# gravity

## orbital motion

The moon moves around Earth in a circular path called an orbit. Why doesnt Earths gravity pull the moon down to the ground instead? The moon has enough forward velocity to partly counter the force of Earths gravity. It constantly falls toward Earth, but it stays far enough away from Earth so that it actually falls around the planet. As a result, the moon keeps orbiting Earth and never crashes into it. The diagram in Figure 13.23 shows how this happens. You can explore gravity and orbital motion in depth with the animation at this URL: http://phet.colorado You can see an animated version of this diagram at: http://en.wikipedia.org/wiki/File:Orbital_motion.gif .

## newtons law of universal gravitation

Newton was the first one to suggest that gravity is universal and affects all objects in the universe. Thats why his law of gravity is called the law of universal gravitation. Universal gravitation means that the force that causes an apple to fall from a tree to the ground is the same force that causes the moon to keep moving around Earth. Universal gravitation also means that while Earth exerts a pull on you, you exert a pull on Earth. In fact, there is gravity between you and every mass around you your desk, your book, your pen. Even tiny molecules of gas are attracted to one another by the force of gravity. Newtons law had a huge impact on how people thought about the universe. It explains the motion of objects not only on Earth but in outer space as well. You can learn more about Newtons law of gravity in the video at this URL:

## law of gravity

People have known about gravity for thousands of years. After all, they constantly experienced gravity in their daily lives. They knew that things always fall toward the ground. However, it wasnt until Sir Isaac Newton developed his law of gravity in the late 1600s that people really began to understand gravity. Newton is pictured in Figure 13.17.

## defining gravity

Gravity has traditionally been defined as a force of attraction between two masses. According to this conception of gravity, anything that has mass, no matter how small, exerts gravity on other matter. The effect of gravity is that objects exert a pull on other objects. Unlike friction, which acts only between objects that are touching, gravity also acts between objects that are not touching. In fact, gravity can act over very long distances.

## gravity and weight

Weight measures the force of gravity pulling on an object. Because weight measures force, the SI unit for weight is the newton (N). On Earth, a mass of 1 kilogram has a weight of about 10 newtons because of the pull of Earths gravity On the moon, which has less gravity, the same mass would weigh less. Weight is measured with a scale, like the spring scale in Figure 13.16. The scale measures the force with which gravity pulls an object downward.

## earths gravity

You are already very familiar with Earths gravity. It constantly pulls you toward the center of the planet. It prevents you and everything else on Earth from being flung out into space as the planet spins on its axis. It also pulls objects above the surface, from meteors to skydivers, down to the ground. Gravity between Earth and the moon and between Earth and artificial satellites keeps all these objects circling around Earth. Gravity also keeps Earth moving around the sun.

## gravity and motion

Regardless of what gravity is a force between masses or the result of curves in space and time the effects of gravity on motion are well known. You already know that gravity causes objects to fall down to the ground. Gravity affects the motion of objects in other ways as well.

## acceleration due to gravity

When gravity pulls objects toward the ground, it causes them to accelerate. Acceleration due to gravity equals 9.8 m/s2 . In other words, the velocity at which an object falls toward Earth increases each second by 9.8 m/s. Therefore, after 1 second, an object is falling at a velocity of 9.8 m/s. After 2 seconds, it is falling at a velocity of 19.6 m/s (9.8 m/s 2), and so on. This is illustrated in Figure 13.20. You can compare the acceleration due to gravity on Earth, the moon, and Mars with the interactive animation called "Freefall" at this URL: http://jersey.uoregon.edu/vlab/ . You might think that an object with greater mass would accelerate faster than an object with less mass. After all, its greater mass means that it is pulled by a stronger force of gravity. However, a more massive object accelerates at the same rate as a less massive object. The reason? The more massive object is harder to move because of its greater mass. As a result, it ends up moving at the same acceleration as the less massive object. Consider a bowling ball and a basketball. The bowling ball has greater mass than the basketball. However, if you were to drop both balls at the same time from the same distance above the ground, they would reach the ground together. This is true of all falling objects, unless air resistance affects one object more than another. For example, a falling leaf is slowed down by air resistance more than a falling acorn because of the leafs greater surface area. However, if the leaf and acorn were to fall in the absence of air (that is, in a vacuum), they would reach the ground at the same time.

## factors that influence the strength of gravity

Newtons law also states that the strength of gravity between any two objects depends on two factors: the masses of the objects and the distance between them. Objects with greater mass have a stronger force of gravity. For example, because Earth is so massive, it attracts you and your desk more strongly than you and your desk attract each other. Thats why you and the desk remain in place on the floor rather than moving toward one another. Objects that are closer together have a stronger force of gravity. For example, the moon is closer to Earth than it is to the more massive sun, so the force of gravity is greater between the moon and Earth than between the moon and the sun. Thats why the moon circles around Earth rather than the sun. This is illustrated in Figure You can apply these relationships among mass, distance, and gravity by designing your own roller coaster at this URL: .

