Moments

Question 1

what is a moment

Answer 1

• it is the turning effect of a force

- caused when a force causes an object to rotate

Question 2

what is the equation for the size of a moment

Answer 2

• moment of a force = force x distance

- Nm = N * m

Question 3

what does a force on a spanner cause

Answer 3

• a turning effect or moment of the nut

- which acts as a pivot

Question 4

what would a larger force or a longer distance mean

Answer 4

a larger moment

Question 5

what is the distance in the equation for the size of a moment actually referring to

Answer 5

the distance normal to the direction of the force

Question 6

what do you do to get the maximum moment or turning effect of a spanner

Answer 6

• you need to push the spanner at right angles

- perpendicular to the spanner

Question 7

why do you have to push at right angles to the spanner in order to generate maximum moment

Answer 7

• because pushing at any other angle would mean a smaller distance
• and a smaller distance means as smaller moment
• this is what the ‘normal to the direction of the force’ means

Question 8

what does the principle of moments state

Answer 8

• that for an object in equilibrium

- the sum of the clockwise moments = the sum of the anticlockwise moments

Question 9

what is the physics based function of a lever

Answer 9

• to transfer the turning effect of a force
• push one end of the lever down
• and the rotation around the pivot causes the other end to rise

Question 10

why do levers make lifting things easier

Answer 10

• they increase the distance from the pivot at which the force is applied
• so the longer the lever, the smaller the force needed to give the same moment
• expressed in the equation moment = force x distance

Question 11

there is a 2.25m long lever with a rock on the very left and the pivot 0.25m from the left of the lever. The rock applies a force of 800N on the very left while someone on the vey right applies a force of 150N downwards. Calculate the moments of each force and the net force and deduct whether the rock would rise or fall from the forces applied

Answer 11

• as the rock is on the very left and the pivot is 0.25m away from the very left, the distance between the rock and the pivot is 0.25m
• moment = force x distance, so 800N x 0.25m = 200Nm ANTICLOCKWISE
• for the right side, the distance between the very right and the pivot is 2.25 - 0.25 = 2m
• 150N x 2m = 300Nm CLOCKWISE
• 300 - 100 = net CLOCKWISE MOVEMENT OF 100Nm
• meaning the rock will rise

Question 12

what are gears

Answer 12

circular cogs with teeth around their edge

Question 13

what do gears do

Answer 13

• their teeth interlock

- so that turning one causes the other one to turn in the opposite direction

Question 14

why does one turning gear cause the other one attached to it to rotate

Answer 14

because the gear transmits the rotational effect of its force from itself to another gear

Question 15

what is the difference in how a small and large gear would react when interlocked

Answer 15

• a force applied to a small gear creates a small moment
• the gear applies the same force to the next gear
• if this gear is larger the force is applied further from the pivot
• and as moment = force x distance
• the moment created by the large gear is bigger

Question 16

what is the correlation between the speed at which a gear turns and its size, assuming the force remains constant

Answer 16

the larger the gear, the slower it spins

Question 17

what do gear ratios allow you to do

Answer 17

work out how the speed and moments will change between gears

Question 18

a small gear with 8 teeth is interlocked with a large gear with 16 teeth. how would the gears move compared to each other using their gear ratios

Answer 18

• the ratio of teeth between the large and small gear is 16 : 8, or simplified to 2 : 1
• this means that for every 1 turn the large gear does
• the small gear does 2 turns

Question 19

why is the ratio of moments of two gears equal to the ratio of the gears radii

Answer 19

• the force applied to each gear is the same
• the radius of the gear is equal to the distance of the applied force to the pivot (from the teeth to the center)
• as moment = force x distance
• the ratio of moments = ratio of gears radii as the force is constant

Question 20

using the knowledge that the ratio of moments = ratio of gears radii, what is the ratio of moments between the 16 and 8 teeth gears

Answer 20

• it is 16 : 8 or 2 : 1
• as the 16 teeth has twice the distance between the pivot and the area that the force is applied (the teeth)
• the force is constant, so the moment will just be twice as big

Question 21

why is lubrication used on gears in real life

Answer 21

• lubrication reduces friction and unwanted energy transfers

- so gears are lubricated to increase the efficiency of machines