Schematic illustration of a pendulum.

Gravity Pull

Why Don't Liquids Flow at the Same Rate?

 

Materials

  • paper hole-punch
  • 3 paper cups
  • masking tape
  • large pan
  • molasses or honey
  • tap water
  • vegetable oil
  • timer
  • helper

Try This

  1. Punch a hole in the side of each paper cup near the bottom. Be sure the holes in all the cups are the same size and the same distance from the bottom.
  2. Place a small piece of masking tape over the hole in each cup.
  3. Place the cups in the large pan.
  4. Pour the molasses (or honey), tap water, and vegetable oil each into a separate cup up to the top.
  5. With your helper, remove the tape from the cups all at the same time. Note the amount of time it takes each liquid to run out of each cup. In what order do the cups empty?

What's Going On?

Gravity acts on each liquid the same, so why does each cup empty at different speeds? The first cup to empty was the one filled with water. The last cup to empty was the one filled with molasses/honey. While gravity pulls on everything with the same force, different liquids can flow at different rates. The viscosity of a liquid is the rate at which the liquid can be poured. One of the thickest of all liquids is Silly Putty.

Schematic illustration of a paper hole-punch, 3 paper cups, masking tape, large pan, molasses or honey, tap water, vegetable oil, and a timer.