Air Resistance & Free Fall Physics: Practice Problems

Air Resistance & Free Fall Physics: Practice Problems
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  • 0:04 Free Fall
  • 0:57 Air Resistance
  • 1:54 Finding Free Fall Velocity
  • 4:00 Mass and Free Fall
  • 5:57 Lesson Summary
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Lesson Transcript
Instructor: Damien Howard

Damien has a master's degree in physics and has taught physics lab to college students.

Learn what we mean by free fall, and how air resistance affects objects falling through the air. Then we'll work though a couple practice problems to see how objects act while they are in free fall.

Free Fall

Projectile motion problems are a common type of physics problem you'll deal with. These involve objects traveling through the air, such as a football being thrown, or a penny being dropped off of a building. One special type of projectile motion is free fall. An object is in free fall when it's falling, and no force is acting on it other than the force due to gravity.

An astronaut orbiting the Earth is a good example of an object in free fall. We normally think of these astronauts as weightless, but that's not really what's going on. Earth's gravity does not immediately cease to exist once you exit the atmosphere. Astronauts still experience a force due to gravity, aka weight. What's actually happening is that the astronaut is constantly falling like a skydiver. They are in constant free fall, and their trajectory causes them to miss the Earth and circle back around in an orbit.

Air Resistance

Now, unlike our astronaut, the skydiver we just mentioned would not be in free fall. You might be wondering what force other than the force due to gravity could possibly be affecting the skydiver. The answer is air resistance. Air resistance is a form of frictional force that acts against an object falling through the air. Astronauts do not experience it because they're out of the atmosphere.

Air is actually a fluid that consists of many tiny particles. As skydivers fall through the air these particles resist the force due to gravity by pushing against them as they fall. To imagine what this is like, let's look at another fluid: water. Imagine wading through chest-high water in a pool. You can feel the water pushing back on you as you try to walk through it. This is similar to what is happening to skydivers with air. Finally, it's worth noting that for introductory physics problems it is common place to ignore air resistance when doing projectile motion problems.

Finding Free Fall Velocity

To solve projectile motion problems, such as an object in free fall, we need to look at the four kinematic equations.

four kinematic equations

kinematic equations key

Which equations we use depend on the scenario the problem gives us. Imagine a 15 kg stone falls for four seconds after being dropped off a cliff. Ignoring air resistance, what's the stone's final velocity right before it hits the ground?

Let's look at what we know. We know the stone falls for four seconds (t = 4 s). We also know the stone's mass (m = 15 kg). There are actually two more things we know, though it might not be apparent at first. We know the stone's acceleration. On Earth all objects that are falling under the influence of gravity alone have the same rate of acceleration. We call this the acceleration due to gravity, and it is 9.8 m/s^2. Finally, we actually know the stone's initial velocity. When the stone is dropped, it starts with an initial velocity of 0 m/s and then starts accelerating.

Final Velocity Problem Diagram
final velocity problem diagram

The two unknowns we have left are displacement and final velocity. Final velocity is what we're looking to find, so we need a formula with final velocity in it but no displacement. We can look at our kinematic equations and see that this is the one that will work for us.

final velocity equation

Now putting in what we know, we can find our final velocity.

final velocity solution

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