Chapter 6 Forces in Motion

Gravity and Motion Q: If you were to drop a marble and a baseball at the same time from the same height which would hit the ground first?

A: They will hit the ground at the SAME time! http://www.youtube.com/watch?v=z_sJ15feNGw

ALL objects fall with the SAME acceleration. Mass DOES NOT affect the rate at which an object falls.

Acceleration at a Constant Rate ALL objects accelerate at 9.8 meters per second per second or 9.8 m/s/s

Examples: What is a penny’s velocity after if has fallen for 2 seconds? The penny hits the ground in 4 seconds. What is its final velocity?

Terminal Velocity A falling object will continue to fall faster and faster….until the upward push of air resistance becomes equal to the downward pull of gravity. The object then falls at a constant velocity known as terminal velocity.

Free Fall Free fall occurs when there is NO air resistance. An object is in free fall only if the gravity is pulling it down and NO other forces are acting on it. Free fall can only occur where there is NO air!

On Earth… Air resistance slows down acceleration. Air resistance: is the friction caused by air that occurs between the surface of the falling object and the air that surrounds it.

Projectile Motion Objects orbiting the Earth are in free fall. Two motions combine to cause orbiting: Forwards/Horizontal Down/Vertical

Projectile Motion Learn360: Projectile Motion

Quiz 6.1 A feather and a rock dropped at the same time from the same height would land at the same time when dropped by: Galileo in Italy Newton in England An astronaut on the moon An astronaut on Earth Falling objects stop accelerating at _________ Free fall Terminal velocity Momentum Inertia

Quiz #6.1 Continued An object in motion tends to stay in motion because it has _________. Mass Weight Inertia None of the above Extra Credit: A boy standing on a high cliff dives into the ocean below and strikes the water 3 seconds later. What is the boy’s velocity when he hits the water? Don’t forget your units?

Newton’s 3 Law’s of Motion Inertia Acceleration Action/Reaction

Newton’s First Law Inertia: An object at rest will stay at rest and an object in motion will stay in motion unless acted on by an outside force.

Newton’s Second Law Acceleration of an object depends on the mass of the object and the amount of force applied.

Example 1:

Example 2 A 50kg skater pushes off from a wall with a force of 200N. What is the skater’s acceleration? F=M x A

Answer A 50kg skater pushes off from a wall with a force of 200N. What is the skater’s acceleration? F = M x A Or 200N = 50Kg x A 200N/50Kg = A A = 4 m/s/s

Newton’s Third Law Action/Reaction: Whenever one object exerts a force on a second object, the second object exerts and equal and opposite force on the first.

Examples

Momentum = Mass x Velocity Momentum: is a property of a moving object that depends on the object’s mass and velocity. Momentum = Mass x Velocity

Examples 1. If a semi-truck and a slug-bug are both traveling down the highway at 50MPH North, then the ___________ has more momentum because it has a greater _______. 2. If the semi-truck and slug-bug are both parked on a hill and the slug-bug begins to roll down the hill, then the __________ has more momentum because ______________________________.

Examples cont. Which has more momentum, a 5kg toy truck moving at 1m/s or a 10 kg toy car moving 2m/s? Momentum = mass x velocity

Quiz 6.2 Newton's first law of motion applies : To moving objects To objects that are not moving To objects that are accelerating Both a and b A golf ball and a bowling ball are moving with the same velocity. Which has more momentum? ____________ is the path that a thrown object follows. (free fall, projectile motion)

Momentum = Mass x Velocity Quiz 6.2 Cont. Name three action reaction force pairs involved in doing your homework. Name which object is exerting the force and which is receiving the force. Which has more momentum, a mouse running 1m/s north or an elephant walking 3m/s east? Explain your answer. Momentum = Mass x Velocity