Free-Fall When something is in free-fall, it is in a state of constant acceleration. This means the three kinematics equations we learned last class can be used to solve free-fall problems.

Free-fall Acceleration  For an object to be in free-fall, means that the only force acting on it is gravity. (memorize this definition)  Gravity always pulls toward the center of the Earth, which we consider to be the down or negative direction.  All things in free-fall accelerate toward the ground at the same rate.

Question??  If you were to drop an elephant and a bowling ball out of the same window at the same time, which would hit the ground first?  Think about it….

Answer…  Naturally, you think the elephant right? Wrong! The two objects are both in free-fall, which means they accelerate at the same rate. Since they both have the same initial velocity (0), the same distance to cover, and the same acceleration, it’s going to take them the same amount of time to do it.

Acceleration Due to Gravity  Like we said, when something’s in free-fall the only force acting on it is gravity, which means gravity is the only reason it’s accelerating.  Therefore, we call the rate at which something accelerates in free-fall the “acceleration due to gravity.” We represent this quantity with a lower case g.  g = -9.8m/s 2  The negative is because things in free-fall always accelerate toward the center of the Earth, or downward

What does this mean?  When you are solving free-fall problems, (like how long does it take for a dropped object to hit the ground?) you will always use -9.8m/s 2 as your acceleration.

Free Fall’in  Free Fallin Free Fallin Please view the free-fall video below to review before moving on to the practice problem on the next slide.

Practice Problem  We’ll do this one together…  A bowling ball is dropped from a third story window. It hits the ground below 2.4 seconds later. How high is the window?

Remember Your Steps to Solving Physics Problems Write down all knowns 1. Write down all knowns given in the problem. Make sure to include their letter designation and unit of measure. the variable 2. Write down the variable you are solving for. the equation 3. Look through the reference packet. Write the equation which has the variable you are looking for and the knowns. Plug in solve 4. Plug in the knowns and solve. 5. You may need to use an interim step if there is not an equation which will solve this directly.

A bowling ball is dropped from a third story window. It hits the ground below 2.4 seconds later. How high is the window?  Knowns:  v i = 0m/s  t = 2.4s  a = -9.8m/s 2  Unknown:  d = ?  Equation:  d = v i t + ½ at 2 Solve! d = (0m/s)(2.4s) + ½ (-9.8m/s 2 )(2.4s) 2 d = m Why is this number negative? Because the ball went down m from it’s original position. Should your final answer be negative? No, height should never be written as a negative number, so you simply drop the negative from the displacement and get h = 28.22m

What if the ball had been thrown up or down instead of simply dropped?  That would mean the ball had a non-zero initial velocity. If the ball were thrown up out of the window, the initial velocity would be positive. If it were thrown down out of the window, the initial velocity would be negative.  You would plug in the initial velocity of the ball to the equation for v i.

Assignment  Complete the practice worksheet given to you by the substitute.  This is due at the end of class. Please place in the tray when you are finished.  If you finish early, you are expected to work on your problem set, which is due Friday.  At this point, you know everything you need to complete the entire problem set assignment.  See you tomorrow!!