If you jump out of an airplane, how fast would you fall? What’s the greatest speed you would reach? Let’s practice figuring it out without jumping out of a plane.
This experiment will help you get the concept of velocity by allowing you to measure the rate of fall of several objects. It’s also a great experiment to record in your science journal.
First, you’ll need to find your materials:
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- stop watch
- feathers (or small pieces of paper, plastic bag or anything light and fluffy)
- a tape measure
- If you’re crunching numbers, you’ll also need a calculator.
Now here’s how to do the experiment:
1. Get 5 or so different light and fluffy objects. Feathers of different size, small strips of paper, parts of a plastic bag, cotton balls, whatever is handy.
2. Make a prediction by writing down the objects you chose in order of how fast you think they will fall. The fastest on top, the slowest on the bottom. Leave space to the right of your prediction so that you can write in your conclusions and then compare the two.
3. Make a table with two columns. Use one column to fill in the name of the items. Use the second column to write down the time it took each object to fall.
4. Drop the different items and time them from the moment you let go to the moment they hit the ground. Be sure to drop each item from about the same height. The higher the better. Just be sure not to fall off anything! We don’t want to measure your velocity!! You might want to drop them two or three times to get an average time.
5. Now compare the items. Which one fell the least amount of time (dropped the fastest)? Which one fell the most amount of time (dropped the slowest)? Write your results next to your hypothesis. By the way, did you find anything that dropped slower than a feather? I have seen very few things that take longer to fall straight down than a feather.
Download Student Worksheet & Exercises
Did you see how many of your objects stopped accelerating very quickly? In other words, they reached their terminal velocity soon after you let them go and they fell all the way to the ground at that same constant velocity. This is why a parachute is a sky-diver’s best friend! A human has a decent amount of air resistance but he or she can reach a lethal dose of velocity (120 mph) if dropped from a great height. The parachute increases the air resistance so that the terminal velocity of that sky-diver is quite a bit safer!
Exercises
- What is velocity?
- How do acceleration and deceleration relate to velocity?
- How do we know when an object has reached terminal velocity?
Taking it Further for advanced students:
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We can do a little math here and figure out the actual velocity of your objects. Here’s how to do it:
Measure the height from which you dropped each object. Now take the height of the drop and divide that by the number of seconds it took for it to drop that distance. That’s the velocity of that object. For example, My “from-under-the-couch-six-month-old-dust-bunny” took 3 seconds to fall 6 feet. I take 6 feet and divide it by 3 seconds to get 2 feet/second.
The velocity of my dust bunny is 2 feet/second downward.
Remember, that velocity has a directional component as well as a number. Add a little more math, and I can predict how long my dust bunny will take to fall 15 feet. Take the distance (15 feet) and divide it by the velocity (2 feet/second) and I get 7.5 seconds. It will take my dust bunny 7.5 seconds to fall 15 feet. Hmmm, maybe we should call it a dust snail.
Have you noticed something here? In this experiment, we used a different formula to find out how far something would fall over a given time.
What’s going on? In Unit 1, we ignored their terminal velocity. Those things were in free fall and accelerating (gaining velocity) all the way to the ground. They were never going the same velocity for the entire trip. So, we needed to use the gravitational constant 32 ft/s² in the equation d=1/2 gt² to determine how far something fell in a given amount of time.
For this unit, we are dealing with things that are at an almost constant velocity, (since they reach their terminal velocity quickly) so we can use the much simpler equation d=vt (d is distance, v is velocity and t is time). In the problems we’ve done in this lesson plan, we have modified that formula to find how long the fall took so we’ve used t=d/v.
If you’d like to solve for v you would use v=d/t. Isn’t algebra fun?
Advanced students: Download your terminal velocity lab here.
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