## einsteins theory of gravity

Newtons idea of gravity can predict the motion of most but not all objects. In the early 1900s, Albert Einstein came up with a theory of gravity that is better at predicting how all objects move. Einstein showed mathematically that gravity is not really a force in the sense that Newton thought. Instead, gravity is a result of the warping, or curving, of space and time. Imagine a bowling ball pressing down on a trampoline. The surface of the trampoline would curve downward instead of being flat. Einstein theorized that Earth and other very massive bodies affect space and time around them in a similar way. This idea is represented in Figure 13.19. According to Einstein, objects curve toward one another because of the curves in space and time, not because they are pulling on each other with a force of attraction as Newton thought. You can see an animation of Einsteins theory of gravity at this URL: http://einstein. theory of gravity, go to this URL:

## projectile motion

Earths gravity also affects the acceleration of objects that start out moving horizontally, or parallel to the ground. Look at Figure 13.21. A cannon shoots a cannon ball straight ahead, giving the ball horizontal motion. At the same time, gravity pulls the ball down toward the ground. Both forces acting together cause the ball to move in a curved path. This is called projectile motion. Projectile motion also applies to other moving objects, such as arrows shot from a bow. To hit the bulls eye of a target with an arrow, you actually have to aim for a spot above the bulls eye. Thats because by the time the arrow reaches the target, it has started to curve downward toward the ground. Figure 13.22 shows what happens if you aim at the bulls eye instead of above it. You can access interactive animations of projectile motion at these URLs: http://phet.colorado.edu/en/simulation/projectile-motion http://jersey.uoregon.edu/vlab/ (Select the applet entitled Cannon.)

## instructional diagrams

No diagram descriptions associated with this lesson

## questions

The SI unit for weight is the

``````a. gram.

b. kilogram.

-->  c. newton.

d. poun
``````

Newtons law of universal gravitation states that the force of gravity

``````a. affects all objects in the universe.

b. is stronger for objects with more mass.

c. is stronger for objects that are closer together.

-->  d. all of the above
``````

Gravity causes all objects to

``````a. attract one another.

b. have projectile motion.

c. accelerate when they fall toward Earth.

-->  d. two of the above
``````

The only reason that a leaf falls to the ground more slowly than an acorn is that the leaf has

``````a. less mass.

-->  b. more air resistance.

c. a weaker force of gravity.

d. less acceleration due to gravity.
``````

On Earth, a mass of 1 kilogram exerts a downward force due to gravity of about

``````a. 1 N.

b. 5 N.

-->  c. 10 N.

d. 15 N.
``````

Satellites orbit Earth because of gravity.

``````-->  a. true

b. false
``````

An object has a greater mass on Earth than it does on the moon.

``````a. true

-->  b. false
``````

Molecules of gas are attracted toward one another by gravity.

``````-->  a. true

b. false
``````

All of the solar systems in the universe formed because of gravity.

``````-->  a. true

b. false
``````

The mass of an object affects its force of gravity.

``````-->  a. true

b. false
``````

Gravity acts only between objects that are close together or touching.

``````a. true

-->  b. false
``````

Objects that are closer together have a weaker force of gravity.

``````a. true

-->  b. false
``````

All objects have the same acceleration due to gravity.

``````-->  a. true

b. false
``````

Earth has stronger gravity than the moon.

``````-->  a. true

b. false
``````

The curved path of an arrow is called its orbit.

``````a. true

-->  b. false
``````

Weight is measured with a balance.

``````a. true

-->  b. false
``````

People have known about gravity for thousands of years.

``````-->  a. true

b. false
``````

The moon has both forward velocity and acceleration toward Earth.

``````-->  a. true

b. false
``````

Einsteins theory of gravity is better than Newtons law of gravity at predicting how all objects move.

``````-->  a. true

b. false
``````

Einstein defined gravity as a force of attraction between objects with mass.

``````a. true

-->  b. false
``````

SI unit for weight

``````a. gravity

b. Isaac Newton

c. orbit

d. weight

e. projectile motion

f. Albert Einstein

-->  g. newton
``````

motion of an object subject to horizontal force and the force of gravity

``````a. gravity

b. Isaac Newton

c. orbit

d. weight

-->  e. projectile motion

f. Albert Einstein

g. newton
``````

force of attraction between two masses

``````-->  a. gravity

b. Isaac Newton

c. orbit

d. weight

e. projectile motion

f. Albert Einstein

g. newton
``````

scientist who proposed that gravity is due to curves in space and time

``````a. gravity

b. Isaac Newton

c. orbit

d. weight

e. projectile motion

-->  f. Albert Einstein

g. newton
``````

measure of the force of gravity

``````a. gravity

b. Isaac Newton

c. orbit

-->  d. weight

e. projectile motion

f. Albert Einstein

g. newton
``````

path of one object around another, such as the moon around Earth

``````a. gravity

b. Isaac Newton

-->  c. orbit

d. weight

e. projectile motion

f. Albert Einstein

g. newton
``````

scientist who proposed the law of universal gravitation

``````a. gravity

-->  b. Isaac Newton

c. orbit

d. weight

e. projectile motion

f. Albert Einstein

g. newton
``````

Gravity always causes objects to

``````a. repel each other.

b. circle each other.

-->  c. attract each other.

d. two of the above
``````

Unlike friction, gravity

``````a. is a force.

b. acts over a distance.

c. acts between objects that are not touching.

-->  d. two of the above
``````

What does weight measure?

``````a. size

b. mass

-->  c. force

d. volume
``````

Jody has a mass of 50 kilograms. What is his weight on Earth?

``````a. 5N

b. 50 N

-->  c. 500 N

d. 5000 N
``````

There is gravity between you and

``````a. Earth.

b. the moon.

-->  d. all of the above
``````

The moon orbits Earth rather than the sun because

``````a. the suns gravity is weaker than Earths.

b. the moon is smaller than Earth.

c. Earth already orbits the sun.

-->  d. the moon is closer to Earth.
``````

An object with greater mass

``````a. has greater acceleration when it falls.

b. has a weaker force of gravity.

c. is less affected by gravity.

-->  d. has greater weight.
``````

## diagram questions

No diagram questions associated with this lesson