Does speed increase or decrease?

Principles illustrated

Acceleration due to gravity
Free Fall
Terminal Velocity
Friction
Balanced and Unbalanced Forces
Projectile Motion

This is a simple demonstration that can foster discussion of many physics topics.

Begin by standing on a desk, holding a book and an open piece of paper. Ask «which will hit the ground first if I drop them?» Typically, students will answer the book. Ask why, the answer you will probably get is because «the book is heavier.» Then, crumple up the paper (usually to remarks such as «cheater») and drop them. Some follow up questions might be.

Does crumpling the paper add mass to it?
When the paper is not crumpled does its mass make it fall faster?
What makes the paper fall more slowly when it is not crumpled? (Note here you are leading students thought toward air resistance)
What force makes things fall toward the ground?

This demonstration can be followed up with discussions on Terminal Velocity to talk about balanced and unbalanced forces.

This can also be followed up with a discussion of projectile motion with the classic question:

«If a bullet is shot forward parallel to the ground without anything in its way, and at the exact same moment another bullet is dropped from the same height, which bullet will hit the ground first?» (Both bullets hit the ground at the same time since gravity is the only force pulling them down)

Projectile Motion Demonstration

Set-up two pennies and a ruler on a table as in the picture below.
Hit the ruler as shown, watch and listen to which penny falls first. (both should hit the ground very close to the same time)

Standards

2b. Students know when an object is subject to two or more forces at once, the result is the cumulative effect of all the forces.

2d. Students know how to identify separately the two or more forces that are acting on a single static object, including gravity, elastic forces due to tension or compression in matter, and friction.

2g. Students know the role of gravity in forming and maintaining the shapes of planets, stars and the solar system.

1f. Students know applying a force to an object perpendicular to the direction of its motion causes the object to change direction but not speed (e.g., Earth’s gravitational force causes a satellite in a circular orbit to change direction but not speed).

Questioning Script

Prior knowledge & experience:

Feathers fall slower than heavier objects.
Parachutists fall slower when the parachute is open.
Heavy objects push with greater force on your hand.
You can feel a force on your hand if you place it out the window of a moving car.

Root question:

Which falls more quickly in free fall, a feather or a hammer? (See answer here.)
Does mass change the acceleration of the object if gravity is the only force acting on it?
If you shoot a bullet parallel to the ground and at the exact same moment, from the exact same height, which will hit the ground first? (Assume the bullet does not hit anything)
What makes a feather fall slower on earth than a hammer?

Target response:

Both objects fall at the same speed.
Mass does not affect the speed of falling objects, assuming there is only gravity acting on it.
Both bullets will strike the ground at the same time. The horizontal force applied does not affect the downward motion of the bullets — only gravity and friction (air resistance), which is the same for both bullets.
Air resistance makes a feather fall slower.

Common Misconceptions:

Objects with a greater mass will fall faster (with a greater acceleration?)
An objects forward motion will change the rate at which objects fall.

References & Links:

DOE Explains. Relativity

Why can’t objects escape black holes? Because special relativity holds that the speed of light is the same across the cosmos. Escaping a black hole’s gravitational pull at its surface (the event horizon) would require an object to move faster than light.

Image courtesy of Sophia Dagnello, NRAO/AUI/NSF

Relativity is two related theories: special relativity, which explains the relationship between space, time, mass, and energy; and general relativity, which describes how gravity fits into the mix. Albert Einstein proposed these theories starting in 1905. By the 1920s, they were widely accepted by physicists.

Special relativity involves two key ideas. First, the speed of light in a vacuum is the same for any observer, regardless of the observer’s location or motion, or the location or motion of the light source. Second, the laws of physics are the same for all reference frames that are not speeding up or slowing down relative to each other. A reference frame can be thought of as an environment in which an observer is at rest. For example, when you drive down the road, your car can be thought of as your reference frame. You are at rest with respect to your car and everything in it. However, if a reference frame is moving relative to another, those two reference frames each has a different perspective on time and space. The three dimensions of space and the one dimension of time as well as how we measure them make up what physicists call the space-time continuum.

Einstein’s most famous equation describes the relationship between energy, mass, and the speed of light. It says energy (E) equals mass (m) times the speed of light (c) squared (2), or E=mc 2 . It means that mass and energy are related and can be changed from one to the other. Mass is basically the amount of material an object contains (which is distinguished from weight, which is the force of gravity on an object). Mass changes depending on the object. In contrast, the speed of light is a constant—it is the same everywhere in the universe.

The speed of light is incredibly high. Because the speed of light is squared in Einstein’s equation, tiny amounts of mass contain huge amounts of energy. Another result of the theory of special relativity is that as an object moves faster, its observed mass increases. This increase is negligible at everyday speeds. But as an object approaches the speed of light, its observed mass becomes infinitely large. As a result, an infinite amount of energy is required to make an object move at the speed of light. For this reason, it is impossible for any matter to travel faster than light speed.

Special relativity describes how the universe works for objects that are not accelerating, called inertial reference frames. However, it doesn’t incorporate gravity. That’s part of the theory of general relativity. Before Einstein, the traditional view was that gravity was an invisible force pulling things together. Instead, general relativity states that gravity is how mass warps space and time. The bigger the mass, the more it warps things. Imagine that the universe is a rubber sheet covered with objects of different weights, each sitting in a curved depression formed by that object’s weight; more massive objects will bend the sheet more. General relativity is why stars, which are incredibly massive, bend the path of light. Black holes, with huge amounts of mass in a small space, bend space so much they actually trap light.

