Pressure

 

Effects of force and area on pressure

A stone or pebble will make a dent when resting on a soft surface, such as sand or mud. The pressure of the stone decides how deep the dent is. The greater the pressure the deeper the dent will be.

The heavier the stone, the higher the pressure it will exert. Also, the smaller the surface area resting on the surface, the higher the pressure it will exert.
We can make more pressure by pushing down on the stone or standing on it, because this increases the downwards force and makes a deeper dent.

 

Calculating pressure

When calculating pressure, it’s important to know the following two things:

 

• The weight or force applied – measured in newtons (N)
• The surface area on which the weight or force is applied – measured in square metres (m²)

 

We can calculate the amount of pressure on an object using the following formula:

 

 

Pressure can be measured in many different units, but scientists usually measure pressure using pascals (Pa), which is equivalent to newtons per square metre (N/m²).

 

• 1 Pa = 1 N/m²

 

Example

Let’s use a balloon and a pencil to illustrate how pressure works. Suppose the blunt end of the pencil has an area of 0.5cm² and it takes 50 newtons of force to pop the balloon with it. We can find the pressure by dividing the force by the area:

 

 

Now, what if we use the sharp end of the pencil, which has a much smaller area of 0.001cm²? It turns out that we only need 0.1 newtons of force to pop the balloon with it. The pressure is still the same, but the force is much smaller:

 

 

This shows that the balloon pops more easily with the sharp end of the pencil, because we need less force to create the same pressure. The smaller the area, the less force we need to apply. Conversely, the larger the area, the more force we need to apply. For example, if we try to pop the balloon with the palm of our hand, which has a much larger area, it would be very difficult.

 

 

Application of Pressure and Surface Area

Drawing pins are flat-headed pins used for attaching paper to a wall or another surface.

Remember, the formula for pressure is:

 

 

When you push a drawing pin, the force from your finger is spread over the wide side of the pin, but the force on the wall is concentrated on the sharp point. This makes the pressure on your finger low and the pressure on the wall high. That’s why you don’t hurt your finger, but the pin goes into the wall. This happens because of the smaller surface area.

 

Question

What would the pressure on the thumb be when pushing a drawing pin into a wall?

 

• The force exerted when pushing the drawing pin is 10 N
• The surface area of the wide side of the pin is 2.5 cm²
  

   

  

   

  Pressure = 4 N

   

  Skis and snowboards have large surface areas to stop them from sinking into the snow.

   

  

   

  The large surface area spreads the snowboarder’s weight and reduces the pressure exerted on the snow. This stops them from sinking, even in very deep snow.

  
  Question

  
  What would be the pressure exerted in the snow by the snowboarder if:

   

  • The force exerted by the snowboarder is 600 N
  • The surface area of the snowboard is 2.0 m²
    

     

    Answer:

   

  

   

  • Atmospheric Pressure

    
    The air around you exerts pressure. This is called atmospheric pressure.

    
    Atmospheric pressure is the pressure exerted by the atmosphere.

     

    Air pressure depends on how much air there is. The more air, the more pressure. The less air, the less pressure. Air pressure changes with height. You should know:

     

    • Pressure is low at higher altitudes.
    • Pressure is high at lower altitudes.

     

    For example, there is less air on a mountain top, so the pressure is low. But there is more air at the ocean level, so the pressure is high.

     

     

    Pressure in liquids

     

    Objects in contact with liquids experience pressure from them. The pressure in a liquid depends on how deep it is. The deeper you go, the greater the liquid pressure. When a liquid has an outlet (e.g., a hole), it can force water to spurt out. The deeper the opening, the faster the water will spurt out. The shallower the opening, the slower the water will spurt out.

     

    

   

  Summary:

   

  • Pressure tells you how much force is on an object.
  • The pressure on an object changes with the force and the area it covers.
  • We can use this formula to calculate pressure: Pressure = force ÷ area

   

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