P11 Forces and Pressure

Question 1

What is the angle of the force exerted on a surface when a particle in a fluid collides with it?

Answer 1

90 Degrees (along the normal)

Question 2

What equation links pressure, force and area?

Answer 2

Pressure (Pa) = Force normal to the surface(N)/

Area of surface(m^2)

Question 3

Why is there a greater pressure closer to the surface of the earth?

Answer 3

There are a greater number of gas particles in a fixed volume, and therefore a greater number of collisions on a surface

Question 4

What happens to atmospheric pressure as height increases?

Answer 4

It decreases

Question 5

Why does a packet of crisps swell up on an aeroplane?

Answer 5

As planes fly, the pressure in the cabin is kept at a level slightly lower than that on the ground, meaning that the pressure in the bag is greater than that inside of the cabin, so the bag inflates as particles try to move down the pressure gradient

Question 6

Which equation links the Pressure in a column of liquid, density, gravitational field strength and height?

Answer 6

Pressure = hpg

Pa) (m) (Kg/m^3) (N/kg

Question 7

Why does the pressure of a liquid increase with depth?

Answer 7

As the depth increases, there are more fluid molecules above a surface, and therefore a greater weight acting downwards

Question 8

A silver necklace is dropped into the ocean. Explain why the necklace sinks

Density of silver is 10,490kgm^-3
Density of water is 1000kgm^-3

Answer 8

Upthrust acting on the necklace is equal to the weight of water it displaces. Water has a much lower density than silver, so the weight of the displaced water is less than the weight of the necklace. This means that the upthrust acting on the necklace is less than its weight, so the object sinks.

Question 9

Describe why atmospheric pressure decreases at increasing altitudes [6]

Answer 9

• Caused by air molecules colliding with a force, which exerts a force on the surface
• As altitude increases, atmosphere becomes less dense
• Less gas particles in a given volume
• Therefore there are less gas particles to collide with a surface, so the force and
  pressure exerted decrease
• Weight also contributes, as there are fewer air molecules at higher altitudes, the weight
  of the air above a surface will also decrease, decreasing the atmospheric pressure