Second Year Science Moments, Levers and Centre of Gravity Ms Rochford
In this topic: Levers Moment of a force Centre of Gravity
Levers
Levers are everywhere!
Levers Lever: a rigid body that can rotate (pivot) around a fixed point Fulcrum: the fixed point when using a lever Examples of levers: Door handle Wheelbarrow Wrench
How a Lever Works Fulcrum fixed point around which the lever turns Load Effort The force (usually a weight) that you want to move Force applied by YOU EFFORT FULCRUM
How a lever works There are 3 terms associated with levers Force (or effort) Load Fulcrum
Where is the Fulcrum?
Moment
What happens when you try to open a door with one finger? Where do you push? Hi, my name is Mr Stick hinges
Do you push near the hinges? I can t do it!
Do you push far from the hinges? That s easier!
The turning effect of a force depends on two things: 1. The size of the force Obviously!
The turning effect of a force depends on two things: 2. The distance from the fulcrum (axis of rotation) Not quite so obvious! Fulcrum
Moment Moment: the turning effect of a force It is equal to the force multiplied by the perpendicular distance from the fulcrum
Moment Moment = Force x Perpendicular distance from fulcrum to line of action of the force Units: Newton metres (Nm)
Calculate the moment of the force in the diagram below Moment = Force x distance from fulcrum to line of force action Force = 7 N distance from fulcrum to force action = 0.8 m Moment of the force = 7 N x 0.8 m Moment of the force = 5.6 Nm
5N 1. What does this Force number Mean? 2. What is Fulcrum this? 3. Which direction will the Anti-clockwise see-saw turn?
5N Force Fulcrum Anti-clockwise
How can we increase the size of the moment (the turning effect)? Fulcrum Distance from force to fulcrum Force (effort)
10cm 5N 50 Ncm Fulcrum Calculate the moment (turning effect) of the force applied to the see-saw. Give the units.
5cm 5N 25 Ncm fulcrum Calculate the moment (turning effect) of the force applied to the see-saw. Give the units.
10cm 10N 100 Ncm fulcrum Calculate the moment (turning effect) of the force applied to the see-saw. Give the units.
Law of the Lever When a lever is balanced, the sum of the clockwise moments equals the sum of the anticlockwise moments
The girl on the left exerts an anti-clockwise moment The girl on the right exerts a clockwise moment When an object is balanced it is in equilibrium
10m 10m Left Moment Right Moment 100 Nm 100 Nm 10N 10N fulcrum The The moments see-saw of is the BALANCED. forces are But balanced. why?
10m 7m Left Moment Right Moment 100 Nm 70 Nm 10N 10N fulcrum But what happens if the blue dragon moves in a little?
10m 7m Left Moment Right Moment 100 Nm 70 Nm 10N 10N fulcrum The moment of force on the left is bigger
10m 10m Left Moment Right Moment 100 Nm 200 Nm 10N 20N fulcrum What happens if the blue dragon gets fatter?
10m 10m Left Moment Right Moment 100 Nm 200 Nm 10N 20N fulcrum The moment of the force is greater on the right
10m 10m Left Moment Right Moment 100 Nm 200 Nm 10N 20N fulcrum But can we balance them both again? How?
10m 5m Left Moment Right Moment 100 Nm 100 Nm 10N 20N fulcrum By reducing the distance from the fulcrum for the blue dragon
Centre of Gravity
Centre of Gravity Centre of Gravity: A point through which all of the weight of an object appears to act Or it can also be described as: The point from which an object balances
Centre of Gravity If a fulcrum is set up at the centre of gravity, it will balance A metre stick s centre of gravity is at 50 cm
Activity: To find the centre of gravity of a thin lamina Pin the lamina to the cork through 1 hole and hang the plumb line Mark out the line Repeat through other 2 holes
Activity: To find the centre of gravity of a thin lamina Centre of gravity = Where all 3 lines intersect
Equilibrium Remember: Equilibrium: when an object is balanced and still A metre stick hung at 50 cm (its centre of gravity) is in equilibrium
3 States of Equilibrium Stable Equilibrium The ball will return to the same position if given a slight push Unstable Equilibrium Neutral Equilibrium The ball will not return to the same position if given a slight push The ball will return to a similar position if given a slight push
Stability If objects are stable, they won t fall over easily If the object s weight acts from the centre of gravity, through its base, it is stable If the object s weight acts from the centre of gravity, outside its base, it is unstable and will fall
Stability To improve stability: 1. Use a wide base 2. Lower the centre of gravity
Chapter 35: Moments, Levers and Centre of Gravity DONE!!