At a park, you see a child on a sled coming down a steep hill.

Gravity

Phenomenon

Observations and wonderings

What are you observing about this phenomenon?

What are you wondering about this phenomenon?

Focus questions

1. What do you think causes a sled to go down a hill? List as many causes as you can.

2. How is walking up a hill different from sledding down a hill?

Gravity

Why is jumping on a trampoline so much fun? Its springy top helps you jump higher. But even on a trampoline, you can’t keep going higher, gravity always pulls you back down.

Gravity is a force, but not like other forces you may know. Gravity is a bit special. You know that a force is a push or pull. If you push a ball, it starts to roll. If you lift a book, it moves upward. Now, imagine you drop a ball. It falls to the ground. Can you see the force pulling it down? That is what makes gravity really cool. It is invisible.

Earth’s Gravity

You are already very familiar with Earth’s gravity. It constantly pulls you toward Earth's center. What might happen if there was no gravity? You could fly off into space. Gravity keeps us firmly down on the ground. Gravity also pulls on objects that are in the sky. When you throw a ball up, gravity pulls it back down. Gravity also pulls a skydiver back down to Earth. Earth’s gravity can even pull objects like meteors from space.

How Can Gravity Be Overcome?

"What goes up must come down." You have probably heard that statement before. At one time this statement was true, but no longer. Since the 1960s, we have sent many spacecraft into space. Some are still traveling away from Earth. So it is possible to overcome Earth’s gravity.

Do you need a giant rocket to overcome gravity? No, you actually overcome gravity every day. Think about when you climb a set of stairs. When you do, you are overcoming gravity. What if you jump on a trampoline? You are overcoming gravity for a few seconds. Everyone can overcome gravity. You just need to apply a force larger than gravity. Think about that the next time you jump into the air. You are overcoming gravity for a brief second. Enjoy it while it lasts. Eventually, gravity will pull you back down.

Putting It Together

Focus questions

1. Why is it harder to walk up a hill than it is to sled down a hill?

2. Besides the sled going downhill, what other evidence of gravity do you see in this phenomenon?

Final task

Construct an explanation using words and a model that shows how gravity affects a sledder on a snowy hill. In your model, show:

- Why a sledder goes downhill

- Why a sledder is able to walk back up the hill even though gravity is pulling on them

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