P11 Force and Pressure

Question 1

What is the equation for pressure?

Answer 1

pressure = force / area

Question 2

What is pressure?

Answer 2

The force per unit area

Question 3

Describe the relationship between pressure and depth (in a liquid)?

Answer 3

The pressure in a liquid increases with increasing liquid depth

Question 4

Describe the relationship between pressure and density (in a liquid)?

Answer 4

The greater the density of a liquid the greater the pressure in a liquid

Question 5

What is the equation for pressure of a liquid column?

Answer 5

pressure = height x density x gfs

Question 6

Describe the pressure along a horizontal line.

Answer 6

The pressure along a horizontal line is constant. A liquid flows until the pressure along the same horizontal line is constant.

Question 7

What is the equation for pressure at sea level?

Answer 7

pressure = height x density x gfs

Question 8

Explain the relationship between atmospheric pressure and altitude.

Answer 8

Atmospheric pressure decreases with higher altitude because there is less air above a given altitude than there is at a lower altitude. Therefore there a fewer collisions between the air molecules and less pressure at a higher altitude.

Question 9

Explain the relationship between density and altitude.

Answer 9

The density of the atmosphere decreases with increasing altitude. This is because the weight of air pressing down on a horizontal surface at any altitude decreases with increasing altitude. So there are fewer molecules per cubic metre and therefore fewer collisions per second at a higher altitude.

Question 10

What is the upthrust of an object in a fluid?

Answer 10

The upward force the fluid exerts on the object. It is caused by the pressure of the fluid.

Question 11

Explain why an object floats or sinks.

Answer 11

An object floats when its weight is equal to the upthrust. An object sinks when its weight is greater than the upthrust.

Question 12

Explain why sharp knives are used in hospitals instead of blunt knives.

Answer 12

• A sharp knife cuts more easily than a blunt knife
• A sharper knife has a much smaller area of contact than a blunt knife does
• So the pressure of a sharp knife is much bigger than a pressure of a blunt knife with the same force
• So when the same force is applied with a sharp knife, it has a much bigger cutting effect than it would have with a blunt knife

Question 13

How do you calculate the force on a flat object due to a pressure difference?

Answer 13

the pressure difference x the area of the flat surface

Question 14

Define perpendicular force.

Answer 14

Component of the force which is at right angles to the surface its colliding with

Question 15

Explain how fluids exert pressure on their containers.

Answer 15

The liquid or gas particles collide with the walls of the container. These collisions exert a force onto the walls of the container, and this force is pressure.

Question 16

What is meant by the term ‘fluid’?

Answer 16

A liquid or gas

Question 17

In any fluid, at what angle do the forces due to pressure act on a given surface?

Answer 17

At right angles to the surface - perpendicular force

Question 18

Factors which determine the pressure exerted by a liquid

Answer 18

DEPTH - at a greater depth, more pressure is exerted
DENSITY OF LIQUID - more dense liquids exert a greater pressure
GFS - which is 9.8N/kg on earth

Question 19

What is upthrust caused by?

Answer 19

The pressure on the bottom of the object being greater than the pressure on the top of the object

Question 20

What is the Earth’s atmosphere?

Answer 20

A thin layer of gas, relative to the magnitude of the Earth, surrounding the Earth

Question 21

How does pressure in fluids increase with depth?

Answer 21

As depth increases, the volume of liquid above that depth also increases. This means that the force due to the volume increases. Since the force has increased whilst the area remains constant the pressure will increase.

Question 22

Why does pressure in a fluid increase with density?

Answer 22

As the density of a fluid increases, the amount of particles in a given volume increases. Consequently, the weight of the fluid is greater. This means the force from the fluid above a certain point is greater. Therefore, the pressure increases.

Question 23

Why are objects in a fluid subject to upthrust?

Answer 23

When an object is submerged in a fluid, it has a higher pressure below it than it does above, as there is more weight above it at the bottom than there is at the top. This leads to the upwards force, upthrust. The object will either float or sink depending on whether its weight is greater or equal to the upthrust.

Question 24

What is upthrust always equal to?

Answer 24

The weight of the fluid that the object displaces

Question 25

What factors influence whether an object will float or sink?

Answer 25

• Upthrust
• Weight
• Density of liquid

Question 26

Explain why an object with a density greater than that of water can never float?

Answer 26

Upthrust is always equal to the volume of fluid that the object displaces. If the density is too high, there would not be enough volume displaced to produce an upthrust greater than the object’s weight. This means that it will sink.