Motion, Speed, Acceleration

Chapter 8

Motion

When you are riding in a car, why don’t the people in the car look like they are moving? Why do the things outside the car look like

they ARE moving? Are YOU moving right now?

Are you sure? When riding in a car, some people go

around you and it looks like you’re going backwards. Why?

Motion

Motion is a change in RELATIVE position: Motion requires a reference point, usually

stationary. Distance measure the path taken:

To measure how far you went, you have to follow the actual path

Displacement is the distance moved, regardless of path Straight line from start to finish

Speed

Speed measures how fast a motion occurs Requires distance and time Speed = distance/time

Miles per hour (mph = miles/hr) Feet per second (feet/second) PER means divided by (usually using time)

SI unit of measure for speed is meters per second (m/s)

Constant speed means same distance is covered in the same amount of time.

Speed

Graphing Speed

Graph Time on X-axis Graph Distance on Y-

axis Slope of line is speed

Assumes constant speed

Faster speed = more slope to line

Slower speed = less slope to line

Average Speed

Average speed is calculated based on starting and ending points (distance) and total time elapsed. Used to describe objects that don’t move at constant

speed Exercise: You travel 100 miles in 50 minutes, then 50

miles in 50 minutes, then 50 miles in 100 minutes. What is your average speed?

Answer: Distance is 100 + 50 + 50 = 200 miles Time is 50 + 50 + 100 = 200 minutes Distance/time = 200/150 = 1 miles/minute

Instantaneous Speed

Instantaneous speed describes the speed and any particular point in time Very difficult to measure

Velocity

VELOCITY measure how fast in a particular direction Velocity is speed with direction 60 mph north 4 m/s South

Velocity is a very important concept for physics The DIRECTION portion makes things act

differently If one direction is considered ‘positive’ The opposite direction is ‘negative’ The velocities can cancel out based on direction

Velocity

When you combine velocities, you get a ‘resultant velocity’: What is resultant velocity of 15 m/s east and 1 m/s

east? Going same direction, so add them 15 m/s + 1 m/s = 16 m/s

What is resultant velocity of 15 m/s east and 5 m/s west?

Going opposite directions, so west is negative direction 15 m/s east + (– 5 m/s west) = 10 m/s east

What is resultant velocity of 10 m/s east and 10 m/s south?

14 m/s southeast (use Pythagorean theorem)

Acceleration

When you accelerate your car, what are you doing to the motion?

Acceleration is a change in velocity Acceleration can be a change in speed

Either up or down (positive or negative) Acceleration can be a change in direction

Velocity has direction, so acceleration does also A uniform circular motion is constant acceleration

(always changing direction)

Calculating Acceleration

Acceleration is the change in velocity, so you have to calculate the change. Acceleration = (Velocityfinal – Velocityinitial)/time Also: v

at

Because final velocity can be less than initial velocity (slowing down), acceleration can be negative

Negative acceleration is called deceleration in normal speech

In science, it is still acceleration with a negative value

Velocity-Time Graphs

Velocity changes can be graphed on velocity/time graphs Velocity on Y-axis Time on X-axis

Constant Velocity

Velocity-Time Graphs

Negative Acceleration (slowing down)

Forces

Force – a push or a pull Science definition: an action on a body

(object) in order to change the body’s motion Many forces act at the same time on any

object Combination results from many forces acting

on an object is called the net force

Forces

Balanced Forces do not cause changes in motion This includes from rest (no motion) to motion

or motion to rest Example: Tug of War with equal teams

Unbalanced Forces result in changes in motion The net force is larger in one direction Example: Tug of War with one side stronger

Net Forces

Friction

Friction is a Force Caused by objects rubbing together

Like hands in winter Acts in direction OPPOSITE the applied force

Why your bike stops rolling! Why a ball stops rolling! Because of friction, a constant force must be

applied to your car to keep it rolling (= gas) Rougher surfaces have more friction

Why do tires have grooves & knobs? Why do court shoes have tread?

Types of Friction

Static Friction - between stationary objects Like brakes on a car when it’s not moving

Kinetic Friction – between moving objects Sliding Friction – two (flat) surfaces rubbing Rolling Friction – round object to flat surface

Like tires on bike or car Fluid Friction – in liquids or gases

Air Resistance is a type of fluid friction

Force of Friction: Static > Sliding > Rolling > Fluid

Friction and Motion

Friction between 2 objects results in HEAT buildup and low efficiency

Reduce Friction by: Make surfaces smoother Use of lubricants (turn it to fluid friction) (oil, grease,

wax. graphite Convert Sliding Friction to Rolling Friction

Increase Friction by: Make surfaces rougher Make more surface area rubbing Increase force pushing objects together

Assignment: Friction and Motion

You are riding your bike to school. Is friction good or bad. Justify your answer using the principles of force, motion, acceleration, air resistance, sliding friction rolling friction, and fluid friction. At least 3 paragraphs At least 3 sentences per paragraph.

Newton’s Laws of Motion

8.3 & 8.4

Newton’s First Law: inertia

Which is easier? Pushing a car from start to moving or keeping

it moving? Starting your bicycle moving or keeping it

moving?

Newton’s First Law: inertia

An object at rest remains at rest and an object in motion remains in motion unless acted upon by an unbalanced force. Inertia is the tendency of an object to maintain

the same motion.

Newton’s Second Law: Force

The unbalanced force acting on an object equals the objects mass times its acceleration F = mass * acceleration (F=m*a) The harder you push, the faster it accelerates It takes more force to move a larger mass

than a smaller mass. Which would you rather try to stop (-

acceleration) a freight train moving slowly or a baseball moving 100mph?

Free-Fall and Weight

Free-fall – when the only force acting on an object is gravity

Free-fall acceleration near the earth’s surface is constant – 9.8 m/s2

After 1st second it is moving 9.8 m/s After 2nd second it is moving 19.6 m/s After 3rd second it is moving 28.4 m/s Etc.

Weight is Free-fall acceleration times mass Weight is a force (mass * acceleration)

Weight and Mass

Mass is the amount of matter in an object It doesn’t change, no matter where you go.

Weight is mass times acceleration due to gravity. It changes depending upon gravity On the moon, your mass is the same as on

earth, but 1/6th of the weight

Terminal Velocity

Terminal velocity is when air resistance (friction due to air) exactly balances the weight. A sky-diver will eventually reach terminal

velocity and not fall any faster About 320 km/h (200 mi/h)

Newton’s Third Law

For every force, there is an equal and opposite reaction force They are on different objects When you kick a ball:

Your foot applies a force to the ball The ball also applies a force to your foot.

Applications: Rockets Jet-skis Ram-jet engines