Force and Motion: Top 10 Questions

March 2016

Thanks to Thanks to Dr. Kathryn Devine, assistant professor of physics, College of Idaho; and John Gardner, professor of mechanical engineering, Boise State University for their answers. for the answers.

1: What is force, and how is it made?

A force can be thought of as a push or a pull that makes something change its speed or direction. So, if you want to speed something up, you have to push or pull it to make it speed up or change direction. If you drop something, gravity acts as a force and makes it speed up toward the ground. Static electricity is another force that can suck your hair toward a balloon. There are many types of forces, but they all act as a push or pull that can make something speed up or down, or make it change its direction. (From Blake at Kamiah Elementary School in Kamiah)

2: Why are Newton's three laws so important to science?

One of the reasons why Newton's laws are so important is because Newton observed a lot of things around him. All of the scientists and naturalists of the age were trying to figure out how things move and why. He was able to look at all of these observations and fold them together into a set of rules that fit with one another and fit the observations. It's a great example of the scientific theory and the scientific method. These laws are very important in helping us understand planets, gravity, and just how things move. Newton laid the foundation for all of this. (From Emily at Owyhee Elementary School in Boise)

3: What makes a ball bounce?

When you throw a ball to the ground, you give it energy. That energy is reflected in its velocity. Think of it as speed. When the ball hits the ground, it stops all of a sudden. That energy doesn't go away but gets absorbed into the ball. The ball squishes up a little bit, but it wants to un-squish. It expands and bounces back up. It won't come back in exactly the same way, because some of that energy gets lost in the material of the ball. (From Creighton at Owyhee Elementary School in Boise)

4: How big is the gravitational force and why?

Technically speaking, a gravitational field would extend away from a mass forever. So, Earth's gravitational field would extend infinitely out. Physicists tend to think of it as how far does it extend until it matters less or some other force matters more. Once you get far enough away from Earth, gravitational forces from other objects, planets or stars start to take over and be more important. So around Earth, our gravitational field matters a lot. Once you start moving out to the solar system, the gravitational field from Earth still exists, but other things are going to play a larger role and their gravitational fields matter more. (From Alex at Sagle Elementary School in Sagle)

5: Why does something drop when I throw it up?

When you throw something, like a ball, up in the air, it has mass. The earth under your feet has mass, and gravity acts between the two masses. So, as you throw the ball in the air, gravity is always acting on the ball and changing the speed of it as it moves up. Gravity is always trying to pull it back to the center of Earth. As the ball goes up, it slows down as the force of gravity slows it down. It will eventually reach its highest point and stop altogether and then come back down toward you. It doesn't get to the center of Earth because the surface, or the ground, stops it. When it sits there, gravity is still acting on it, but the surface of Earth is pushing back on the ball with the same amount of force, so it doesn't move anymore. (From Austin at West Park Elementary School in Moscow)

6: I was wondering why a slinky follows itself down the stairs?

A slinky, like everything else, has mass and is affected by Earth's gravity. So, when you push a slinky off the stairs, Earth's gravity is going to pull the top end of the slinky over and create the force that pushes the front end down. Because the slinky is connected, that connection force causes the rest of the slinky to go with it. If it's a little bit off center, the force pulls the slinky with it, causing it to continue down the stairway and you start the whole process over again. Gravity is the key force working here. (From Ayden at Willow Creek Elementary School in Meridian)

7: How did Earth's gravity form, and where did it ever change?

Gravity is directly related to mass. Earth has gravity because it is made up of matter and has mass. Planets, including Earth, formed because there were a couple of smaller pieces of mass that were attracting each other. They came together and they stuck together because that's what gravity does. Gravity pulls objects together. So two objects together have more mass than when they are apart. Those two objects together then create a greater gravitational force pulling more objects toward them. Once there is enough mass, other types of molecules are pulled upon that make up gas. This makes up the atmosphere. We need gravity to keep an atmosphere on a planet as well. (From Jacob at Basin Elementary School in Idaho City)

8: How does the moon circle around Earth?

The moon is being pulled toward Earth at all times. So, why doesn't the moon just crash into Earth? Well, the moon is being pulled toward Earth, but it also has speed around Earth, or velocity. If Earth would vanish, the moon would start traveling in a straight line, because things travel in a straight line unless acted on by another force. Earth exerts a gravitational force on the moon, pulling it a little bit, causing it to change direction and rotate around Earth. (From Jaden at Owyhee Elementary School in Boise)

9: How does a magnet work?

