Forces

Chapter 3

Newton’s 2nd law

• 2nd – an object accelerates in the direction of the net force acting upon it

• A= net force/Mass

• a= Fnet/m

• A force on an object is equal to the change in momentum of the object

• Force = mass * accel. • F=m*a

Practice

• Your push your friends trike with a force of 150N, his mass is 35kg the trikes is 15kg, what is the acceleration?

• 150N/50kg=• 3m/s/s

Friction

• Friction is the force that opposes motion between two opposing surfaces

• Friction always acts against motion

• Causes of friction – unsmooth surface, deformations, molecular attraction

2 types of friction

• Static friction – force of a resting object

• Sliding or Kinetic friction - force on a moving object

• Rolling Friction-Ever notice how it takes

more force to get an object moving

                           

   

Types of Friction

• Static friction – force of a resting object

• Sliding or Kinetic friction - force on a moving object

• Rolling Friction- Friction or a rolling object –similar to kinetic friction

Stuff falls

• Apollo 15 Hammer/Feather gravity demonstration – Google

• http://nssdc.gsfc.nasa.gov/planetary/image/featherdrop_sound.mov

• All objects near the surface of the earth accelerate at a rate of 9.8m/s/s

• Objects of different masses will fall at the same rate regardless of their mass

Gravitational acceleration

• Velocity of a freefalling object can be found with the formula v= g*t

• Where gravities acceleration is 9.8m/s/s

• How can you find falling distance

• D= ½ g *t2

Air resistance

• When drag is equal to weight, there is no net external force on the object.

• The object then falls at a constant velocity as described by Newton's first law of motion.

• The constant velocity is called the terminal velocity.

• This is due to the air resistance – Friction force caused when objects fall through air

• Dependant upon the speed, shape, mass, and size of the object, and the density of the air.

Where is he falling fastest?Where is accelerating the most?Where is he not accelerating?

More massive objects fall faster than less massive objects because they are acted upon by a larger force of gravity; for this reason, they accelerate to higher

speeds until the air resistance force equals the gravity force. 

Centripetal

• Centripetal acceleration: acceleration toward the center of a curved path

• Centripetal means center seeking- it is merely a net force

Centripetal ex.

As a car makes a turn, the force of friction acting upon the turned wheels of the car provide the centripetal force required for circular motion.

As a bucket of water is tied to a string and spun in a circle, the force of tension acting upon the bucket provides the centripetal force required for circular motion

As the moon orbits the Earth, the force of gravity acting upon the moon provides the centripetal force required for circular motion

Weight

• Weight – The measure of the force of gravity on a body.

• On earth your weight in Newtons (N) is equal to your mass in kg times gravity’s acceleration (9.8m/s2)

• W= m * g

Weightlessness

• Weightlessness is sensation experienced when there are no external objects touching one's body and exerting a push or pull

• Astronauts on the orbiting space shuttle are weightless because..

• a. there is no gravity in space and they do not weigh anything.

• b. space is a vacuum and there is no gravity in a vacuum.

• c.  space is a vacuum and there is no air resistance in a vacuum.

• d. the astronauts are far from earth's surface at a location where gravitation has a minimal effect.

Freefallin’

• Orbit is a state of constant freefall – the Earth’s surface is falling away at the same rate of orbit

• 400 km above the earth's surface, then the value of g at that location will have been reduced from 9.8 m/s/s (at earth's surface) to approximately 8.7 m/s/s

Projectile motion

• Projectile motion: Anything that has horizontal motion is a projectile

• Horizontal and vertical motion act independently of one another-

• Projectiles follow a path called a trajectory

So a bullet dropped and a bullet fired from the same height will hit the ground at the same time.

Newton’s 3rd law

• For every action (force) in nature there is an equal and opposite reaction.

• Force pairs: Action and reaction forces do not act on the same object.

• What would happen if they did?

Many reaction forces are not noticed due to other forces or smaller accelerations.

Introduction to Rocket Performance - Level 3 - Jump Animation

Mometum

• Momentum: property of a moving object based on its velocity and mass.

• p = mv• kg x m/s – label

Momentum is conserved

• Law of Conservation of Momentum: Momentum may be exchanged but the total amount of momentum remains the same.

In a game of pool where is the momentum lost

Practice

• A scooter and a big red truck, both moving at 20m/s. Which has more momentum.

• Mass of red truck 3000kg

• Mass of scooter 8kg• Momentum of truck:• of Scooter:

Force and momentum

• Objects with momentum can apply force to other objects when they strike each other

• F=mvf – mvi/t

• This is a formula combining the 2nd and 3rd laws

• A baseball strikes your glove with a velocity of 49m/s, its mass is .145kg. The ball comes to a stop in .01s. What force is applied?

Artificial Satellite vs. Natural Satellite

• Sputnik: 1957 Russian (October 4th) “Fellow Traveler or Satellite”

• What do we use satellites for?

• Weather TV Communications Science and Research

Satellites

• How many satellites currently orbit the earth?

• Geosynchronous Satellites: Orbital speed matches the earth’s rotation

• Where? 35,790 km above the equator.

• International Space Station: 390km