# 04 Forces

## Hydrostatic Pressure and Upthrust

This app is used to demonstrate how a spherical object with a finite volume immersed in a fluid experiences an upthrust due to the differences in pressure around it.

Given that the centre of mass remains in the same position within the fluid, as the radius increases, the pressure due to the fluid above the object decreases while the pressure below increases. This is because hydrostatic pressure at a point is proportional to the height of the fluid above it.

It can also be used to show that when the volume becomes infinitesimal, the pressure acting in all directions is equal.

The following codes can be used to embed this into SLS.

<iframe scrolling="no" title="Hydrostatic Pressure and Upthrust" src="https://www.geogebra.org/material/iframe/id/xxeyzkqq/width/640/height/480/border/888888/sfsb/true/smb/false/stb/false/stbh/false/ai/false/asb/false/sri/false/rc/false/ld/false/sdz/false/ctl/false" width="640px" height="480px" style="border:0px;"> </iframe>

## Centre of Gravity and Stability – Geogebra App

This Geogebra app allows students to explore how the position of the centre of gravity as well as the width of its base affect the stability of an object.

## Siphoning water

This video is taken during my IP4 class today. My students are making use of rubber tubes to demonstrate how we can use gravity to siphon water from one place to another. As long as the level of the source is higher than the level of the receiving end, we can do so.

## Hooke’s Law

Here are two applets students may like to try:

A. The first is a simple “experiment” showing that the extension of a spring is proportional to the force applied:

http://www.absorblearning.com/media/attachment.action?quick=5l&att=394

B. For the second applet, you can try:

1. looking for the unknown masses.
2. observing the potential energy at different extensions of each spring.
3. varying the gravitational field strength.

## Water in an Inverted Cup

This demonstration can be modified for use as a magic trick.

Materials:

1. Glass of water
2. Piece of cardboard that is larger than the mouth of the glass.

Procedure:

1. Fill the glass up with water.
2. Place the piece of cardboard over the mouth of the glass.
3. Holding the cardboard against the mouth of the glass, invert the glass.
4. Release the hand slowly.

Explanation

Water can remain in an inverted glass with the piece of cardboard underneath because atmospheric pressure is acting upward on the cardboard, holding it up together with the water. There is little air pressure within the g;ass, so the downward force acting on the cardboard is mainly the weight of the water, which is to the order of several newtons whereas atmospheric pressure exert an upward force of several thousand newtons.

Modification:

1. Drill a small hole in a plastic cup, near the base.
2. Seal the hole with your thumb and fill the cup with water.
3. Place the cardboard over the mouth of the cup.
4. Invert the cup together with the cardboard, while keeping your thumb over the hole.
5. Using a magic word as the cue, shift your thumb slightly to allow a little air into the cup. This will cause the cardboard and water to fall. As the air pressure within the cup is equal to that of the atmosphere.