Special and general relativity come together to show how time is measured differently in different frames of reference, called time dilation. This effect happens because different frames of reference perceive time and space differently. Let’s look at an example: the muon . Muons are subatomic particles that are created when cosmic rays hit the Earth’s atmosphere. They decay after just 2.2 microseconds. Although muons travel at nearly the speed of light, they decay so fast that they shouldn’t reach the Earth’s surface. But many do. To an observer whose reference frame is standing on the Earth’s surface, a muon should travel only .4 miles in its 2.2 microsecond life. But because muons travel so close to the speed of light, from their reference frame time passes for them about 40 times slower than viewed by an earth observer. This means, from our perspective on Earth, a muon has a lifetime of close to 90 microseconds, during which it can travel 16 miles. This effect is known as time dilation.

DOE Office of Science: Contributions to Special and General Relativity

As fundamental theories of physics, special and general relativity underpin all the work supported by the Department of Energy Office of Science. Relativity is particularly important to the research of the DOE Office of Science Nuclear Physics and High Energy Physics programs. In addition, relativity is essential to many of the scientific facilities the DOE Office of Science supports. For example, DOE’s particle accelerator user facilities, which speed subatomic particles to nearly the speed of light, must take relativity into consideration.

Relativity Fast Facts

In keeping with relativity, as particle accelerators speed subatomic particles, they also make those particles incredibly massive.
Global positioning system (GPS) satellites fly in different orbits around the Earth. These orbits are different frames of reference, so GPS has to take special relativity into consideration to help us navigate.

Resources

DOE SC Nuclear Physics program
DOE SC High Energy Physics program
Fermilab video: What’s relativity all about?
DOE Office of Science Nuclear Physics user facilities
DOE Office of Science High Energy Physics user facilities

Scientific terms can be confusing. DOE Explains offers straightforward explanations of key words and concepts in fundamental science. It also describes how these concepts apply to the work that the Department of Energy’s Office of Science conducts as it helps the United States excel in research across the scientific spectrum.

Does speed increase or decrease?

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Who’s in the Video

Dr. Michelle Thaller is an astronomer who studies binary stars and the life cycles of stars. She is Assistant Director of Science Communication at NASA. She went to college at[…]

Light exists outside of time.
4 min
with

MICHELLE THALLER: So, Tom, you asked the question, «How does mass increase as you go faster?» And this is really a wonderful part of Einstein’s theories. It actually is also relatively slippery and kind of complicated because to even answer this question at all, we have to ask the rather strange question: «What do you mean by mass? What is your definition of mass?» You may have heard that nothing with mass can possibly go at the speed of light. The only things that travel at the speed of light are photons pure energy, light, the speed of light. Nothing with any mass at all can travel at the speed of light because as it gets closer and closer to the speed of light, its mass increases. And if it were actually traveling at the speed of light, it would have an infinite mass.

So think about that. Even if you had a tiny little particle that was, say, billions of times less massive than an electron just a tiny, tiny little piece of mass if for some reason, that tiny thing accelerated to the speed of light, it would have an infinite mass. And that’s a bit of a problem. So let’s talk about this. One of the things that you really have to realize is the speed of light is very, very special. It’s not just simply a speed of something moving through space. As you go faster and faster and closer to the speed of light, time itself begins to slow down. And space begins to contract. As you go close to the speed of light, the entire universe becomes smaller and smaller until it basically just becomes a single point when you’re going at the speed of light. And time, as you go closer to the speed of light, gets slower and slower until basically time is a single point at the speed of light.

Light does not experience space or time. It’s not just a speed going through something. All of the universe shifts around this constant, the speed of light. Time and space itself stop when you go that speed. So the reason you can’t accelerate to the speed of light, and the reason we say you have an infinite mass is a little complicated because the idea that mass actually is a measurement of energy. You may remember Einstein’s famous equation, E equals MC squared. Energy equals mass times the speed of light squared. Energy and mass are equivalent. Mass is basically a measurement of how much energy there is in an object. When you’re moving, you have the energy of your motion, too. That’s called kinetic energy, energy of motion. So E equals MC squared, now your mass has not just the stuff that’s in you but also the energy of your motion.

And that’s why mass seems to increase as you go faster, and faster, and closer to the speed of light. It’s not that you are actually getting any heavier. The increase in mass is something that’s only observed by people that are watching you go by. If you were on a spaceship going very fast at the speed of light, you don’t notice anything getting heavier. You are on your spaceship. You could jump up and down. You can skip rope. You can do whatever you want. You don’t notice any change at all. But if people try to measure your mass as you go by, they not only are measuring your rest mass — your mass when you were still — but this added energy of this huge speed that you have through space. And that’s called a relativistic mass. And it’s a complex idea to think that mass itself is a measurement of energy, so that changes depending on how fast you’re going. If you were to slow down on your spaceship, you would not keep that mass.

You would go back to being the same mass you were before you started moving that quickly. So as you can see, it’s a complicated answer depending on how you define mass. Because as you’re going very close to the speed of light and you have a huge speed, you need to take into account that energy because of the equation E equals MC squared.