Unit 2

Pages 248-249, 272-277, and 280-287

Gravity• Sir Isaac Newton

• Why do objects fall?

• A force pulls objects downward, towards the center of the Earth

• All objects are attracted to each other to some degree due to gravity

Mass• Amount of mass effects force of gravity

• Larger the mass, greater the gravitational force it exerts

• An apple’s center is attracted to Earth’s center

• Earth has more mass, so the apple moves and not Earth

Distance• The force of gravity decreases as

distance increases

• Closer together the objects are, more force they exert on each other

• Farther apart they are, the less force they exert

Motion• Change in an object’s position

• Speed = distance divided by time

• Average speed = total distance divided by time

• Velocity = involves speed and direction

• Acceleration = change in velocity divided by time

Speed • My dog runs 50 meters in 5 seconds.

• What is my dog’s speed?

• What are the units?

Average Speed• It takes you 3 hours to travel 50 miles to

Omaha.

• What was your average speed?

• What are your units in this problem?

• What do we usually want our units of speed to be?

Average vs. Instantaneous Speed

• Average speed is how fast you traveled for the entire trip.

• Instantaneous speed is how fast you are going at the moment that the police officer uses his radar gun on your vehicle and gives you a speeding ticket.

Acceleration• My dog began his walk at 10 meters per

second. 2 hours later, he was walking at 2 meters per second.

• What was his acceleration?

• What are the units?

Momentum• Mass x velocity

• Mass will be in grams, kg, mg, etc.

• Velocity will be in meters per second

• Conservation of Momentum = total momentum of system always stays the same

Momentum• The 2000 kg truck is driving at 30

meters per second.

• What is the truck’s momentum?

• What are the units?

Newton’s 1st Law of Motion• An object will remain at rest or move at a

constant speed in a straight line unless acted upon by a force

• Also known as the Law of Inertia

• Inertia is the resistance to change in motion.• It is hard for en elephant to stop quickly

because it has a lot of inertia carrying it forward.

Newton’s 2nd Law of Motion• The force acting on an object is equal to

mass of the object times the acceleration

• Force = mass x acceleration

• F= m x a

• Mass must be in kilograms

• Acceleration must be in m/s2

Calculating Force• A 10 kg object accelerates at a rate of

45 m/s2.

• Calculate its force.

• What are the units?

• A new unit you will use is Newtons!

• 1 N = 1 kg x m/s2

Newton’s 3rd Law of Motion• For every action force, there is an equal and

opposite reaction• When a rocket is launched into space, its jets

must first ignite its fuel pushing down against the ground.

• This is the action.• The force of the explosion causes the rocket

to fly upward.• This is the reaction.