Concepts of Science

Concepts of Science Topics


Core-Science Sequence

Plagiarism and Honesty Policy

















































 Log out


Kinetic Energy (From The Sciences, 6th ed., by Trefil and Hazen)

Think about a cannonball flying through the air. When it hits a wooden target, the ball exerts a force on the fibers in the wood, splintering and pushing them apart and creating a hole. Work has to be done to make that hole; fibers have to be moved aside, which means that a force must be exerted over the distance they move. When the cannonball hits the wood, it does work, and so a cannonball in flight clearly has the ability to do work—that is, it has energy—because of its motion. This energy of motion is what we call kinetic energy.

You can find countless examples of kinetic energy in nature. A whale moving through water, a bird flying, and a predator catching its prey all have kinetic energy. So do a speeding car, a flying Frisbee, a falling leaf, and anything else that is moving.

Our intuition tells us that two factors govern the amount of kinetic energy contained in any moving object. First, heavier objects that are moving have more kinetic energy than lighter ones: a bowling ball traveling 10 m/s (a very fast sprint) carries a lot more kinetic energy than a golf ball traveling at the same speed. In fact, kinetic energy is directly proportional to mass: if you double the mass, then you double the kinetic energy.

Second, the faster something is moving, the greater the force it is capable of exerting and the greater energy it possesses. A high-speed collision causes much more damage than a fender bender in a parking lot. It turns out that an object’s kinetic energy increases as the square of its speed. A car moving 40 mph has four times as much kinetic energy as one moving 20 mph, while at 60 mph a car carries nine times as much kinetic energy as at 20 mph. Thus a modest increase in speed can cause a large increase in kinetic energy.

These ideas are combined in the equation for kinetic energy.

In words: Kinetic energy equals the mass of the moving object times the square of that object’s speed (v2).

In equation form: kinetic energy (joules) = 1/2 x mass (kg) x velocity2 (m/s)

In symbols: KE = 1/2 x m x v2

Examples: Bowling Balls and Baseballs What is the kinetic energy of a 4-kg (about 8-lb) bowling ball rolling down a bowling lane at 10 m/s (about 22 mph)?

Compare this energy with that of a 250-gram (about half-a-pound) baseball traveling 50 m/s (almost 110 mph). Which object would hurt more if it hit you (i.e., which object has the greater kinetic energy)?

Reasoning: We have to substitute numbers into the equation for kinetic energy.

Solution: For the 4-kg bowling ball traveling at 10 m/s:

kinetic energy (joules) = 1/2 x mass (kg) x [speed (m/s)]2

=1/2 x 4 kg x (10 m/s)2 = 1/2 x 4 kg x 100m2/s2 = 200 kg-m2/s2.

Note that: 200 kg-m2/s2 = 200 (kg-m/s2) x m = 200 N x m = 200 joules

For the 250-g baseball traveling at 50 m/s:

kinetic energy (joules) = 1/2 x mass (kg) x [speed (m/s)]2

A gram is a thousandth of a kilogram, so 250 g = 0.25 kg:

kinetic energy (joules) = 1/2 x 0.25 kg x 2500 m2/s2 = 312.5 kg-m2/s2 = 312.5 joules

Even though the bowling ball is much more massive than the baseball, a hard-hit baseball carries more kinetic energy than a typical bowling ball because of its high speed.

<< End of material from textbook, what follows has been added from other sources.>>

Asteroid impacts on Earth:

Everything in the solar system goes around the Sun. These paths around the Sun are called orbits. An orbit is a delicate balance between the forward motion of the orbiting body and the gravitational attraction between the Sun and the orbiting body.

Because of the gravitational attraction between and among all the orbiting bodies in the solar system, no two orbits are the same. These small differences in orbit do not affect large planets very much, but small bodies orbiting the Sun - like asteroids - can be strongly affected. Asteroids - packed relatively close to one another in the asteroid belt - can collide with one another or graze past each other as their orbits shift over time. This may result in an asteroid bumping or bouncing out of Its previous orbit and changing into a different orbit that is called 'Earth-crossing.' The diagram below shows a typical Earth-crossing orbit for an asteroid. The Sun is shown in red, the Earth in green, and the asteroid in yellow. NOTE - This diagram and the object sizes are NOT in proper scale.


Also, in this figure, objects like Mercury, Venus, Mars, and the asteroids are not included for simplicity.

As you can see, as the Earth and the asteroid go around the Sun, there is some chance that they may be in the same place at the same time one day and thus an energetic collision may take place.

The typical velocity of an asteroid in an Earth-crossing orbit and in the vicinity of Earth is about 20 km/sec. The kinetic energy equation says that KE = 1/2 m x v2. Squaring the velocity makes a large number like 20 km/sec much, much larger.

We are going to consider what happened in Alabama during the age of dinosaurs (specifically about 83 million years ago) when an asteroid about 380 m in diameter struck central Alabama. This event occurred about 20 km north of Montgomery, Alabama, near the town of Wetumpka. You should go to this link to read more about this event before continuing this laboratory: Click Here

When you have finished reading this on-line article at the Encyclopedia of Alabama web site, you will be ready to do the data collection activity in your lab book. Please read and study the article above, because it will be covered (along with the material above) on your laboratory quiz.