P11 - Force and Pressure

Question 1

What is Pressure?

Answer 1

Pressure is the force normal to a surface, divided by the areas of the surface.

Unit: Pascals

Question 2

What is the equation for Pressure?

Answer 2

Pressure = Force / Area

P = F / A

(Pa) = (N) / m^2

Question 3

What 1 Pascal equal to?

Answer 3

1Pa is equal to 1N/m^2

Question 4

What 2 factors increase the pressure on a surface?

Answer 4

1) Bigger force

2) Smaller surface area

Question 5

How and why does depth affect pressure in a liquid?

Answer 5

The pressure in a liquid increases with depth.

This is because there is a greater *weight (N) of water above.

*remember that p is bigger when f (N) is bigger.

Experiment - 3 holes in a water bottle:
-the water jet at the bottom shoots the furthest at the greatest pressure.

Question 6

How and why does density affect pressure in a liquid?

Answer 6

The pressure in a liquid increases with greater density.

This is because the particles are closer together and there will be more collisions between particles.

Question 7

How does height affect pressure in a liquid?

Answer 7

The pressure in a liquid increases as the height of the liquid column increases.

There is greater pressure at the bottom of the column.

Question 8

What equation used to calculate pressure in a liquid uses density, depth (height) and gravity?

Answer 8

pressure = height x density x gravity

p = h x ρ x g

(Pa) = (m) x (kg/m^3) x (N/kg)

Question 9

Why is the turbine in a dam placed at the bottom of the dam rather than at the top?

Answer 9

Because pressure is greatest at the bottom and therefore the greatest amount of force can be exerted from the bottom of the dam (F = P x A)

Question 10

Calculate the pressure exerted at the bottom of a fresh water lake that is 120m deep, has a density of 1000kg/m^3 and where gravity is 10N/kg.

Answer 10

p = hρg

p = 120 x 1000 x 10 = 1,200,000 Pa

Question 11

What is air pressure also known as?

Answer 11

Atmospheric pressure

Question 12

What is the atmosphere? How does the density of the atmosphere change with altitude?

Answer 12

The atmosphere is a thin layer of air surrounding the earth.

The atmosphere gets less dense with increasing altitude.

Question 13

What is atmospheric pressure caused by?

Answer 13

It is caused by the collisions of gas particles with eachother and surfaces.

Question 14

Why does atmospheric pressure decrease with increased altitude?

Answer 14

The density of the atmosphere and the weight of the air above decreases - there are fewer collisions between particles which means less pressure.

Question 15

How can air pressure be increased?

Answer 15

• Increasing the temperature
• Increasing the number of particles - increases the frequency of collisions
• Decreasing the volume of gas - increases frequency of collisions

Question 16

What is upthrust and what is it caused by?

Answer 16

Upthrust is an upwards resultant force caused by a larger pressure on the bottom and a smaller pressure on the top.

This applies to a partially or totally submerged object.

Question 18

What principle describes why the water level in a container rises when an object is submerged in it?

Answer 18

Archimides’ Principle

When an object is fully immersed in water, the volume of water displaced is equal to the volume of the object displacing it.

Reasoning: An object immersed in a fluid experiences an upthrust equal to the weight of the fluid displaced.

The bigger the volume of water displaced —> the bigger the weight —> the bigger the upthrust.

Question 19

When does an object float?

Answer 19

When the object’s weight is equal to the upthrust.

Less dense than water - 1g/cm^3

Question 20

When does an object sink?

Answer 20

When the object’s weight is greater than the upthrust.

More dense than water - 1g/cm^3