Gravity: Newton’s Falling Apple

Connecting Gravity with Science History and
A Scientist’s Contributions.

Sir Isaac Newton (1643-1716) a scientist and mathematician wasn’t the first person to observe an apple fall from a tree.  The story that the apple hit Newton on the head resulting in Newton’s Law of Gravity is not very likely.

My guess is that Newton had seen many hundreds of apples fall from trees. He may have even stood in an apple tree and shook it causing the apples to fall. But for the first time, Newton was viewing the falling apples though the eyes of a scientist.

Newton was in his critical thinking and problem solving mode. This means that Newton would identify a problem and start collecting evidence to solve it.

The problem: Why causes apples to fall downward (vertically)?

Empirical Research:
A way of gaining knowledge by means of direct and indirect observation or experience.

Much of Newton’s evidence was collected by observations,directly and indirectly, then with logical reasoning, he made informed decisions.

Direct Observations
Newton noted that apples always fall  vertically (straight down). Apples never fall up or sideways.

Since the apples are falling in one direction, vertically, instead of speed, scientists call the motion velocity.

Velocity is the speed of a moving object in a certain direction.

Newton’s hypothesis was that some force was pulling the apples vertically downward.  Newton coined the word “gravity” to name this force.

A force is any action (a push or pull) that causes an object to change in speed, change in direction, or change in shape.

As long as the force is applied, the speed of the object continues to change; and/or the direction of the object continues to change; and/or the shape of the object continues to change.

Inference: Indirect Observation

I am guessing that Newton discovered that  apples falling from the top of tall apple trees hurt more it they hit him than apples falling from low branches. This is because the farther the apple fell the greater was their speed.

Another guess, but as a scientist, Newton would have been experimenting with falling things. Living on a farm, I can see him dropping things–apples–from the hay loft, maybe even from the top of the barn. He could have counted to see how long it took the apples to fall. But all of the objects would have fallen vertically. With this settled, Newton directed his attention on how far from the Earth’s surface the force of gravity reached. Was Earth’s gravity pulling on the Moon? If so, why didn’t the Moon fall to the Earth’s surface?

Newton had a vivid imagination and the idea came to him that maybe the Moon was falling around the Earth.

This thought is explained with an imaginary cannon experiment. The pretend canon is positioned at a distance above Earth’s surface. The character being shot form the cannon is suppose to be me. Notice that the path I follow is curved. This is because the cannon propels me in a direction horizontal to the Earth’s surface. With a horizontal velocity and the force of gravity pulling me vertically toward the Earth, the results is a curved path like any projectile would have.

The arc of the path increases as the horizontal velocity increases. Notice that if the horizontal velocity is too great, I would zip out into space making a curved path that in time it would circle the Earth unless some other celestial body’s gravity pulled me toward it.

If the horizontal velocity is too small, my curved path takes me to the Earth’s surface. But, with just the right horizontal velocity, gravity continues to pull me toward the Earth’s surface resulting in a curved path that circles Earth. Technically, I would be constantly falling toward the Earth.

This models how the Moon as well as satellites continually orbit around Earth.

Orbit is the curved path around a celestial body. It is also used to describe the motion of a satellite following a curved path around a celestial body.

Scientific Investigation and Reasoning

Design, plan and implement an experiment to investigate the effect of the horizontal speed of a satellite.

The diagram gives a clue for the experiment. The white tube needs to be sturdy enough so that you can hold and rotate it so that the object on the end (green ball) moves in a circular motion. The hanging weight represented by the black box represent gravity pulling the satellite downward.

What happens if you spin the “satellite” slowly? Fast?

Design, plan and implement an experiment to investigate the effect of different gravitational the horizontal speed of a satellite.

Physics investigations for middle school and/or high school students. Ideas for developing investigations into a science fair project.