Forces and Motion I

1. Forces and Motion I • Motion-linear and circular • Newton’s Three Laws of Motion • Position • Distance • Speed • Velocity • Acceleration

2. Newton’s Laws of Motion An object at rest stays at rest. An object in motion stays in motion with the same rate and in the same direction, unless acted upon by an unbalanced outside force. http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/newtlaws/u2l4a.html

3. Newton’s Second Law • The relationship between an object's mass m, its acceleration a, and the applied force F is ……………F = ma. • Things accelerate in the direction you push or pull it. • It accelerates twice as fast if you push it twice as hard. • If you double the mass it accelerates half as fast.

4. Newton’s Third Law For every force there is an equal and opposite force. Forces act in pairs equal in magnitude, but opposite in direction. http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/newtlaws/u2l4a.html

5. Position, Distance, Speed, and Acceleration • Position: Location in space • Distance: a scalar quantity that describes how far an object is from the origin • Amount of space between two points • Speed: Rate. A change in distance per unit time • Example: Meters per second or miles per hour • Velocity: The rate of change of position. Includes both speed and direction, can be positive or negative. • Example: Measured in meters per second or miles per hour • Acceleration: A change in velocity per unit time • Vector quantity • Example: Measured in meter’s per second2

6. Speed • There are two basic types of linear speed: • Instantaneous Speed • Average Speed • Your automobile speedometer measures Instantaneous Speed • Most people think in terms of average speed which is the total distance covered in a particular amount of time. There are stop lights, traffic and even intentional stops for food or fuel that can cause us to slow down during our trip.

7. Speed Ave. Speed (Rate) = Distance/Time http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/kinema/trip.html

8. Speed on a Graph Actual Speed Average Speed • This graph shows actual speeds, and average speed • Each time the slope changes there is a change in speed as shown by the pink line • The average speed is shown as a straight line shown by the blue line http://mathdemos.gcsu.edu/mathdemos/jogger/jogger.html

9. Speed Problems 1. Calculate the speed of an object that has covered a distance of 250 miles in 10 hours. 2. Calculate the distance an object can travel if it’s speed is 25 miles per hour and travels for a time of 3.5 hours. 3. How long would it take an object traveling at 12 miles per hour to travel 68 miles?

10. 4. Use the following data to create a graph of the speed of the object. Distance (m) Time (s) 0 5 10 15 20 25 0 1 2 3 4 5

11. Acceleration Which car or cars (red, green, and/or blue) are undergoing an acceleration? Study each car individually in order to determine the answer. Watching all cars carefully you will see that only the blue and the green care are accelerating, the red car is maintaining a constant speed. http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/kinema/acceln.html

12. Acceleration on a Graph“It’s all about the Curve” There is no change in velocity. There is zero acceleration. http://www.physicsclassroom.com/Class/1DKin/U1L3a.cfm

13. Acceleration on a Graph“It’s all about the Curve” Since velocity is changing over time, the object is accelerating. http://www.physicsclassroom.com/Class/1DKin/U1L3a.cfm

14. Acceleration on a Graph“It’s all about the Curve” The car is showing a positive acceleration. For every second of time, it is moving a greater distance. http://www.physicsclassroom.com/Class/1DKin/U1L3a.cfm

15. Acceleration Problems Calculate the acceleration in each data table using the equation for acceleration. A a = (8 m/s - 0 m/s) / (4 s) a = (8 m/s) / (4 s) a = 2 m/s/s B a = (0 m/s - 8 m/s) / (4 s) a = (-8 m/s) / (4 s) a = -2 m/s/s http://www.glenbrook.k12.il.us/gbssci/phys/Class/1DKin/U1L1e.html

16. Acceleration Problems Average Velocity and Average Acceleration: 1. A kayaker paddles up a river at 2 m/s and then turns around and floats downstream at 4 m/s. The turn-around time is 8 s. a) What is the average acceleration of the kayak?

17. Acceleration Problem 2. An ice skater first accelerates from 0.00 m/s to 6.0 m/s in 5.5 s, then continues at his speed for another 5.5 s. What is the total distance traveled?

18. Circular Motion • Although the direction is changing, an object traveling in a circular path is always accelerating. This is called centripetal acceleration. • The distance of one complete cycle around the perimeter of a circle is known as the circumference. • Average speed = d/t = circumference/time • circumference = 2πr • Average speed Sa= 2πr/t

19. Circular Motion • Notice the arrows and their directions. • These show that each motion is a vector and has a direction to it. • Animation of an object undergoing uniform circular motion. • What would be an example of an object that experiences uniform acceleration? http://www.physicsclassroom.com/mmedia/circmot/ucm.cfm

20. Questions for Circular Motion: Speed 1. How long would it take to complete a circle with a circumference of 20 m at a speed of 5 m/s? 2. At what speed would it only take 2 seconds? 3. How long would it take to complete a circle with a radius of 5 m at a rate of 5 m/s?