1. Torque

2. Torque - definition
Torque – The ability of a force to cause an object to rotate about an axis
Or, “a force that causes rotation”
Force must act perpendicularly to the object in order to cause the rotation
Ex: to push open a door, you push perpendicularly on the door to rotate it. If you pushed parallel with the door (or along its face), it wouldn’t open!

3. Torque – more info Fulcrum – the point about which the object rotates
Can you identify the fulcrum in the following pictures?
3 m
x = ? meters
30 kg
45 kg

4. Pair & Share
Each person will be assigned an example. Take 30 seconds to consider your answers and be prepared to share with your group.
Identify the force in each of the following situations.
Is the force perpendicular or parallel to the object?
Does it cause torque (or rotation about an axis?)
 Pushing a child on the back while he/she is on a swing.
 A child’s weight acting downward on the face of a see-saw.
 Pulling down on a wrench as you try to unscrew a bolt.
 Wiping dust off the face of your TV screen with the palm of your hand.

5. Pair & Share - Answers
Were you correct? If not, what misconceptions led you astray?
1. Identify the force in each of the following situations. Is the force perpendicular or parallel to the object? Does it cause torque (or rotation about an axis?)
Pushing a child on the back while he/she is on a swing. Force is the push from your hand – it acts perpendicularly to the child’s back. Yes, it causes torque because the child rotates about the axis of the swing.
A child’s weight acting downward on the seat of a see-saw. Force is the child’s weight – it acts perpendicularly to seat of the see saw. Yes, it causes torque because the see saw rotates about its fulcrum.
Pulling down on a wrench as you try to unscrew a bolt. Force is the pull of your hand downward against the wrench – it acts perpendicularly to wrench. Yes, it causes torque because the wrench rotates about the bolt.
Wiping dust off the face of your TV screen with the palm of your hand. Force is your hand pushing dirt off the screen - it acts parallel to the TV. It does NOT cause torque because you are not rotating the TV – the force is not perpendicular to the surface.

6. Torque - Equation τ = Torque (N*m) F = Force (N)
r = Perpendicular distance from the applied force to the axis of rotation (m)

7. Which way will it Twist?
Torque will cause an object to twist clockwise (CW) or counterclockwise (CCW).
CCW twist = positive torque
CW twist = negative torque
+ τ
- τ

8. “Right-Hand Rule”
Hold your right hand up into the air. This won’t work with your left hand!
Curl your fingers into a backwards “C”
Twist your hand/wrist to the left (counter-clockwise) – this feels natural, right?
Now, turn your hand (still your right hand), so that your fingers are facing to the right. Try to twist your hand/wrist to the right – this feels totally unnatural (and may hurt a little bit), right?
The CCW twist (to the left) means positive torque because it feels natural
The CW twist (to the right) means negative torque because it hurts and is unnatural.

9. Sample Problem
Two children are sitting on a see saw as shown in the picture. The see saw is supported directly in the middle by a fulcrum. How far from the fulcrum should the 45 kg child sit in order to balance the see saw?
3 m
x = ? meters
30 kg
45 kg

10. Sample Problem, cont’d
For the see saw to stay balanced, net Torque (Σ τ) must = 0!
This is called “static equilibrium” – a condition where objects are not rotating
The 45 kg child’s torque must be equal (and opposite) to the 35 kg child’s torque

11. Sample Problem, cont’d Στ = 0 Τ30 + (-T45) = 0 (F30)(r) - (F45)(r) = 0
30 kg child causes a CCW twist – positive torque
45 kg child causes a CW twist – negative torque
Στ = 0
Τ30 + (-T45) = 0
(F30)(r) - (F45)(r) = 0
(mg30)(r) - (mg45)(r) = 0
(30*9.8)(3 m) - (45*9.8)(x) = 0
(x) = 0
882 = 441(x)
882 / 441 = x = 2m
x = 2m
the 45 kg child must sit 2 m away from the fulcrum in order for the see saw to not rotate 

12. Archimedes: "Give me but one firm spot on which to stand, and I will move the earth."