Mechanics

Question 1

define moment

Answer 1

A turning effect of a force.

Question 2

what moments does a body in equilibrium (balanced object) experience about any pivot?

Answer 2

equal sums of clockwise and anticlockwise moments about any pivot

Question 3

why does distance, d have to be taken as the perpendicular distance from the line of action of the force to the pivot?

Answer 3

because only perpendicular forces cause moment

Question 4

what 3 things does a lever consist of? explain each one

Answer 4

• a pivot- A point around which something can rotate or turn.
• an effort- Force used to move a load over a distance.
• a load- The overall force that is exerted, usually by a mass or object, on a surface.
  these can be in different orders, causing different effects and uses

Question 5

describe the relationship between the length of a lever, the distance the effort acts from the pivot and the force on the load. give an example

Answer 5

The longer the lever, and the further the effort acts from the pivot, the greater the force on the load will be. It is easier to use a longer spanner when trying to turn a nut, and easiest to push furthest from the hinge when opening a door.

Question 6

what do levers do and what does this mean?

Answer 6

• they increase the distance from the pivot at which the force is applied since M=Fd this means less force is needed to do the same moment
• this means levers make it easier to do work e.g. lift a load or turn a nut

Question 7

how do gears transmit rotational effects? include the effect of the size of the gear

Answer 7

1. their teeth interlock so that turning one causes another to turn, in the opposite direction
2. they are used to transmit the rotational effect of a force from one place to another
3. different sized gears can be used to change the moment of the force. a force transmitted to a larger gear will cause a bigger moment, as the distance to the pivot is greater
4. the larger gear will turn slower than the smaller gear

Question 8

define fluid

Answer 8

A substance that can flow, such as a liquid or a gas.

Question 9

what does pressure in a fluid cause?

Answer 9

a force normal (perpendicular) to a surface/ the surface of it’s container

Question 10

why does the pressure at a point in a fluid increase with the height of the column of fluid above?

Answer 10

• water at the bottom of a container is pushed on by the weight of the water further up, which causes it to be under higher pressure
• therefore pressure increases as the depth increases because pressure in a liquid is due to the weight of the column of water above

Question 11

define upthrust

Answer 11

Upwards force exerted by a liquid or gas on an object floating in it.

Question 12

why does the density of a fluid have an effect on the upthrust experienced by an object submerged in it?

Answer 12

If the density of the object is greater than the density of the fluid, the object can never displace enough fluid to create an upthrust that will hold its weight up so it sinks

Question 13

explain why an object floats or sinks

Answer 13

• If the upthrust is less than the weight of the object, the object will sink.
• If the upthrust is larger than the weight of the object, the object will float.
• If something has a density that is less than the density of the water, the object will float.
• And if something has a density that is greater than the density of the water, the object will sink.

Question 14

why does atmospheric pressure vary with height above a surface?

Answer 14

Atmospheric pressure decreases as the height of a surface above ground level increases.
This is because, as the altitude increases:
* the number of air molecules decreases
* the weight of the air decreases
* there is less air above a surface

Question 15

what is stopping distance equal to?

Answer 15

thinking distance + braking distance

Question 16

what is a typical human reaction time?

Answer 16

between typically 0.2 s and 0.9 s

Question 17

what is momentum? what type of quantity is it and what is it measured in?

Answer 17

• A quantity relating to a moving object that is calculated by multiplying its mass by its velocity.
• vector quantity
• kg m/s

Question 18

how do you complete a conservation of momentum question involving two objects?

Answer 18

1. First calculate the momentum of both trolleys before the collision
2. add them together to get a total
3. this is equal to the momentum after the collision
4. rearrange p=mv to find what you want to find

Question 19

what happens to momentum when a force acts on an object that is moving and what happens if this force is increased?

Answer 19

• When a force acts on an object that is moving, or able to move, there is a change in momentum (m∆v)
• an increased force delivers an increased rate of change of momentum

Question 20

how do you calculate calculate the force involved when a change in momentum happens?

Answer 20

• because Acceleration (α) appears in both the equations
• force=mass x acceleration
• and acceleration= change in velocity/ time taken
• we can make force= change in momentum/ time taken (given)

Question 21

explain how car safety features work in general

Answer 21

• During a collision there is a change in momentum.
• The force of the collision is equal to the rate of change of momentum.
• Car safety features (seatbelts, airbags and crumple zones) all work to change the shape of the car, which increases the time taken for the collision.
• These different safety features decrease the rate of change of momentum, which decreases the force of the collision on any people within the car.

Question 22

how do air bags promote safety?

Answer 22

• inflate before you hit the dashboard of a car
• the compressing air inside it slows you down more gradually than if you had just hit the hard dashboard
• this decrease the rate of change of momentum
• which decreases the force of the collision on any people within the car

Question 23

how do seat belts promote safety?

Answer 23

• they stretch slightly
• this increases the time taken for the passenger to stop
• this decrease the rate of change of momentum
• which decreases the force of the collision on any people within the car

Question 24

how do helmets promote safety?

Answer 24

• contain a crushable layer of foam which helps to lengthen the time taken for your head to stop in a crash
• this reduces the impact on your brain
• because of the decrease in the rate of change of momentum which decreases the force of collision

Question 25

how do cushioned surfaces promote safety?

Answer 25

• increase the time taken for you to stop if you fall on them
• because they are made from soft, compressible materials
• of the decrease in the rate of change of momentum which decreases the force of collision