Balanced Forces
Levers
Write out the statements that are true.
• a The longer the lever, the bigger the force that is needed to move an object. • b It is easier to close a door if you push the door close to the hinge • c The shorter the lever, the bigger the force that is needed to move an object • d Joints are examples of pivots. • e Bones are examples of levers.
C, D and E
Learning Objective
To investigate, through practical experimentation, the principle of moments.
Recording your results
• What do we need to record? • How many columns will we need in our table?
Recording your results
Weight and Mass
YouTube - Eureka! Episode 6 Gravity
• YouTube - Eureka! Episode 7 - Weight vs. Mass
Racing Balls
Write out each term along with its correct description
unbalanced system
Descriptions • anticlockwise moments = clockwise moments • two boys of different weights sit opposite each other on a see saw, both the same distance from the pivot Lever Principle GCSE PHYSICS: • the turning effect of a force Moments
Moment calculation
Gina weighs 500 N and stands on one end of a seesaw. She is 0.5 m from the pivot.
What moment does she exert?
moment = 500 x 0.5
= 250 Nm 0.5 m
500 N
pivot
Moment equation
The moment of a force is given by the equation:
moment = force (N) x distance from pivot (cm or m)
moment
f
x d
Moments are measured in Newton centimetres (Ncm) or Newton metres (Nm).
Principle of moments
The girl on the left exerts an anti-clockwise moment, which equals... her weight x her distance from pivot
The girl on the right exerts a clockwise moment, which equals... her weight x her distance from pivot
Principle of moments
If the anticlockwise moment and clockwise moment are equal then the see-saw is balanced. This is known as the principle of moments. When something is balanced about a pivot: total clockwise moment = total anticlockwise moment
Principle of moments – calculation
Two girls are sitting on opposite sides of on a see-saw. One girl weighs 200 N and is 1.5 m from the pivot. Where must her 150 N friend sit if the seesaw is to balance? When the see-saw is balanced:
total clockwise moment = total anticlockwise moment
200 N x 1.5 m = 150 N x distance 200 x 1.5 = distance 150 distance of second girl = 2 m
Anagrams
Why don’t cranes fall over?
Tower cranes are essential at any major construction site. trolley load arm
counterweight
loading platform tower Concrete counterweights are fitted to the crane’s short arm. Why are these needed for lifting heavy loads?
Why don’t cranes the crane balanced? fall over? Using the principle of moments, when is
3m 6m ? moment of load =
10,000 N moment of counterweight
If a 10,000 N counterweight is three metres from the tower, what weight can be lifted when the loading platform is six metres from the tower?
Why don’t cranes fall over?
moment of load = load x distance of load from tower = ? x 6 moment of = counterweight x distance of counterweight counterweight from tower = 10,000 x 3 = 30,000 Nm moment of load = moment of counterweight ? x 6 = 30,000 ? = 3,000 6
? = 5,000 N
Crane operator activity
Where should the loading platform be on the loading arm to carry each load safely?