CG Freelancer Magazine has partnered with Physics for Animators to bring you a Gravity IQ Quiz to test your essential knowledge of gravity as it’s used in animation. If you haven’t taken the quiz, you can do so here before reviewing the answers below. No peeking!

### How did you do on the Gravity IQ Quiz?

• 100 points or more: You’re a Gravity Master! Take a bow and pat yourself on the back. You’re ready to animate gravitational effects convincingly.
• 70-100 points: You can probably animate scenes with gravity pretty well, but you’d best look through the answers below to complete your education.
• 40-70 points: You’re not a total noob, but you could do with a refresher course before attempting animation involving gravity.
• 0-40 points: You can get around without falling down, but best not to attempt animation until you’ve reviewed the answers below.

Here are the correct answers, and partially correct ones.

1. Which of the following statements are true about gravity at the surface of the Earth?

• Gravity pulls things toward the center of the Earth. The force of gravity pulls toward the Earth’s center. This is why people on the other side of the Earth don’t fall off!
• Gravity keeps our atmosphere in place. Our atmosphere is held to Earth by gravity. That’s a lucky thing for us, otherwise we wouldn’t be able to breathe.
• Atmospheric pressure is greater on the surface of the Earth than it is several miles up because gravity presses down on the atmosphere. Gravity continually pulls the atmosphere toward the center of Earth. As part of this activity, the air at the outer edges of the atmosphere continually presses down on the air below it, compressing it. The pressure this generates down at the surface is our everyday “air pressure”. It’s also why air pressure is lower for air high above the Earth; way up there, there isn’t as much air pressing down on it.
• Gravitational pull is slightly different at the equator than it is at the North Pole and South Pole. This is true, though it’s not very well known. The spin of the Earth, as experienced at the equator vs. at the North or South Pole, affects how gravity is experienced in these places. At the equator, gravitational pull is slightly less. If you got this one right, you truly are an avid student of gravity!

2. In classical physics, gravity is…

• a type of acceleration (partial points)
Gravity is considered a force in classical physics, and forces cause acceleration.

3. In what units is gravitational acceleration expressed?

• Gravitational acceleration is expressed Same as any other acceleration, as Distance per time per time.

4. What is the approximate acceleration of gravity at the surface of the Earth?

• 9.8m/sec/sec
• 32ft/sec/sec
If you’re going to animate falling people or objects, you need to know this one cold!

5. The gravitational pull that a planet (or moon) exerts at its surface depends on its:

• mass – This is the correct answer. Mass is related to size and density.
• size – Partial points given for a related answer.
• weight – Weight is essentially mass affected by gravity, so this is a related answer (partial points).
• density – Partial points given for a related answer.
• rotational speed – Gravitational pull is very slightly affected by the rotational speed at the planet’s equator vs. its poles (see Question 1). Partial points.

6. Choose the best description of the relationship between mass and weight.

• Weight is a measure of the gravitational pull on a mass.
Don’t feel bad if you got this one wrong. Unless you’ve done a study of the difference between mass and weight, it can be very confusing!

7. Suppose you stand on your roof and, at exactly the same time, drop an orange and a handkerchief. The orange will hit the ground first because:

• Light cloth is affected more by air resistance, which slows the handkerchief’s fall more than the orange’s.
This question is often answered incorrectly, even by animators who have been working in the biz for decades. Air resistance, the result of air pushing back up on a falling object, is responsible for most of the “anti-gravity” effects we see in life, such as flying birds and airplanes and parachutes that slow a jumper’s fall. Every object is affected by gravity equally, but can just seem to be affected differently because of upward-pointing forces like air resistance.

Now that you know, want to give it another go? Take the quiz again! And download the free CG Freelancer Magazine app on your iOS or Android device. The next issue of CG Freelancer magazine is all about gravity!

And if you want to argue with me about any of these answers, you can email me at editor@cgfreelancer.com.