# Gravity - PowerPoint PPT Presentation

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Gravity. Force at Impact. A falling ball on a paper surface can break through. The ball is exerting a force A falling ball on a hard surface rebounds. The surface is exerting a force There are forces both ways at contact. force of the ball on the surface. force of the surface on the ball.

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Gravity

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## Gravity

### Force at Impact

• A falling ball on a paper surface can break through.

• The ball is exerting a force

• A falling ball on a hard surface rebounds.

• The surface is exerting a force

• There are forces both ways at contact.

force of the ball on the surface

force of the surface on the ball

### Force is Not Motion

• The ball breaking paper doesn’t stop.

• The force needed to break through is small.

• But that small force is also exerted on the ball.

• On a rebound the ground doesn’t move.

• The force needed to break through is large

• A force acted in both ways

force of the ball on the surface

force of the surface on the ball

### Third Law: Law of Reaction

• For every action there is an equal and opposite reaction.

• Forces between two objects act in pairs.

• F12 = -F21 (Newton’s third law)

reaction force acts on the rocket

rocket

hot gas is forced out

rocket and gas are both affected

### Equal and Opposite

• Newton’s law of reaction also applies to the force of gravity.

• The Earth pulls the Moon

• The Moon pulls the Earth

• Newton used this to describe a Law of Gravity.

FEM

Moon

FME

Earth

Newton realized that all objects obey that Law.

Other planets

Apples

People

The gravitational force is universal.

The gravitational constant is G = 6.67 x 10-11 Nm2/kg2.

What is the gravitational force between two students sitting in adjacent seats?

Assume the students have a mass of 70 kg each.

Assume that they are separated by 1m.

F = (6.67 x 10-11 Nm2/kg2) x (70 kg)2 / (1 m)2

F = 3.3 x 10-7 N.

### Universal Gravity

The force of gravity on a mass is its weight.

The force of gravity compared to the mass is the field strength.

Consider the force on an unknown mass m.

Use Earth’s mass M and Earth’s radius R

Calculate the field strength.

g = F/m = GM/R2

M = 5.97 x 1024 kg

R = 6.37 x 106 m

g = 9.81 N/kg = 9.81 m/s2

This field strength g only applies at Earth’s surface.

Approximately, g is equal to 9.8 N/kg or 9.8 m/s2.

### Variations

• Gravity varies over the surface of the Earth.

• The height of the surface varies – so the radius does, too

• The material under the surface is not uniform

• The earth isn’t exactly round

• The tides affect the earth as well as the oceans

• One unit of gravitational acceleration used on the Earth is the Galileo.

• 1 Gal = 1 cm/s2 = 0.01 m/s2 = 0.01 N/kg.

• g = 981 Gal

### Effect of Height

• High areas have a greater distance from the center of the Earth.

• An increase of 1 km should decrease g by 300 mGal.

• Type of rock affects g.

USGS

### Latitude Changes

• As the Earth spins the equator slightly bulges.

• The radius is about 22 km bigger compared to the pole.

• Expect a few Gal difference from equator to pole.

• Equator: 9.780 m/s2

• North Pole: 9.832 m/s2

### Earth’s Rotation

• The earth is made of layers of different types of rock.

• These rocks can move due to daily tides.

• Much less than ocean tides

• Period is 12 hours like the ocean

• The force of gravity from the Moon is countering some of the force of the Earth.