Gravity’s effect on objects in motion
Inquiry 15.1
Directions: Following the procedures below. Write down the purpose and answer any of the questions on
a sheet of notebook paper. Remember to include part of the question in your answer.
Purpose: How does gravity affect an object that is in motion?
Procedure:
1. Hold the marble over the sand at height of 30 cm, with your fingers
let it go. What happens to the marble?
2. Repeat step 1, three more times. Does the marble fall the same way
each time?
3. What force is acting on the marble?
4. Gravity: a force that pulls objects towards each other. Gravity pulls
all objects to the center of that object. If the sand were not there to
stop the marble, what would the marble do?
5. Where would the marble fall to eventually if there was nothing
underneath it?
6. Law of Inertia: Objects will not change their motion until an
unbalance force acts on them. Unbalanced forces: a force that
changes an object’s motion. How does question 4 show the law of
inertia?
7. Roll the marble gently down the center of the ruler into the sand. Keep the ruler almost at no angle
at all. How does the marble move as it falls off the ruler? (does the marble fall straight down or
fall at an angle?)
8. Copy the data table below
Speed
Trial
1
Trial 2
Trial 3
Average distance marble
traveled in centimeters
Slow
(just letting go)
Medium
(pinky flick)
High
(Pointer finger
flick)
9. Keep the ruler nearly flat (and at the same angle for ALL trials). Mark a line directly under the
edge of the ruler (like a starting line) in the sand. Let the marble go at the 30 cm mark of the ruler.
Record your marbles distance in the table below, using the measuring tape. Remove the marble
with the magnet. Smooth out the surface of the sand. Repeat this step for 3 trials. Repeat steps
except flick the marble with your pinky at the 30 cm line, and record your distance. Remember to
try and use the same force for all 3 trials and to smooth surface out for each trial. Again repeat
steps this time flick the marble with your pointer-finger at the 30 cm line, and record your
distance.
10. How does the forward speed of the marble affect the motion of the marble once it leaves the ruler?
11. How much farther does the marble travel at your high-speed average from your low speed
average?
12. All planets that orbit the Sun are traveling forward due to inertia and falling towards the Sun due
to gravity. Describe the path of something that has forward motion (like your marble) but is also
being pulled down by gravity.
You don't feel the sun's gravity because you and the earth are both in orbit around the sun. Being in
orbit means that you are accelerating into the sun the same way you would be accelerating in a free fall.
Say you have a small marble. If you drop the marble, it accelerates towards the ground due to the pull of
the earth's gravity. Now say you are in an elevator at the top floor of a tall building and the elevator cable
snaps. Assuming no friction or air resistance, if you tried to drop the marble as you and the elevator are
falling, the marble would seem to float weightlessly - i.e. you cannot feel the force of gravity since you,
the marble and the elevator are all accelerating at the same rate. Similarly, if you are in space orbiting the
sun and you drop the marble, you and the marble are accelerating towards the sun at the same rate and
you will not feel the force of the sun's gravity.
Things get a little tricky because the force of gravity varies with distance, so in the case of the marble, the
bottom of the marble will feel a stronger pull than the top of the marble. What if the marble were as big as
the earth? The sun's pull is stronger on the near side and weaker on the far side. This difference (gradient)
in gravitational pull is what is responsible for tides. It turns out that even though the moon's gravitational
force on the earth is much smaller than the sun, it has a larger gravitational gradient) and therefore a
bigger effect on the earth's tides.
To address some of the previous answers:
As pointed out, the gravitational acceleration due to the sun is actually pretty small, around 0.006 m/s^2
compared to the earth's 9.8 m/s^2. One-way of thinking about these numbers is as how powerful a rocket
you'd need to hover at a fixed distance above each. You'd need a pretty powerful one to hover above the
earth's surface, and not such a strong one to hover 150 million km above the sun.
However, if you are in orbit around the sun at the same distance (150 million km), you don't need a rocket
at all. You are free falling into the sun. Fortunately, while in orbit you have a perpendicular velocity that
ensures that while you are free falling, you maintain the same distance from the sun.
Question:
13. Which part of an object (say a marble) that if falling towards the earth feels a stronger
gravitational pull?
14. Why does a rocket have to work harder when it is not orbiting an object?
15. Why doesn’t a Satellite we put into space come crashing back into the earth or float away?
16. Write a couple of sentences answering the purpose question to this inquiry.