P11 Force and pressure Flashcards
define pressure
the force (normal to a surface) per unit area
equation for pressure (3 things)
pressure = force / area
P (Pa) = F (N) / A (m^2)
1Pa = ???N/m2
1Pa = 1N/m2
give the units for pressure
pascals (Pa) or Newtons per metre squared (N/m2)
Chefs say that using a sharper knife is safer.
If the pressure to cut something is always constant, why would chefs say this?
If a knife is sharper the area is smaller.
Using the equation:
Force = Pressure x Area
So, a smaller area and the same pressure means less force.
explain why you can push a drawing pin into a board without hurting your thumb. you should refer to pressure in your answer.
- Your thumb exerts the same force on the pin as the pin exerts on the board.
- The area for your thumb is larger than the area in contact with the board.
- pressure = force/area
- So same force and larger area means less pressure.
A tube is filled with water. It has 3 holes in the side at different heights.
Will the water get pushed further out of the top, middle or bottom hole?
- bottom because that’s where the pressure is highest
- the pressure in a liquid increases with increasing depth
- because the weight of the water above is greater as there is more water above
- so the water is pushed out furthest through the bottom hole
the pressure in a liquid increases with increasing ___ ?
depth
“the pressure in a liquid increases with increasing depth”
a liquid will flow until ___ is constant
pressure
a liquid will flow until pressure in constant
how does the pressure in a liquid change as depth increases
also increase
the pressure in a liquid increases with increasing depth
a liquid will flow until pressure is ___
- constant
- a liquid will flow until pressure is constant
- this means for the same liquid it will be along the same horizontal level
explain how density affects the pressure in a liquid (refer to particles)
- higher density means there are more liquid particles per unit volume
- this means they will collide with each other (and the walls of their container) more frequently (–> “collision frequency increases”)
- particles exert more force on each other and on walls of container in a given time
- causes the pressure to increase
- therefore as density of a liquid increases, the pressure in the liquid also increases
the greater the density of a liquid, the ___ the pressure in the liquid
greater
the greater the density of the liquid, the greater the pressure in the liquid
equation for pressure (4 things)
- Pressure = height x density x gravitational field strength
- p = h x ρ x g
- (Pa), (m), (kg/m3), (N/kg)
if the internal gas pressure and external atmospheric pressure of a balloon are balanced, what happens to its size?
stays the same
why / how do gases exert pressure?
gases exert pressure because the particles collide with surfaces, causing forces on them
what would happen to a balloon’s size as it gained altitude and why?
- altitude increases
- density of air decreases
- fewer particles hit the outside of the balloon, causing pressure
- balloon expands
- the balloon expands to balance its internal gas pressure with the external atmospheric pressure
why does a balloon increase in size / expand as it gains altitude?
- this is because when the balloon expands, there is more space between gas particles / distance between gas particles increases
- so there are less frequent collisions between particles and the inside of the balloon
- so gas pressure decreases as the balloon expands
- to balance the external atmospheric pressure
what are the symbol and unit for density
symbol is ρ, measured in kg/m3
how does a suction cup stick to a wall without any adhesive
- when someone presses on suction cup, air underneath it is pushed out
- the cup forms a seal on the outside
- this creates a vacuum
- if the cup is pulled away from the wall, air can’t get in underneath it
- this means there is lower pressure in the area underneath the suction cup than the pressure outside
- A suction cup sticks to a wall by creating a partial vacuum between the cup and the surface.
- When the suction cup is pressed against the wall, air is forced out from the inside of the cup, lowering the air pressure within the cup compared to the outside air pressure.
- This difference in pressure creates a force that pulls the suction cup tightly against the wall, allowing it to stick.
- The higher atmospheric pressure outside the cup then pushes against the cup, holding it in place.
a dusk floats in a pond. what two forces act on the duck?
- weight (downwards)
- upthrust (upwards)
how does a duck float in a pond if it
its weight and upthrust are equal
A ball floats on the water.
If you pushed the ball fully under the water, how would the size of the upthrust change?
- As the ball is submerged the upthrust increases.
- This is due to the ball displacing a greater volume of water.
- If the ball was released it would be pushed upwards back to the surface.
explain how the upthrust of a ball changes as it is submerged deeper into water
- pressure increases with increasing depth
- this means there is higher pressure acting against the bottom of the ball than the top
- therefore as the depth of the ball increases, the upthrust also increases
what is upthrust
upthrust is an upwards force due to a fluid
what causes upthrust
upthrust on an object is caused by the pressure of the fluid the it is in, as the pressure is greater on the bottom than top of the object
the larger the volume of fluid an object displaces, the ___ the upthrust
larger
the larger the volume of fluid an object displaces, the larger the upthrust
how do you know whether an object will float or sink?
the object’s density
if an object is less dense than the fluid it is in, then it floats
if an object is denser than the fluid it is in, then it sinks
describe an object’s upthrust and weight (relative to each other) when it is fully immersed in a fluid
an object’s upthrust will be greater than its weight when it is fully immersed
