Unit 2 Reflection

During this unit we learned about free fall, projectile motion and falling through the air. Many of these are considered hard topics but are surprisingly easy to understand. Newton’s second law is very important to know for this topic. This law states that acceleration is proportional to force and inversely proportional to mass. We can derive an equation stating a=f/m. It is important to know the objects mass rather than its weight! We will now look at what physics are in play when throwing a ball upward. You may be thinking to yourself that if you drop different objects with different weight they will hit the ground at the same time. In our world this may be true however in a vacuum, were air resistance is negligible, both the objects will hit the ground at the same time. This is known as free fall. Free fall is when air resistance is negligible therefore the only force acting on an object in free fall is gravity. This means that in a vacuum, if you dropped a bowling ball and a feather they would both hit the ground at the same time! Weird right? When we throw a ball straight up it has an initial velocity. When the ball leaves your hand the force of gravity is constantly accelerating the ball in an opposite direction. This is why the ball will begin to slow down at the top of its path. If a ball is thrown in the air for with an initial velocity of 40 m/s how long is the flight time? You may want to draw a diagram to help with this problem; The diagram here shows that the ball is in the air for 8 seconds. When the ball is released gravity pulls the ball to the ground therefore the velocity decreases by 10m/s every minute. When the ball reaches the top of the path it accelerates and will hit the ground with the same velocity when it was thrown. In order to calculate the maximum height of this problem we use the equation; d=.5(g)(t)2. It is important to remember that this formula only works when an object is going downward/start with a velocity of 0m/s. Because of this we start when t=4. Since we are calculating the maximum height our t in this case would be 4 seconds. Now consider a ball is released from a height, undergoing free fall, but is moving in the horizontal direction. An example of this is an airplane dropping a bomb (sorry). When released, the bomb still only has one force acting on it. This is the force of gravity. However, since the bomb was moving in the horizontal direction, we know that an object in motion wants to stay in motion! This means that the bomb will have a constant velocity in the horizontal direction and a constant force of gravity in the vertical direction, It is important to remember that the vertical velocity and the horizontal velocity are in-dependent from each other. If a bullet is fired and a bullet is dropped at the same height and at the same time which will hit the ground first? They both would!! This happens because although the bullet fired covers more distance in the vertical direction, it still undergoes the same force of gravity as the bullet dropped. Pretty crazy stuff!! This is why snipers always aim higher than the target. This is known as ‘bullet drop’. Now to calculate the vertical distance an object travels for projectile motion we use the formula d=.5(g)(t)2. We need to know the time the object is in the air in order to calculate the horizontal distance. We do this because the object is in the air for a certain amount of time therefore it will travel at a constant horizontal velocity during that time. We calculate this by using the equation v=d/t. We use the same time that the ball is in the air for in this equation.

Now in our world we don’t experience free fall. This only occurs in a vacuum where air resistance is negligible. When we are falling through the air we experience air resistance. When a skydiver jumps out of a plane, he will accelerate due to the force of gravity also known as his force of weight. As the skydivers velocity increases so does his air resistance. Therefore we can say that air resistance is directly proportional to velocity. The initial acceleration of gravity is 9.8 m/s2. This acceleration will decrease due to the opposing force of gravity acting upon the skydiver. The skydiver will eventually reach a point of equilibrium where his force of air resistance is equal and opposite to his force of weight. This point is referred to as terminal velocity. In many James Bond movies we see the hero moving through the sir by changing his body position. This actually happens! If we change our surface area we change the air resistance. By reducing you surface area you are able to reduce your air resistance. If you increase your surface area, your air resistance will also increase. When a skydiver deploys his parachute he is increasing his surface area. This causes him to get in increase in air resistance and therefore he accelerates in the opposite direction. This causes the skydiver to slow down. The skydiver wants to get back to equilibrium so he will eventually slow down to a point where the force of air resistance is again equal this force of weight. The net force acting on him is the same however his velocity has changed. I know this may seem strange but it is true physics! Know why does a crumple piece of paper hit the ground before a flat sheet of paper? It is because of the difference in air resistance. The flat sheet of paper has a higher surface area therefore the sheet of paper is unable to gain as much velocity as the crumpled sheet of paper. If a lead ball is dropped of a building and a Ping-Pong ball is dropped of the same height, which will hit the ground first? If you said the lead ball you are correct! This happens because the lead ball has a higher force of weight than the pin pong ball therefore it will take longer for the air resistance to be equal and opposite to its weight. This will allow the lead ball to have an increased terminal velocity than that of the Ping-Pong ball.