A Balancing Act

1. A Balancing Act

2. When is an object in Equilibrium? • When its state of motion is not changing What are the conditions for Equilibrium? 1) the net force acting on the object must be zero 2) There is a second condition necessary to ensure that the object does not rotate – there must be no net turning effect of forces rotating an object around a pivot point

3. What happens when there is a net turning effect? • The object turned or twisted • turned in the direction of the side that had more of a turning force

4. Redefining the Turning Force • The turning force that makes objects rotate or turn when not in equilibrium is torque. • Torque can be thought of as the twist applied to an object to make that object turn about its axis of rotation.

5. Finding Torque • Think back to the balancing acts activity: the two things that matter the most when trying to balance the system, were: • 1) the number of paper clips • 2) the distance of placement of paper clips from the fulcrum Restating what we know: The two factors that determine torque are 1) the perpendicular component of the force 2) the distance from the point of application to the axis of rotation

6. Torque • Torque= F * D (very important to make sure you are using the perpendicular component of the force) • Force is measured in Newtons or pounds • Distance can be measured in meters or feet • Therefore the units for torque are N*m or lbs*ft

7. Weight on the end of a stick • When the weight is far from the point of rotation, the torque is greater than if the weight is moved closer to the pivot point • Note the weight of the object is the perpendicular component of the force

8. Opening the Door • Challenge –try to open the • door with one finger Why is it hard to open this way when you apply a force close to the hinges? Why are door handles placed on the side of the door farthest from the hinges? You will create more torque on the door by applying a force at the greatest distance possible from the pivot point (hinges).

9. P W W F D F D W W D P F W W a D2 D1 P F1 F2 Diagrams of Torque T= F*D T=0 T= F*D T= F*D cos a T1= T2 or F1*D1=F2*D2

10. Examples of Torque • Changing your car tire • Car Engine http://science.howstuffworks.com/fpte4.htm http://science.howstuffworks.com/engine1.htm

11. Calculating Torque • Find the torque exerted by a wrench on the bolt T= F*d t=(50 lb)(0.5 ft) = 25 lb*ft

12. Back to Equilibrium • We have talked about how to get a seesaw to balance with a person who weighs twice as much as the other - Have the person which weighs half as much sit twice as far from the pivot point of the seesaw. When the seesaw became balanced, the net torque would equal zero. When we balanced our pieces of cardboard with paper clips, the net torque was also zero.

13. Back to Equilibrium • As stated before - There is a second condition necessary to ensure that the object does not rotate – there must be no net turning effect of forces rotating an object around a pivot point We can restate this as a situation in which the sum of all the clockwise (CW) torques must equal the sum of all the counterclockwise (CCW) torques.

14. Problems of Equilibrium A 20 ft seesaw has a fulcrum in the center. If you have a 100 lb person sitting at one end, at what distance should the 175 lb person get on so that the seesaw is balanced? 100 lb 175 lb 20 ft (175 lb)d =(100 lb)(10 ft) d= (100 lb)(10 ft) 175 lb d= 5.71 ft T CW=T CCW

15. How can I get more Torque? • What are three ways that you could increase the torque? • Increase your force T= F * d • Make your lever bigger (get a bigger wrench) T= F * d 3) How you apply the force (not changing the force magnitude or size of the lever)

16. Force Application How you apply the force matters as well as the size and the distance. This diagram helps us to understand which scenario can give maximum torque.