PhysicsEnergy on the Move: The Big Bump
Lumi the glowing firefly hovers over a smooth floor where a rolling blue marble bumps into a still red marble, sending it scooting forward.
- Explain that a moving object carries energy of motion.
- Describe how energy of motion passes from one object to another during a collision.
- Predict how a faster moving object affects a still object it hits.
- Predict whether a still object will move when nothing collides with it.
Key terms
- energy of motion
- the energy a moving thing carries
- collision
- when one thing crashes into another
- still object
- a thing that is not moving
- transfer
- to pass energy from one thing to another
Energy on the Move
When something moves, it carries energy of motion. The faster it moves, the more energy of motion it carries. A still object is not moving, so it has no energy of motion right now. Picture a marble rolling across a smooth floor. While it rolls, it carries energy. The moment it stops, that energy of motion is gone because nothing is moving anymore. Only moving things have energy of motion.
Passing Energy in a Bump
A collision is when a moving thing crashes into a still thing. During the bump, the moving thing passes some of its energy of motion to the still thing. The energy does not disappear; it moves over to the new thing. That is why a rolling marble can make a still marble scoot away. A faster marble passes more energy, so the still marble scoots even farther after the bump.
Worked examples
A rolling marble bumps a still marble and the still one zooms off. Why?
- The rolling marble carries energy of motion.
- During the collision, it passes some energy to the still marble.
- The still marble now has energy of motion, so it moves.
Answer: The rolling marble transferred its energy of motion to the still marble.
A fast marble and a slow marble each hit the same still block. Which block goes farther?
- The fast marble carries more energy of motion than the slow one.
- More energy of motion can transfer during the bump.
- More transferred energy makes the block move a longer way.
Answer: The block hit by the fast marble goes farther because more energy transferred.
Activity
A rolling ball is about to hit each still object below. Predict what each still object will do after the bump.
Practice
Explain why a still block stays put when nothing bumps it.
Tell what happens to a still ball when a fast ball hits it.
Common mistakes to avoid
- still objects hold hidden motion energyA still object has no energy of motion until something moving bumps it.
- energy is made from nothing in a crashEnergy is never made from nothing; it only transfers from a moving thing.
Check your understanding
A rolling ball hits a still ball that is sitting on the floor. Why does the still ball start to move?
Two rolling balls hit the same still block. One ball is fast and one ball is slow. They are the same size and made of the same material. Which one makes the block move farther?
A wooden block is sitting still and nothing is moving toward it. What will the block do?
Recap
Moving things carry energy of motion, and faster ones carry more. In a collision, that energy transfers to a still thing and makes it move. A still object has no energy of motion on its own and stays put unless something bumps it.
Reflect
Which marble would you make move first, and why does it move?