A dense metal sphere is rolled toward another dense metal sphere of the same size and makeup that's sitting still. The sphere at rest rolls forward slowly and the moving sphere continues forward at a much slower pace.


In this activity, students make predictions about what will happen when two objects collide and compare their hypotheses to their observations. They also compare what happens when objects made of different materials collide to determine how the makeup of objects affects the impact.



  • This activity can be done as a whole-class demonstration with a discussion or in small groups, depending on student ability, material availability and classroom aide support.
  • Desktops or smooth flooring work best for rolling. Carpeted flooring can affect the way the spheres roll and collide. Be sure that the surface is large enough that objects have room to roll around and deflect off one another.
  • Consider using the Whip Up a Moon-Like Crater lesson before or after this activity to develop pre-existing knowledge about asteroid and comet impacts, or extend learning beyond this lesson.


In 1686, Sir Isaac Newton published his three laws of motion. Together, they describe the relationships between objects and the forces acting upon them.

Newton’s Laws of Motion:

  1. An object at rest remains at rest, and an object in motion remains in motion at constant speed and in a straight line unless acted on by an unbalanced force.
  2. The acceleration of an object depends on the mass of the object and the amount of force applied.
  3. Whenever one object exerts a force on another object, the second object exerts an equal and opposite force on the first.

Newton’s laws are used when launching rockets, conducting spacewalks, and landing spacecraft at places like the Moon and Mars. They also play an important role in NASA’s planetary defense strategy.

While there are no known objects currently posing a threat to Earth, NASA is proactively researching and planning for ways to prevent or reduce the effects of a potential impact from an asteroid or comet. The DART mission is the first test of such a plan – in this case, whether it's possible to divert an asteroid from its predicted course by slamming into it with a spacecraft. The impact does not need to destroy or even greatly change the path of the asteroid, as even slight changes in the asteroid's orbit over great distances would help keep Earth safe from potentially hazardous objects.

This animation shows conceptually how DART's impact is predicted to change the orbit of the asteroid Dimorphos around a larger asteroid called Didymos. Credit: NASA/Johns Hopkins APL/Jon Emmerich | Watch on YouTube

The DART mission will use principles outlined in Newton’s laws to slightly change the velocity of a small asteroid that will be located 6.8 million miles (11 million kilometers) away from Earth at the time of impact. The asteroid, known as Dimorphos, and the predicted change in its orbit pose no threat to Earth. You can learn more about the DART mission in this Teachable Moments article.

Beyond space exploration and planetary defense, Newton’s laws are also present in our everyday life. For example, scientists and engineers use them to build safer cars and bike helmets, and students can use them to describe, predict, and understand the movement of objects in the world around them.


  1. Ask students to think about and give examples of things that can bump into each other. Introduce the verb "collide" and the nouns "collision" and "impact," and encourage students to use those terms when giving their examples. Ask students to describe what happens when their described objects collide with each other. If responses don’t include things like falling down, moving, or getting pushed aside, prompt students to consider those reactions.
  2. Explain to students that objects in space can collide. For example, comets and asteroids can collide with other asteroids, moons, or planets, and spacecraft can collide with asteroids, comets, and moons.
  3. Pick two spheres and ask students to predict, then observe what happens in the following scenario: One sphere is at rest, and another sphere rolls into it. In their prediction, students should consider things like the direction and speed of each object before and after the collision. After the collision, ask students if they were correct in their prediction and to identify if anything was different than they predicted. Consider recording the collision with a mobile device for students to review at regular and slow-motion speeds.
  4. A dense metal sphere is rolled toward another dense metal sphere of the same size and makeup that's sitting still. The sphere at rest rolls forward slowly and the moving sphere continues forward at a much slower pace.

    A sphere collides with another sphere that's sitting still. Image credit: NASA/JPL-Caltech | + Expand image

  5. Repeat step 3 with the following scenario: Both spheres are rolled toward each other at the same speed.
  6. Two metal spheres are rolled toward each other. They collide and roll back slower in the opposite direction.

    Two spheres in motion collide with one another. Image credit: NASA/JPL-Caltech | + Expand image

  7. Ask students to compare and contrast how the spheres reacted in the two collisions.
  8. Have students consider ways they can change the collision between objects and share them with the class. Ask the class to predict how changing the collision will affect the motion and speed of the objects. Then, conduct the collisions, making a single change at a time. If necessary, prompt students to consider the following changes:

    • Speed of the collision (rolling the spheres faster or slower)
    • Direction of the collision (head-on collision vs. a glancing or side-on, collision)
    • Size of the objects (two large objects, two small objects, or one small and one large object)
      A small marble is rolled toward a large metal sphere. The marble bounces off the metal sphere and rolls back in the other direction while the metal sphere wobbles slightly.

      Two spheres of different sizes collide. Image credit: NASA/JPL-Caltech | + Expand image

    • Mass of the objects (two heavy objects, two light objects, or one heavy and one light object)
      A foam ball is rolled toward a metal sphere. The foam ball shoots off in a diagonal direction after colliding with the metal sphere while the metal sphere wobbles slightly.

      Two spheres of different masses collide. Image credit: NASA/JPL-Caltech | + Expand image

    • Makeup of the objects (two steel marbles colliding, two foam balls colliding, a foam ball colliding with a steel marble)
      A small wooden ball is rolled toward a puffball. Both spheres roll forward at a similar speed.

      Two spheres made up of different materials collide. Image credit: NASA/JPL-Caltech | + Expand image


  • Ask students to explain how they would make a marble speed up or slow down in a given direction or how to make it change directions.
    • Answers should include details about the direction and speed of objects in a collision, and the makeup of objects, if appropriate.
  • Ask students to predict other types of possible collisions, such as two objects colliding and sticking together. Consider demonstrating this with modeling clay or play dough. In what situation would this take place in space? How would changing directions be affected?


  • Students should learn and use the terms "collide," "collision," and "impact" correctly.
  • Student predictions should incorporate existing knowledge about collisions and new knowledge as more test collisions are observed.
  • Student observations should accurately reflect what occurred.


  • Create a game-like scenario in which a rolling sphere is heading toward a target representing Earth. Challenge students to roll another sphere toward the oncoming sphere and deflect it from its path toward the target.
  • Using a pile of sand or aquarium gravel to represent an asteroid of loosely-bound material, have students roll a small metal ball into the pile. Ask students to predict what will happen, and then describe and explain what they observe.

Explore More