Forces in Motion

Newton’s Laws

Free Fall and Acceleration

• Galileo’s experiment

• http://en.wikipedia.org/wiki/Galileo's_Leaning_Tower_of_Pisa_experiment

Free Fall

• Acceleration towards Earth for all objects is 9.8 m/s/s =g

0 m/s9.8 m/s

19.6 m/s

29.4 m/s

V=gt

Gravity and Mass?

• F = GMm/r2

• F = ma

• Ma = GMm/r2

• A = GM/r2

• F = mg

Questions

1) What is the velocity of a penny that is dropped from a building after 2 secs?

2) If the penny strikes the ground in 4.0 secs, what is its final velocity?

3) Can you determine the height of the building?

What about Air Resistance• Air creates friction – it is a

force that is pushing up against a free fall object. It increases as the object falls faster and faster.

• Net force = acceleration due to gravity – force created by air

• Terminal velocity – when the upward force of air resistance matches the downward force of gravity.

Drop a Penny from the Empire State Building

• Could it kill someone?• The building is 1000 feet

(305 meters)• Penny weighs about 1 gram• Penny reaches terminal

velocity at about 50 ft (15 m)• Free fall really only occurs in

a vacuum

Orbiting Objects

• Orbiting objects are in free fall• Orbiting is caused by the

combination of 2 motions (moving forward and free falling)

• The motions cancel each other out)

• Centripetal force – unbalanced force that causes objects to move in a circular path

• “toward the center”

Projectile Motion

• Horizontal Motion

• Vertical Motion – 9.8 m/s

Newton’s First Law:

• An object at rest remains at rest and an object in motion remains in motion at constant speed and in a straight line unless acted on by an unbalanced force

• Friction is the ultimate unbalanced force

Inertia

• Inertia – the tendency of all objects to resist any change in motion.

• How does inertia effect you at the amusement park? (roller coaster, scrambler, free fall, spinner, standing on the tram))

• Mass is a measure of inertia (small mass = less inertia)

Newton’s Second Law

• The acceleration of an object depends on the mass of the object and the amount of force applied

1) Acceleration depends on mass – full shopping cart vs an empty one.

- for the same force an object’s acceleration decreases as its mass increases & its acceleration increases as its mass decreases

•

1) Acceleration depends on force

• Acceleration increases as the force on it increases and decreases as the force on it decreases

a=F/m or F=ma

Why do all objects fall to the Earth with the same

Acceleration?

• Weight is offset by inertia

Apple

m= 0.102 kg

F=1N

1N = 1 kgm/s/s

a = 1kgm/s/s = 9.8m/s/s

0.102 kg

Watermelon

m= 1.02 kg

F=10N

10N = 10 kgm/s/s

a = 10kgm/s/s = 9.8m/s/s

1.02 kg

Newton’s Third Law of Motion

• Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first

• “all forces act in pairs” – the action force and the reaction force

The action force is the swimmer’s hand pushing on the water

The reaction force is the water pushing on the swimmer’s hands

What about gravity• Action – the Earth pulls the ball toward it• Reaction – the ball pulls the Earth

towards it.

• Why don’t you notice the ball’s attraction to the Earth

• F = ma• F= mg

• The rabbit’s legs exert a force on Earth. Earth exerts an equal force on the rabbit’s legs, causing the rabbit to accelerate upward. Does the Earth move when the rabbit jumps upwards?????

What would happen if everyone on the planet went to Mexico and

started jumping• Actually if that were to

happen, at the precise moment when the people jumped, the loss of all that weight from the earths surface would cause the earth to move out of orbit, but the moment the people's feet landed back onto the ground, that force would put the earth back into normal orbit.

Momentum• A property of a moving object that depends

on its mass and velocity

• Conservation of Momentum – anytime 2 or more objects interact they may exchange momentum but the total momentum stays the same