Activities

1. What is friction?
   
   
2. What is gravity?

3. Explain the forces that keep the moon in orbit around the Earth.  Are these forces balanced or unbalanced? 

4. A wagon rolls down a small hill.  There is a ball inside the wagon.  At the bottom of the hill, the wagon hits a large rock.  Write a paragraph to explain the different forces that are acting on the wagon and the ball and how their velocities change based on these forces.

Answer Key/Rubric

1. Friction is a force that resists motion between two surfaces that are touching.
   
   
2. Gravity is the force object exert on each other due to their masses.
   
   
3. Acceptable responses may include, but are not limited to:
   
   
   • Newton’s third law of motion states that every time one object exerts a force on another object, the second object exerts a force that is equal in size and opposite in direction back on the first object.
   • The Earth exerts a force on the moon.
   • The moon exerts a force on the Earth.
   • The forces between the Earth and the moon are balanced.
   • Centripetal force is any force that keeps an object moving in a circle.
   • Centripetal force helps keep the moon in orbit around the Earth.
   • Gravity also helps keep the moon in orbit around the Earth.

4. Acceptable responses may include, but are not limited to:
   
   
   • Gravity pulls the wagon down the hill.
   • The wagon will accelerate as it rolls down the hill.
   • The ball and the wagon will have the same velocity and acceleration as they roll down the hill.
   • When the wagon hits the rock, the wagon will stop.  Its velocity will be zero.
   • The ball will fly out of the wagon due to its inertia.
   • Eventually, the ball will hit the ground because gravity will pull it down.
   • Friction will stop the ball from rolling.
   • Its velocity will decrease until it reaches zero.