Chapter 3: Gravity, Friction, and Pressure

12.1 Gravity is a force exerted by masses12.2 Friction is a force that opposes motion12.3 Pressure depends on force and area12.4 Fluids can exert a force on objects

12.1 Gravity is a force exerted by masses

• Masses attract each other– Gravity is the force that objects exert on each

other because of their masses– Gravity accelerates all masses equally

• Gravity is a universal force: acts between any two masses anywhere in the universe– Between the earth and moon, sun and moon– Between dust and gas particles in space

helped form the solar system

The force of gravity• Recall: the net force on you

determines how your motion changes– Force between you and the

desk is extremely small compared with other forces constantly acting on you (friction, muscles, Earth’s gravity, other objects)

• Strength of the gravitational force between two objects depends on mass and distance

• Greater mass results in greater force

• Smaller distance results in greater force

Gravity on Earth

• Gravity acts on both masses equally, even if the masses are very different

• Every action force has an equal and opposite reaction force– Earth’s gravity exerts a force on a coin, the coin

exerts an equal upward force on Earth• Small coin mass means it can easily be accelerated• Large Earth mass means it is much more difficult to

accelerate

• g = acceleration due to gravity = 9.8 m/s2 at Earth’s surface (moon: 1.6 m/s2)– Newton’s 2nd law: F=ma F=mg

Gravity• In a vacuum:

– Quarter vs penny falling• Quarter has more mass so gravity exerts more force on it• But it also has more inertia, so the greater force does NOT

produce a larger acceleration• Therefore: objects with different masses fall with the same

acceleration

         

             

At a given location on the earth and in the absence of air resistance, all objects fall with the same uniform acceleration. Thus, two objects of different sizes and weights, dropped from the same height, will hit the ground at the same time.

http://www.classzone.com/books/ml_science_share/vis_sim/mfm05_pg79_vacuum/mfm05_pg79_vacuum.html

Gravity

• In a vacuum:– Coin dropped falls at the same rate as one thrown

forward• Horizontal velocity does not affect acceleration due to gravity• Gravity is directed downward so it changes only the

downward velocity of the coin, not its forward velocity

An object is controlled by two independent motions. So an object projected horizontally will reach the ground in the same time as an object dropped vertically. No matter how large the horizontal velocity is, the downward pull of gravity is always the same.

                 

                

Gravity

At the instant a cannon fires a cannonball horizontally over a level range, another cannonball held at the side of the cannon is released and drops to the ground.

Which strikes the ground first?

Distance Fallen

Distance fallen in one second is 5 meters.

This distance fallen is the same whether falling straight down or in projectile motion.

0.5 s

1.0 s

1.5 s

5 meters

• http://scienceblogs.com/dotphysics/2009/10/mythbusters-bringing-on-the-physics-bullet-drop.php

• http://www.youtube.com/watch?v=D_JbgP8mpsE

Weight and Mass

• Mass is a measure of how much matter an object contains– Same no matter where the object is located

• Weight is the force of gravity on that object– Depends on the force of gravity acting upon it

• On Earth: Mass = 50 kg, Weight = 490 N

• On Moon: Mass = 50 kg, Weight = 82 N

Gravity keeps objects in orbit

• Orbit: elliptical path one body follows around another body due to the influence of gravity

• Centripetal force keeps one object in orbit around another due to the gravitational pull– Between Earth and Moon, Earth and Sun– “falling around Earth”

Projectile Motion & Curvature

For initial speeds that are faster and faster, the range of the projectile is farther and farther.

For very large speeds, the curvature of Earth starts to be noticeable.

Earth’s Curvature

Curvature of the Earth is about 5 meters over a distance of 8000 meters (which is about 5 yards over 5 miles).

Missing the Ground

8000 m

NOT toScale

5 mCurvature

Suppose you throw a ball at a speed of 8000 m/s (about 18,000 mph).After one second, ball travels 8000 meters and falls 5 meters.In that distance, Earth curves by same amount (5 meters).

If nothing stops the ball, what happens?

Orbits and Centripetal Force

Gravity provides the centripetal force required for a satellite to move in a circle.

Getting into Orbit-Rocket needs to lift abovethe atmosphere and thenfire thrusters to acquire therequired orbital speed ofabout 8000 meters (8 km) per

second.-path of the falling object matches the curve of Earth’s

surface-greater than 11,000 m/s andthe spacecraft will escape thepull of Earth’s gravity!

Elliptical Orbits

For speeds higher than 8 km/s, the orbit is elliptical instead of circular.

Escape Speed

If speed exceeds 11.2 km/s then object escapes Earth because gravity weakens (as object gets further away) and never slows the object enough to return it back towards Earth.

Circular

Elliptical

Hyperbolic

People in Orbit

• Elevator: on a scale during downward acceleration, you would appear to weigh less – the scale is also moving downward, and you are pushing on it less– If in free fall (fall entirely due to gravity), you would not

press against the scale at all

• Spacecraft in orbit is in free fall (around Earth)– Astronauts weight does not press against the floor of

the spacecraft, so objects behave as if there were no gravity

In-class activity p.394Time (s) Velocity

(m/s)

0 0

2 18

4 29

6 33

8 35

10 36

12 36

14 36

16 36

18 36

1) For both variables, decide the scale that each box on your graph will represent and what range you will show for each variable.

2) Determine the dependent and independent variables.

3) Plot the independent variable along the horizontal axis, and the dependent along the vertical axis.

Challenge: alter the scale you chose to use in #1. Graph again. How do different scales give different impressions of the data?

• http://www.edinformatics.com/math_science/solar_system/gravity_mass_weight.htm