Chapter 3 Notes

What we have been studying: So far all of the motion we have studied as been in a straight line. This is also called linear motion. Not everything always moves in a straight line. A football that is thrown will move in both the vertical and horizontal directions at the same time.

Vectors and Scalars A vector is a quantity that has magnitude and direction. A scalar is a quantity that has only magnitude. Velocity is a vector that we have already been using. Two other vectors that we have studied are displacement and acceleration.

Vectors All vectors have magnitude and direction 7 m/s west magnitude direction

Vectors Vectors are represented as arrows. Every vector has a tail and a head.

Vectors The length of the vector indicates the magnitude

Vector Addition Vectors can be added together. The resultant is the sum of vectors.

Vector Addition

Vector Addition Vectors can even be added if they are not in the same or opposite directions. We can add vectors if they are at right angles. To add vectors at right angles we will use the Pythagorean theorem.

Practice Problem Bobby leaves the base camp and hikes 11 km, north and then hikes 11 km east. Determine Bobby's resulting displacement.

Vector Addition When vectors are at right angles to one another the magnitude of the resultant is determined with the Pythagorean theorem. The direction is a combination of the two contributing vectors.

Projectile Motion

What is a projectile? A projectile is an object that moves in two dimensions at the same time. One of the dimensions must be vertical. A projectile is acted on only by gravity in the vertical direction. An example of a projectile is any thrown object.

Why does a projectile move the way it does? A projectile moves only under the influence of gravity. This is not to say that there was no force that got it moving horizontally but as soon as the projectile is thrown or fired or released, there is no longer a horizontal force present.

Horizontal Motion When we neglect air resistance (which we will always do for problem solving), a projectile moves at a constant speed in the horizontal direction. The horizontal acceleration of a projectile is zero.

Vertical Motion A projectile is influenced by gravity in the vertical direction. This means an object will slow down on the way up and speed up on the way down just like an object that was thrown straight up would do. The vertical acceleration of a projectile is 10 m/s2.

Horizontal and Vertical Motion The horizontal motion and the vertical motion are independent of one another. This means that gravity is going to work the same whether a ball is thrown softly, thrown very fast or even just dropped.

Check for Understanding A ball is dropped off a cliff at the same time a cannon ball is fired horizontally off the same cliff. Which will hit the ground first?

Video1

Check for Understanding A ball is dropped off a cliff at the same time a cannon ball is fired horizontally off the same cliff. Which will hit the ground first? They will both hit the ground at the same time. Gravity does the same thing to both of them.

Check for Understanding Suppose a rescue airplane drops a relief package while it is moving with a constant horizontal speed at an elevated height. Assuming that air resistance is negligible, where will the relief package land relative to the plane? a. below the plane and behind it. b. directly below the plane c. below the plane and ahead of it

Check for Understanding Suppose a zookeeper must shoot a banana from a banana cannon to a monkey who hangs from the limb of a tree. This particular monkey has a habit of dropping from the tree the moment that the banana leaves the muzzle of the cannon. The zookeeper is faced with the dilemma of where to aim the banana cannon in order to hit the monkey. If the monkey lets go of the tree the moment that the banana is fired, then where should she aim the banana cannon?

The Monkey and the Zookeeper http://www.physics.umn.edu/video.html?url=/media/outreach/pforce/circus/videos/MechanicsProjectileMotion-MonkeyAndHunter.flv&vidname=Physics%20Force:%20Monkey%20and%20Hunter&goback=/outreach/pforce/circus/

Fast Moving Projectiles Satellites A satellite is any object which is orbiting the earth, sun or other massive body. Satellites can be categorized as natural satellites or man-made satellites.

Fast Moving Projectiles The moon, the planets and comets are examples of natural satellites. Accompanying the orbit of natural satellites are a host of satellites launched from earth for purposes of communication, scientific research, weather forecasting, intelligence, etc

Fast Moving Projectiles A satellite is a projectile. That is to say, a satellite is an object upon which the only force is gravity. Once launched into orbit, the only force governing the motion of a satellite is the force of gravity. Newton was the first to theorize that a projectile launched with sufficient speed would actually orbit the earth.

Fast Moving Projectiles

Fast Moving Projectiles

Projectile Motion Sample Problem Frank stands on the balcony of a building and kicks a pebble off with a horizontal velocity of 5 m/s. The balcony is 12 m high. How long will it take the pebble to reach the ground and how far horizontally will it travel?

Sample Problem First pick out the given numbers in the problem and organize them as either vertical or horizontal. Next write which equation matches which information Horizontal Vertical v = 5 m/s Dy = 12 m a = 10 m/s2

Sample Problem s = Dx/t Dy = ½ at2 Horizontal Vertical v = 5 m/s Dy = 12 m a = 10 m/s2

Sample Problem Why have I changed the equations from s = d/t and d = ½ at2 To s = x/t and y = ½ gt2 ??????????????????????????????????????????

Sample Problem Now find time with either the vertical or horizontal information. NEVER NEVER EVER mix horizontal and vertical information into the same equation!

Sample Problem WHICH IS BEST TO FIND TIME? s = Dx/t Dy = ½ at2 Horizontal Vertical v = 5 m/s Dy = 12 m a= 10 m/s2 THE ONE WITH THE MOST INFORMATION.

Sample Problem Dy = ½ at2 Vertical y = 12 m g = 10 m/s2 Dy = ½ at2 12 = ½ (10) t2 12 = 5 t2 5 5 2.4 = t2 t = 1.55 s Dy = ½ at2 Vertical y = 12 m g = 10 m/s2

Sample Problem Have we answered the problem? Frank stands on the balcony of a building and kicks a pebble off with a horizontal velocity of 5 m/s. The balcony is 12 m high. How long will it take the pebble to reach the ground and how far horizontally will it travel? Not quite through.

Sample Problem s = Dx/t Horizontal v = 5 m/s Now we also know time; t = 1.55 s 5 = x/1.55 5 * 1.55 = x Dx = 7.75 m

Now you try A plane flying at 70 m/s drops a package from a height of 5000 m. How long will it take the package to reach the ground and how far horizontally will it travel?