Projectile Motion Reflection on Learning

This week's Big Questions:

What is a projectile? What is the general path of motion? Why?

In this lab we explored projectiles by shooting an "airball" with a basketball in the gym. Unfortunately, I was busy taking my driving test so I was not present for this part of the lab. Each person in our group video taped one another shooting the basketball with the app Video Physics. This app allowed us to track the ball's motion as it was shot using dots shown in the image below:

The dots showed us that the ball moves in an arc, which is the general path of motion. A projectile is any object projected into space with the use of a force but when the object is in motion, the only force acting on it is gravity. This can be anything from a cannon to a bullet shot from a gun to a volleyball being served over a net. Although there is air resistance in this experiment  it only really matters with sports such as baseball and golf. When we analyze the projectile of the basketball, we must first separate dimensions. In the x dimension (horizontally) , the ball is moving at constant speed because the ball has a constant slope which is shown below:

This is how a free body diagram looks with a projectile:

The ball never stops moving on the x-axis but on the y-axis, the ball decelerates, stops, and accelerates. This is where you can really see the arc shape of the ball. As it is moving upwards, the ball is losing speed because gravity is pulling the ball down. At the top, for one instant, the ball stops moving then proceeds downward. As it moves down, the ball begins to accelerate again and then hits the ground.How do we find the acceleration of the ball in the y-axis? Acceleration is the slope of velocity. So we use rise over run to find the slope. This was done by our ipads and is shown below:
        

We found the acceleration to be roughly -10N/kg 

Another expiriment that we performed in class tested Galileo's theory that all objects fall at the same rate, despite the amount of mass or matter. This is the only projectile that moves in a straight line, not an arc. We dropped a series of objects at the same time that have different masses. For example, we used a block of wood and a sticky note. The block of wood fell faster because the sticky note had more air resistance. When we crumpled the sticky note ( decreased the air resistance), the block of wood and the paper fell at the same rate. This also happened on the moon , where air resistance is not not a factor except the objects used were a hammer and a feather. Proving Galileo's theory right, they fell at the same rate.

Hover Disc--Centripetal Force

This Week's Big Questions:
1. What does it mean to analyze forces in 3D?
2. How do forces cause objects to move in a circle?
3. What does it mean to be in orbit? How do satellites orbit planets? How do planets orbit the sun?

 What does it mean to say that we live in a 3D universe? By using the term 3D, we are saying that there are 3 dimensions of space (x, y, and z). Examples of this on earth include the latitude, longitude and altitude of a particular spot on Earth. Actually, it may be better to say that we live in a 4D world. This includes time ( t ). The important thing to remember is that when we analyze these dimensions, we can analyze them separately or independent of one another using SIN and COS (or SOH CAH). Using these, we can analyze a piece of wire by making it a triangle (portrait problem). This week, we explored the use of forces involved in causing objects to move in a circle using a hover disk, which eliminated friction force (unlocked the ridges between the disk and the floor). We moved this disk in a circle using constant speed. When we drew the free body diagram of the disk as it was spinning, there were 3 forces acting on the disk (gravitational, normal, and tension) the normal and gravitational forces cancelled each other out because they were balanced--meaning that because the disk was travelling at constant speed (Newton's 1st law) the normal force was the same as the gravitational force. This left tension. This central pointing net force (centripetal) is what made the disk move in a circle. Without this net force, the disk proceeded in a straight line at constant speed. Since the disk felt a net force towards the center, it moved towards the center . But because it was moving fast enough, the disk began moving around the center. You can be at constant speed in a circle while accelerating. The only thing that changes is direction. Remember, acceleration does not necessarily mean speeding up. When an object is moving in a circle, it is accelerating because its direction of motion is changing and whenever the acceleration is perpendicular to the velocity, the object changes direction but is still at constant speed.

This relates to Earth and how we orbit around the sun. We are moving at constant speed (about 60,000 mph) around the sun with nothing to stop us. In this case, the centripetal force is the sun. Earth has a gravitational rope pulling us around the sun. This is what disproves the idea that there is no gravity in space. If there was no gravity (or no rope) the earth would move away from the sun in a straight line at constant speed. We got the initial speed that propelled us around the sun (just like the force that we applied to the disk in order to get it moving)  from the creation of our solar system. Even though the gravitational force is weaker, it still exists in space. In regards to the moon, the  center is Earth and the center pointing net force  (centripetal force) is gravity which it to fall around the earth. This is how orbits happen.