then it's an inelastic collision. Please note that in many collisions, one of the bodies is the Earth, such as when one body is falling to the ground. One way to remember which type of collision means which type of action, is to think of darts going into a dart board. The word "into" and the word "inelastic" both start with the prefix "in." These three rubber bands are tied together and so are these three rubber bands, but only one group bounces. This is a reminder that we classify the collision as elastic or inelastic based on whether the bodies bounce apart or not, rather than classifying them based on what the bodies are made of or other factors. Next, we'll introduce predictions for how objects move after collisions. It can be very difficult to predict how bodies will behave after a collision. In a more advanced video, we'll describe how to solve these problems using formulas. But for an introduction, you may wish to do some simulations at this free website. You can adjust the mass, speed, direction and elasticity of the balls, then the computer will do the calculations and show you what will happen. There is one type of collision which is famous and we suggest you memorize. That is the case in which two bodies are equal mass and shape, then the moving one hits the still one in an almost perfectly elastic head-on collision. What do you think will happen immediately after this collision between these curling stones? The moving stone completely stops and the one that was stationary now moves quickly. So then why is it that immediately after a collision the white cue ball is still moving if all the pool balls are the same weight? The reason is because in some sets, the white pool ball is heavier than the other balls. Here's a situation in which the white cue ball does have the same mass as the other balls. Here's a summary of this topic plus some additional information. Please pause the video if you wish to read this. If you're interested in practice tests that are similar to State exams but with detailed, colorful explanations for each answer, then please see our apps in the App Store. Many of these are free and none expire or limit their function. 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