The force at work is called a magnetic force. It is related to the electrostatic force. If you were to look at the tiny pieces that make a magnet, like a bar of iron that has been magnetized, you would see individual atoms that make up the iron in the magnet. All of the electrons in those atoms are lined up so their orbits are oriented in the same direction. That's the key to how a magnet works. All of the electric fields of the electrons are in the same direction, and all of those electrons are moving. This generates a magnetic field. So, because all of the electrons are aligned, a magnetic field is generated. If they are not aligned, there is not a magnetic field. (From William at Owyhee Elementary School in Boise)

10: Are all metals affected by magnets?

No, not all metals are affected by magnets. It has to do with how the atoms line up in an individual grain of metal. We classify metals as ferrous metals, which is related to iron containing metals, and nonferrous. So, it's mostly metals that contain iron that are attracted by magnets. (From Jenson at Owyhee Elementary School in Boise)

February 2011

Thanks to Kathryn Devine, Assistant Professor of Physics, College of Idaho for the answers.

1: What is a force?

A force is what puts an object in motion. In order to create motion, you need to a have a force. (From Chloe in Mrs. Evans' class at Littleton School in Heckmondwike, England)

2: Why do some planets have more gravity than others?

Some planets have more mass or matter to them. The more matter a planet has, the stronger the pull of gravity. A very small planet will exert less gravity than a large planet. That's why we see astronauts bouncing around on the moon, because there is less gravity. (From Dylan in Mrs. Hudson's class at Dalton Elementary School in Hayden)

3: How does gravity hold up the Earth?

Gravity provides a centripetal force (a force that makes a body follow a curved path). Earth has a velocity and it's always traveling in a circle. The gravitational force between the Earth and the sun keeps them bound together, but Earth's velocity around the sun keeps it from falling into the sun. (From Clara at Longfellow Elementary School in Boise)

4: Why doesn't anything on a table just fall through the table?

The table is solid. Solid objects can provide a normal force. That's a force that pushes back. So, if a coffee mug is sitting on a table, gravity is acting upon it as well as the normal force. These forces cancel each other out, adding up to zero force. Therefore, there would be no acceleration on the mug and it would stay put. (From Darian in Mrs. Hudson's class at Dalton Elementary School in Hayden)

5: How do you measure force?

It depends on the type of force you are trying to measure. The one we know the most about is the measurement of the gravitational force. We measure the force of gravity by getting on a scale and seeing what we weigh. Your weight tells you how much gravitational force is acting on your mass. (From Patrick, a student in Weiser)

6: Why does water not have too much friction, but it hurts when you do a belly flop?

If you hit water going fast enough, it acts a little like hitting a solid. You are accelerating downward and when you hit the water it exerts an equal and opposite force back on you. If you hit it with your tummy, it can hurt quite a bit. (From Alex in Mrs. Hayes' class at Owyhee Elementary School in Boise)

7: What's the difference between gravity and magnetic force?

Magnetic force and electric forces are caused by charged particles and things on the atomic level. Gravity is a force that exists because of mass, the amount of matter. It comes down to what matter is made out of that causes different forces to exist. (From Cassidy in Mrs. Hayes' class at Owyhee Elementary School in Boise)

8: Is air part of a force?

Air is made out of a lot of small molecules and particles. It is basically a gas. When those tiny molecules hit you, you can feel the force of them interact with you. When you are feeling wind, you are feeling the collision force of air (or the molecules and particles) hitting you. The faster the wind, the more force behind it. (From Quinn at Sage International School in Boise)

9: How does gravity keep us on the ground?

Gravity keeps us on the ground because there is a huge amount of mass to the planet Earth. That means the gravitational force of the planet is very high. We are constantly accelerating down toward Earth. The reason we don't fall straight toward the center is because Earth is a solid creating a normal force. These forces cancel each other out, so we stay here, on the surface. (From Cooper at Joplin Elementary School in Boise)

10: How does a frictional force work?

You can experience friction if you rub your hands together and feel the warmth that is created. If you push really hard, it becomes harder to scoot your hands along one another. The force of friction depends on how much downward pressure you have (or how much force there is) between the two things that are in contact. (From Natalie in Mrs. Woodall's class at Dalton Elementary School in Dalton)

January 2008

Thanks to Steve Shropshire, Professor of Physics, Idaho State University; and Jim Dull, Associate Professor of Physics, the College of Idaho for the answers.

1: What is gravity made of?

