Momentumhttps://www.youtube.com/watch?v=y2Gb4NIv0Xg

EQ: How can an object’s motion be described?

Momentum

• Momentum is defined as the quantity of motion of a body.• According to Newton, momentum causes an object in motion to

remain in motion unless it’s acted on by some other force. • So the momentum of a moving object is related to its mass and its

velocity. • For example, if you were driving a car, and wanted to stop quickly,

you couldn’t just take your foot off the accelerator.• If you did this, the car’s momentum, as defined by the product of

its mass and velocity, would cause it to keep moving forward. • If you wanted to stop the car, you’d have to hit the brake, which

uses the force of friction to counteract the car’s momentum.

Momentum

• Because momentum is related to mass and velocity, it takes a lot of force to stop the momentum of a heavy object that’s traveling at high speed.

• Imagine how much force it’d take to stop an 18-wheeler that was speeding along at 70 miles per hour.

• It’d certainly be a lot more force than it would take to stop a VW Bug traveling at the same speed.

• It’d also be a lot more force than it would take to stop the same truck traveling at 5 mph instead of 70.

Momentum-- Defined• All moving objects have what

Newton called, “a quantity of motion”

• This is called momentum: a characteristic of a moving object that is related to the mass and the velocity of the object.

• Mass: the amount of matter in an object

• Velocity: the speed in a given direction of an object

• One way to think of momentum is that momentum measures how hard it will be to stop the object.

Calculating Momentum

• Mass is measured in kg/lbs

• Velocity is measured in meters per second

• The unit is kg*m/s• Which hammer has

more momentum?• Momentum = massx velocity• Answer:• Hammer 1: 4.5 kg*m/s• Hammer 2: 3.6 kg*m/s

Hammer 1 Hammer 2

Mass 3.0 kg 4.0 kg

Velocity 1.5 m/s 0.9 m/s

Momentum

Momentum --- Other Facts

• An object with a large mass or a fast velocity has a large amount of momentum.

• The more momentum an object has, the harder it is to stop.

• A speeding truck has a large amount of momentum.

• A mouse does not

Law of Conservation of Momentum

• Moving objects sometimes bump one another.

• When this happens, some momentum can move from one object to another.

• However, the total amount of momentum stays the same.

• This is called the Law of Conservation of Momentum

Law of Conservation of Momentum

• The law of Conservation states that in the absences of outside forces, the total amount of momentum of objects that interact does not change.

Law of Conservation of Momentum

• The amount of momentum is the same before and after objects interact.

• The total momentum of any group of objects remains the same, or is conserved, unless outside forces act on the object.

• In the absence of friction, momentum is conserved when 2 objects collide.

Acceleration

EQ: How can you describe an object’s motion?

Acceleration

• The pitcher throws.• The ball speeds

toward the batter. • Off the bat it goes. • It’s going, going,

gone!• A home run!

Acceleration

• Before landing, the ball went through several changes in motion.

• It sped up in the pitcher’s hand, and lost speed as it traveled toward the batter.

• The ball stopped when it hit the bat, changed direction, sped up again, and eventually slowed down.

Acceleration

• Most examples of motion involve similar changes. In fact, rarely does any object’s motion stay the same for very long.

Acceleration

• Suppose you are a passenger in a car stopped at a red light. When the light changes to green, the driver steps on the accelerator.

• As a result, the car speeds up, or accelerates.

• In everyday language, acceleration means “the process of speeding up.”

Acceleration, a definition• Acceleration has a more precise

definition in science.• Scientists define acceleration as

the rate at which velocity changes. • Recall that velocity describes both

the speed and direction of an object.

• A change in velocity can involve a change in either speed or direction—or both.

• In science, acceleration refers to increasing speed, decreasing speed, or changing direction.

Increasing Speed• Whenever an object’s speed

increases, the object accelerates.• A softball accelerates when the

pitcher throws it, and again when a bat hits it.

• It is sometimes referred to as positive acceleration

• A car that begins to move from a stopped position or speeds up to pass another car is accelerating.

• People can accelerate too.• For example, you accelerate

when you coast down a hill on your bike.

Decreasing Speed

• Just as objects can speed up, they can also slow down.

• This change in speed is sometimes called deceleration, or negative acceleration.

• For example, a softball decelerates when it lands in a fielder’s mitt.

• A car decelerates when it stops at a red light.

• A water skier decelerates when the boat stops pulling.

Changing Direction

• Even an object that is traveling at a constant speed can be accelerating.

• Recall that acceleration can be a change in direction as well as a change in speed.

• Therefore, a car accelerates as it follows a gentle curve in the road or changes lanes.

• Runners accelerate as they round the curve in a track.

• A softball accelerates when it changes direction as it is hit.

Calculating Acceleration

• Acceleration describes the rate at which velocity changes. If an object is not changing direction, you can describe its acceleration as the rate at which its speed changes.

• To determine the acceleration of an object moving in a straight line, you must calculate the change in speed per unit of time.

• This is summarized by the following formula:• Acceleration = Final Speed – Initial Speed/Time

Calculating Acceleration

• If speed is measured in meters per second (m/s) and time is measured in seconds, the SI unit of acceleration is meters per second per second, or m/s2.

• Suppose speed is measured in kilometers per hour and time is measured in hours. Then the unit for acceleration is kilometers per hour per hour, or km/h2.

Calculating Acceleration

• To understand acceleration, imagine a small airplane moving down a runway.

• The figure to the left shows the airplane’s motion after each of the first five seconds of its acceleration.

• To calculate the average acceleration of the airplane, you must first subtract the initial speed of 0 m/s from the final speed of 40 m/s.

• Then divide the change in speed by the time, 5 seconds.

•

Calculating Acceleration

• The airplane accelerates at a rate of 8 m/s2.

• This means that the airplane’s speed increases by 8 m/s every second.

• Notice in the figures that, after each second of travel, the airplane’s speed is 8 m/s greater than it was the previous second

Calculating Acceleration

• As a rollercoaster starts down a slope, it’s speed is 4m/s.

• 3 seconds later, at the bottom, it’s speed is 22 m/s

• What is its average acceleration

• Acceleration = Final speed – initial speed/time

• Initial speed=• 4 m/s• Final speed =• 22 m/s• Time =• 3 seconds• Acceleration = 22m/s-4m/s / 3s• Acceleration = 18 m/s / 3 s• Acceleration = 6 m/s2• The rollercoaster’s average

acceleration is 6 m/s 2

Calculating Acceleration

• A falling raindrop accelerates from 10 m/s to 30 m/s in 2 seconds. What is the raindrop’s average acceleration?

• Answer: 10 m/s²

• A certain car can accelerate from rest to 27 m/s in 9 seconds. Find the car’s average acceleration.

• Answer: 3 m/s²

Graphing Acceleration

• Suppose you ride your bicycle down a long, steep hill.

• At the top of the hill your speed is 0 m/s.

• As you start down the hill, your speed increases.

• Each second, you move at a greater speed and travel a greater distance than the second before.

• During the five seconds it takes you to reach the bottom of the hill, you are an accelerating object.

Graphing Acceleration

• To the left, there is a speed-versus-time graph for your bicycle ride down the hill.

• What can you learn about your motion by analyzing this graph?

• First, since the line slants upward, the graph shows you that your speed was increasing.

Graphing Acceleration

• Next, since the line is straight, you can tell that your acceleration was constant.

• A slanted, straight line on a speed-versus-time graph means that the object is accelerating at a constant rate.

Graphing Acceleration

• You can find your acceleration by calculating the slope of the line.

• To calculate the slope, choose any two points on the line. Then, divide the rise by the run.