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 |
straw |
level table |

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:Donottry to answer these questions in the space provided on this sheet. Invest in a sheet of paper, please.

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

- 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?)

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.

- 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.
- Sphere at rest:

- Force in the same direction as the sphere's velocity:

- Force in the opposite direction to the sphere's
velocity:

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

- Sphere at rest:
- When you stop applying the force, what does the sphere do?

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

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

- 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