It's not so much what gravity is made out of, it's what gravity does. Gravity is simply the interaction between objects that are big, usually. But it can be between objects that are small, too. So anytime there's stuff (material), then you're going to have a gravitational interaction between them. (From Nicole in Mrs. Woodall's class at Hayden Meadows Elementary School.)

2: Who discovered gravity?

Galileo did the first experiments with gravity and was able to describe how it affected motion, but Newton was the first one to really spell it out into a well-defined theory. The idea that not only do things with mass attract each other, but that attraction decreases with distance and is proportional to how much stuff you have with each object. So Newton was the first one to really nail down something close to the current theory we have. (From Aspen in Mrs. Hunt's class at Cynthia Mann Elementary School.)

3: How far do you go into the atmosphere until gravity stops?

The pull of gravity from our planet affects even distant planets. However, gravity from the earth has to be plenty strong, even as far out as the moon, to keep something as big as the moon moving around our planet. The reason that the moon doesn't shoot off away from us is it's constantly being pulled towards the earth from the force of gravity. Also, the earth is being pulled toward the moon. And, we're being pulled toward the sun because of its huge mass. You have to go way, way far away before you can start to really neglect the pull of the earth; you have to go way beyond Jupiter before you can neglect it to any reasonable degree. (From Janely at Westside Elementary in Idaho Falls)

4: What makes studying force and motion interesting?

We want to understand forces and we study them because basically any motion that occurs is going to occur because of a force that's on it. So we'll study big forces like gravity or smaller forces like electricity and magnetism. And then we also study forces that are very small - not necessarily small in size but small over the distance in which they interact with one another, such as nuclear forces. What we try to understand is why things move the way they do or predict how things will move in the future based upon these forces. (From Tom at Valley View Elementary)

5: Is there gravity in space?

Space pretty much starts where our atmosphere ends. And you certainly have gravity from the earth pulling the moon in. When people go up in the space shuttle or in rockets they're constantly being pulled back toward the earth. You often think of astronauts floating around. That's not because there's no gravity; there's actually quite a bit of gravity there, not much different than what we have on the surface of the planet. It's just that they're falling. And if you're falling you don't notice it so much especially if everything around you is falling too. People in the space shuttle are pulled very strongly by the earth's gravity, but they're falling all the time. They're actually going in a big circle in what we call an orbit around the planet. (From Carter, a third grader in home school)

6: What do magnets have to do with force and motion?

There is a force between two magnets. The North Pole will attract the South Pole, the South Pole will attract the North Pole, or they'll repel each other. So magnetism is a little bit different than gravity in the sense that there are two types of forces, attraction and repulsion, whereas gravity is only attractive. (From Marlee in Mrs. Schweitzer's class at Riverside Elementary School)

7: What is stronger, gravity or friction?

Friction is basically sticky things. When you try to rub one thing against another, they're kind of sticky and that's what we call friction. But if you had no gravity there would be no contact force between the objects. Without gravity there really wouldn't be friction, at least not much of it. The greater the contact force between the surface the greater they're being pushed together. And that's usually resulting from things being supported around gravity. Without that there would be very little contact force and very little friction. So it really depends on the circumstance. (From Bryan at Hayden Meadows Elementary School)

8: Does the sun have gravity?

The sun is huge and it's made of lots of "stuff." Scientists refer to that "stuff" as mass. All of the planets will revolve around the sun because of the gravitational force between the sun and the planets. The more mass of the object, the bigger it is, the greater the gravitational force. That's why the planets revolve around the sun because the sun's mass is so great. That's part of it. Let's go ahead and look at Newton's Law, number two. Forces make things move. The bigger the force or the lighter the object, the greater the motion. (From Todd is Mrs. Wysong's class at Rupert Elementary School)

9: Can gravity be measured?

Yes, gravity can be measured. Any force can be measured. We measure gravity all the time by simply weighing ourselves on the scale. It's telling us what the force of interaction between us and the earth is. So if we weigh less that means that force of direction is less. That's a pretty simple way to figure out gravity. (From Dallin in home school in Idaho Falls)

10: How do roller coasters stop and go using gravity?

Usually there's some chain or something, some force that's pulling it up the hill. What happens when they go down the hill is that there is a sideways force maybe, but usually they use gravity, at least on the old roller coasters. So you're going down and falling at an angle in this case. It uses gravity to go down. To stop a roller coaster they're going to increase the friction. So they have this little, well, block of some sort, usually of wood or rubber that pushes against the roller coaster cart and slows it down gradually so that you don't pop forward too quickly. (From Hannah in Mrs. Woodall's class at Hayden Meadows Elementary School)

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