Lesson 4: Pressure in liquids

Question 1

What is a “fluid?”

Answer 1

• A fluid can either be a liquid or a gas.

Question 2

Finish the sentence..

The force acting on the walls of a container from liquid/ gas particles are at……

Answer 2

….. Right angles/ normal to any surface.

Question 3

How can pressure at the surface of a fluid be calculated?

Answer 3

Pressure = force normal (at right angles) to a surface/ area of that surface.

Question 4

Why is the pressure of a liquid in a column higher at the bottom of the column than the top of the column?

Answer 4

• As depth (height of the column increases), there is a greater weight of liquid acting downwards.

Question 5

Why does the pressure of a liquid increase with the density of a liquid?

Answer 5

• Liquids with a greater density have a greater weight acting downwards.

Question 6

What causes an object to float?

Answer 6

• The object (fully/ partially submerged in water) experiences a greater pressure on the bottom surface than on the top surface.
• This creates a resultant force upwards (known as upthrust)
• For object to float, the upthrust must be equal to the objetct’s downwards force of weight.
• If an object can displace its own weight of water then the upthrust will equal the object’s weight.

Question 7

What causes an object to sink?

Answer 7

• The upthrust force must be less than the object’s weight.

Question 8

If an object is submerged into water, what is the size of the upthrust equal to?

Answer 8

• The size of the upthrust is equal to the weight of water that has been displaced by the object.

Question 9

If an object is less dense than water, how much water will the object need to displace before the weight of the water displaced is equal to the weight of the object?

Answer 9

• Only a small volume of water.
• The upthrust equals the weight of the object and the object floats high in the water.

Question 10

Why do objects more dense than water sink?

Answer 10

• The object can’t displace a volume of water equal to its own weight.
• Therefore, weight of object is greater than the upthrust and the object sinks.

Question 11

What is the “atmosphere?”

Answer 11

• The atmosphere is a thin layer of air round the earth.

Question 12

What happens to the density of the atmosphere with increasing altitude/ height?

Answer 12

• The atmosphere gets less dense with increasing altitude.

Question 13

What causes atmospheric pressure?

Answer 13

• Air molecules colliding with a surface creates atmospheric pressure.

Question 14

Why does the pressure decrease with an increase in height?

Answer 14

• The number of air molecules (above a surface) decreases as the height of the object above ground level increases.
• Less molecules = less collisions
• So, atmospheric pressure decreases with an increase in height.