Motion and Momentum

Chapter 7

What is Motion? Section 1• an object is in motion if it changes position

relative to a reference point. • An object moves when its distance from

another object is changing. • Whether an object is moving or not depends

on your point of view (reference point).

Reference point

• a place or object used for comparison to determine if something is in motion.

• Suppose you are standing on a sidewalk and your friend rides past you on her skateboard. – Which one of you is moving relative to the Earth?– Are you moving relative to your friend?

Distance and Displacement• Distance – the total length of the route

you will travel.– Odometers measure distance traveled.– It is 4 miles to drive from my house to

school.

• Displacement – the distance between the starting and ending points and the direction in which you traveled.– “As the crow flies.”– It is 1.3 miles from my house to school as

the crow flies. A straight line distance.

Speed

• The distance an object

travels in one unit of time

is it’s speed. • Speed = Distance

Time

Units of Speed = (m/s) or (km/hr)• Average speed = total distance

total time

The 3-Step Process

• FOR SHOWING MATH WORK IN SCIENCE

• Step 1: Formula

• Step 2: Substitute (with units)

• Step 3: Solve (with units!!)

• Ex: – F=ma– F=20kg x 0.5m/s/s– F=10kgxm/s/s = 10N

Sample Problem: • At what speed did a plane fly if it traveled

1760 meters in 8 seconds?

• Step 1: Formula

–v = d/t

• Step 2: Substitute

–v = 1760m / 8s

• Step 3: Solve

–v = 220 m/s

• At what speed did a plane fly if it traveled 1760 meters in 8 seconds?

Speed Practice Problems:

• A car travels 240 kilometers in 3 hours. What is the speed of the car during that time?

• The speed of a cruise ship is 50 km/hr. How far will the ship travel in 14 hours?

• A cyclist travels 32 km during the first 2 hours of riding, and 13 km during the next hour. What is the average speed of the cyclist?

Average vs Instantaneous Speed• Average speed

– Speed can change during the time traveled.

• Speeding up and slowing down• Stopping at traffic lights• Getting behind slower drivers• Stopping to use the rest-room or to get a drink

at the convenient store• Passing the semi-truck in front of you• Traffic jam

– Average speed is TOTAL distance traveled during the TOTAL time traveled!!

• Instantaneous Speed– The speed of an object at one instant of

time• Right NOW!• Shown on the speedometer!

Velocity • speed in a given

direction.• Velocity = Distance

Time• Note that the velocity

equation is the same as the speed equation

• A change in v happens when:– 1 it’s speed changes– 2 it’s direction

changes– 3 they both change

Graphing motion

• Always use a distance vs. time graph (distance on the vertical or y-axis and time on the horizontal or x-axis).

• Slope – tells you the rate of change (speed).• slope = rise

run

Acceleration Section 2 • the rate of change in velocity.

– Refers to: • increasing speed (speeding up) – positive accel

• decreasing speed (slowing down) – negative accel• or changing direction (turn)

• (a)=Final Velocity (vf)-Initial Velocity (vi)Time (t)

• a = vf – vi

t• Units for Acceleration: km/hr2, km/hr/s or

m/s2

Acceleration Practice Problems:

• A roller coaster is moving at 25m/sec at the bottom of a hill. Three seconds later it reaches the top of the next hill, moving at 10m/sec. What is the deceleration of the roller coaster?

• A car is traveling at 60km/hr. It accelerates to 85km/hr in 5 seconds. What is the acceleration of the car?

Graphing Acceleration• Velocity vs. Time

– Velocity on y-axis– Time on x-axis

• The slope of the line gives you the acceleration of the object.

Momentum Section 3• When a collision occurs, changes in motion

of the colliding objects depend on their masses and their velocities before the collision.

Mass and Inertia

• Would it be harder to stop a large adult from rushing at you or a small child?

• Why?

• Inertia is a property of matter which states that objects have the tendency to resist a change in its motion!

• Objects with more mass have more inertia!

Momentum

• This is a measure of how hard it is to stop an object.

• Momentum (p) = mass (m) x velocity (v)– p=mv

• Mass is measured in kg

• Velocity is measured in m/s

• Units of momentum are kg*m/s

Momentum Practice Problems

• Calculate the momentum of a 14kg bicycle traveling north at 2.0m/s

• A 10,000kg train is traveling east at 15m/s. What is the momentum of the train?

• What is the momentum of a car with a mass of 900kg traveling north at 27m/s?

Final Practice Problems• One jet plane is flying east at 880 km/h, and

another plane is traveling north at 880 km/h. Do they have the same velocities? The same speeds? Explain.

• A swimmer speeds up from 1.1 m/s to 1.3 m/s during the last 20 seconds of the workout. What is the acceleration during this interval?

• Describe three different ways to change your velocity when you’re riding a bicycle.

• An object is said to be accelerating if it ___________.

a) speeds up c) changes direction b) slows down d) all of these