Everything that has mass is affected by gravity.

Everything that has mass exerts a gravitational attractive force on everything else that has mass.

The attractive force depends on:

the masses of the objects

the distance between them.

As the mass increases, the force increases.

As the distance increases, the force decreases.

The sun is much more massive than the earth, so why doesn’t its gravitational attraction pull us off of the earth?

The sun is much more massive than the earth, so why doesn’t its gravitational attraction pull us off of the earth?We are so much closer to the earth; the sun is very far away. The earth’s gravity has a greater effect on us.

The moon is much closer than the sun, so why doesn’t its gravitational attraction pull us off of the earth?

The moon is much closer than the sun, so why doesn’t its gravitational attraction pull us off of the earth?

We are closer to the earth than the moon, and the earth has more mass and more gravity.

When people have been on the moon, why hasn’t the gravitational attraction of the earth pulled them off into space?

When people have been on the moon, why hasn’t the gravitational attraction of the earth pulled them off into space?

Even though the earth is larger and has 6 times more gravity than the moon, it is too far away to affect people who are on the moon.

Your mass is much greater than all of the pencils and pens in this room, and you’re relatively close to some of them. Why isn’t your gravitational attraction pulling them all towards you?

Gravity is what keeps the atmosphere around the earth.

It keeps the moon orbiting the earth,

Apollo 8

the earth orbiting the sun,

Photo taken from the space shuttle

and gives the universe its structure.

Galaxy M100

Without air resistance, all falling objects near earth’s surface accelerate at the same rate, 9.8 m/s 2. The direction is towards the center of the earth.

If a bowling ball and a marble were dropped from a bridge at the same time, which would hit the water first?

They would hit at (about) the same time.

(The bowling ball might hit just a second later because of the increased air resistance on its larger surface area.)

The acceleration due to gravity is the same for all falling objects near the earth’s surface.

The gravitational forces would be different because of the different masses (F = ma), but they still fall at the same rate.

(Even though the force of gravity exerted on the bowling ball is more than on the marble because of its greater mass, it also has more inertia because of its greater mass, so it takes more force to change the motion of the bowling ball than the marble. The interaction of the force of gravity and the inertia balance out and they fall at the same rate.)

A falling object is accelerating toward the surface, but air resistance is working in the opposite direction.

As the downward speed increases, the upward force of air resistance also increases. Eventually they are equal and opposite…and then what happens?

The object stops moving?

The object stops accelerating?

The object begins to rise back up in the air?

If the force of gravity and air resistance are equal and opposite, then the net force = ________and the forces are __________.

Newton’s First Law says that the object will keep moving in the same way unless acted on by an outside unbalanced force.

So our falling object will_____________.

Recall that mass is the amount of matter in an object.

Weight is the measure of the force of gravity exerted on an object.

Weight (W) = mass (kg) x 9.8 m/s2.

Since weight is a force, it is reported in Newtons (N).

When astronauts went to the moon, their weight changed because the moon’s gravity is less, but their mass stayed the same. Buzz Aldrin, July 20, 1969

What does it mean to be “weightless”?

NASA astronauts training aboard the Vomit Comet.

To be “weightless”, an object would have to be so far away from large masses that gravity no longer affects them.

Since the moon is held by earth’s gravity, clearly that is not far enough away…so if they are not weightless, why are they floating?

Everything in orbit -

satellites, the International Space Station, the Space Shuttle, the people on the ISS and shuttles, the moon –

is falling at the same rate.

The acceleration is towards the center of the earth, but the velocity is “forward”; gravity keeps the moon from continuing in a straight line path away from us.

Since the astronauts and shuttle are all falling together at the same rate, the sensation is “weightless”.

If you were standing on a scale when you were pushed out of an airplane, as you fell together, the scale would register no weight.

Projectile motion is the result of two interacting motions: the horizontal and vertical.

Newton’s 1st Law says that an object thrown forward will continue to move forward unless an outside unbalanced force acts on it.

Gravity begins pulling the ball down as soon as its released.

These two motions don’t cancel each other out, they both continue. The ball moves forward and it moves down – at the same time.

The result is projectile motion; the object moves forward and down in an arc.

An object traveling forward still falls at the same rate.

They will hit the ground at the same time.

Centripetal Force is the net force experienced by an object traveling in a circular path.

Centri petal = center

Centripetal Force towards the center

Centripetal Force towards the center

The centripetal force is the friction between the tires and the road.