Integrated Natural Science

Integrated Natural Science

for Detroit Public

Schools

LeversKat Woodring

Key Questions: 4.2.1 Analyze and label the parts of a lever

and evaluate how a lever multiplies force. 4.2.2 Provide examples of first, second and

third class levers. 4.2.3 Compare parts of the human body to

the types of levers. 4.2.4 Calculate and determine the

mechanical advantage of a lever.

District Outcomes

Qualitatively and quantitatively explain forces and charges in motion.

Observe and explain forces as push and pull, acting on an object and exerted by the object.

Analyze the operations of machines in terms of force and motion.

Lever Assembly

SAFETY NOTE:

WATCH for FALLING weights on bare toes or sandals or table tops!

DO not place the fulcrum higher than hole 3 of stand!

4.1 Forces in Machines

A simple machine is an unpowered mechanical device, such as a lever.

Introducing… The Lever

A lever includes a stiff structure (the lever) that rotates around a fixed point called the fulcrum.

fulcrum

Anatomy of the lever Fulcrum – point around which the lever

rotates

Input Force – Force exerted ON the lever

Output Force – Force exerted BY the lever

Levers and the human body

Your body contains muscles attached to bones in ways that act as levers.

Here the biceps muscle attached in front of the elbow opposes the muscles in the forearm. Can you think of other

muscle levers in your body?

Three Classes of Levers

First Class - fulcrum between Input and output

Second Class – output between fulcrum and input

Third Class – input between fulcrum and output

CPO Lever – First Class All The Way

Here we have a first class lever

The fulcrum is between the input and output

Can you get two weights to balance?

Levers in Equilibriu

m

Hang your weights like shown here

Does the lever balance?

What variables can be changed to balance a lever?

Four Variables in a Lever

Amount of Input Force

Amount of Output Force

Length of Input Arm

Length of Output Arm

Lever Challenge Hang weights

from the lever and get it to balance.

Use at least 3 strings!

Do 4 trials and record how many weights to hang and where you hang them.

Lever Challenge

Lever Modification Hang 1 weight

10 cm from the fulcrum.

Where does the output force need to be to oppose our input force?

1 1

Basic Lever Investigation If we move the

input force 10 cm, how much more do we need to add for the same output force?

Try it...

1

Basic Lever Investigation If we move the

input force 10 more cm, how much more do we need to add for the same output force?

Add two masses at 20 cm.

HINT: you will need two strings

1

Basic Levers Investigation

Mathematical Rule for Balancing the Lever

What mathematical relationship can you find that will balance the lever every time?

Put your rule in terms of input and output and forces and distances.

What if there is more than one location on either side of the lever?

What is the Relationship?

Force x Distance = Force x Distance

Input Force

x Length of Input Arm

# of Weights x Distance

Output Force

x Length of Output Arm

=

# of Weights x Distance=

What if there several groups of weights ?

Sum of Input = Sum of Output

(F1 x D1) + (F2 x D2) (F3 x D3) + (F4 x D4) =

Mechanical Advantage We use the same kind of relationship for all simple

machines to calculate Mechanical Advantage.

Output Force / Input Force

4.1 Mechanical Advantage

MA = Fo

Fi

Output force (N)

Input force (N)

mechanicaladvantage

Michigan Content Expectations

P4.1c Explain why work has a more precise scientific meaning than the meaning of work in everyday language.

P4.1d Calculate the amount of work done on an object that is moved from one position to another.

P4.1e Using the formula of work, derive a formula for change in potential energy of an object lifted in a distance h.