2. Forces and Loads GRADE SEVEN SCIENCE

3. Introducing Forces A force is a PUSH or PULL THINKING QUESTION: –Why don’t bumper cars cause serious injuries while cars hitting each other on the road do? KEY CONCEPT The effect of a force depends on its magnitude, direction, and the point of application of the force

4. Magnitude of Force Magnitude is a measure of how strong the force is. In diagrams, the magnitude of a force is indicated by the length of the arrow. A weak force (low magnitude) is not enough to make the block move to the right. A strong force (high magnitude) moves the block to the right.

5. Direction of Force The direction of a force affects how a structure will react to it. EXAMPLE: –If a door says “push”, it won’t open if you use a pulling force

6. Point of Application of Force Where and how a force is applied will affect the possible movement of the structure THINKING QUESTION: –How does a dresser react if you push it at the top compared to the bottom

7. ACTIVITY: Applying Force Moving Desks Wrecking Ball Demo Strong Finger

8. ACTIVITY: Forces in the Class Complete the following table. In first column list nine objects in the classroom. In the second column list one force that does or could act on each of them. ObjectForce that Acts upon the Object Teacher DeskCoffee Mug that pushes on top

9. Introducing Loads THINKING QUESTION: –What happens when you place too many groceries in a single plastic bag? Loads are usually grouped into two categories: –Static loads – caused by force of gravity –Dynamic loads – caused by other forces KEY CONCEPT The LOAD, the effect of the forces acting on the bag is too great for the structure

10. Classifying Loads LOADS STATIC - caused by gravity LIVE - can change DEAD - always the same DYNAMIC caused by other forces

11. Static versus Dynamic Loads Consider a bicycle….. DEAD STATIC force = LIVE STATIC force = DYNAMIC force =

12. Structural Design and Loads The engineer designing a structure must try to anticipate of the loads that the structure will bear If a structure cannot even bear the dead load of its own weight – it will fail Engineers must also think of possible dynamic forces that could exist EXAMPLE – Tacoma Narrows Bridge

13. EVENT: Tacoma Narrows Bridge At 11:00 a.m. on 7 November 1940, Barney Elliott captured footage of the Tacoma Narrows Bridge collapsing due to wind force. This video has gone on to become famous among engineers and high school physics classes. Movie Footage of Tacoma Narrows Bridge

14. Thinking about your Coaster What static and dynamic loads must engineers consider in designing real roller coasters? How about bridges? What types of loads must you consider in the design of your coaster and bridge?