Physics 1
Dynamics Lab Activity
Investigating Newton's First and Second Laws



To investigate Newton's First and Second Laws by studying the motion of a small sphere.


Newton's First Law tells what happens to an object when no unbalanced forces act on it, and Newton's Second Law tells what happens to an object when an unbalanced force does act on it. In this simple activity, you will observe what happens to a small sphere when forces are applied, as well as when no force is applied at all. Of course, the situation in which no forces act can only be approximated here - there is a small friction force that the table exerts on the ball - but try to extrapolate your observations to the situation where no friction acts. (In plain English: "Try to ignore friction.") In other words, try to be clear about whether the motion of the ball is due to no forces acting, or due to the friction force.


assorted small spheres


level table

Part 1: Newton's First Law:

What happens to the motion of an object when no unbalanced forces act on it? A smooth sphere rolling across a smooth, level tabletop almost meets the requirements of Newton's First Law. The earth pulls downward on it (its weight) and the table pushes upward on it, but these 2 forces balance (cancel each other out). There is a small friction force acting to slow the ball down, but this friction force is very small (Try to ignore it.). Therefore, the sphere moves almost as if no forces act on it. Investigate the motion of a rolling sphere until you can answer the following questions.

Hint: Do not try to answer these questions in the space provided on this sheet. Invest in a sheet of paper, please.

  1. Observe and describe the motion of a sphere rolling across a level table. Does the sphere accelerate? If so, why? If not, why not?

  2. Does the mass of the sphere affect its motion? In other words, do rolling spheres of different mass behave differently as they roll? Why or why not? (Note: The question does not ask about differences in getting the spheres rolling (or in stopping their motion) - if 2 spheres of different mass are rolling at the same velocity, do they behave differently?)

Part 2: Newton's Second Law:

[Image of Applying a Force on a Sphere]You can apply a relatively constant force to a sphere by blowing on it through a straw. Newton's Second Law describes what happens to a motion if an unbalanced force acts on it.

  1. While you apply a force to a sphere, what happens to its motion? Make a diagram showing the direction of motion of the sphere, and the direction of the force. Also show any changes in the motion of the sphere on the diagram.
    1. Sphere at rest:

    2. Force in the same direction as the sphere's velocity:

    3. Force in the opposite direction to the sphere's velocity:

    4. Force at right angles to the direction of the sphere's velocity:

  2. When you stop applying the force, what does the sphere do?

  3. Are spheres of different mass affected differently by the same force? Explain.

  4. If a force is applied to an object, 2 factors affect its acceleration. What are they, and how do they affect the acceleration?

  5. Is it possible for an object with a large mass to have the same acceleration as an object with a small mass? How?


last update October 27, 2007 by JL Stanbrough