Chaptsr 14: Description of motion - grade10.pptx

This is the course of chapter 14 in grade 10 lebanese curriculum in physics.

1. Description of motion Grade 10 | Chapter 14

2. Objectives: • Choose a frame of reference. • Distinguish between average speed and instantaneous speed. • Distinguish between speed and acceleration. • Represent velocity and acceleration by vectors.

3. 14.1. Rest and Motion The notion of rest or motion is relative. The same object may be at rest with respect to a certain observer and in motion with respect to another .

4. App 14.1.Rest or Motion?? A B 1) A with respect to B: 2) A with respect to C: motion rest C

5. 14.2. Trajectory The trajectory of a moving object is the line or path described by this object during its motion Form of the trajectory Type of trajectory and motion Straight line Rectilinear Circle Circular Curve Curvilinear

6. In each of the following cases, indicate the type of the trajectory of the body: Motion of a freely falling body between the point where it is dropped and the ground . Motion of a ball after a free kick . Motion of a lift between two floors of a building . Motion of a satellite around Earth. Motion of a ball on a billiard table. rectilinear circular rectilinear rectilinear curvilinear App 14.2.

7. 14.3. Frame of reference A frame of reference is a system of "guideposts" relative to which all positions and motions can be determined.

8. For a frame of reference we may also use a set of axes attached to the object of reference. The position of a point is then determined by its co-ordinates. . In grade 10, we study rectilinear motion only, so we use the axis of abscissa O M x  

9. 14.4. Instant and duration Origin of time (t =0): is the moment at which we start observation (not necessarily motion). Instant (t): is the moment at which the event occurs Duration (∆t): is how long the event lasts between two instants: ∆t= t final - t initial

10. Instant and duration (Example) Vs. At 9:00 p.m. Start of match (t ) Half time Start of second half End of match 9:00 p.m. 9:45 p.m. 10:00 p.m. 10:45 p.m . 45 60 105 Origin of time: Instant of half time: Duration of break: Instant of start of second half: Duration of whole match: t =0 min (at 9:00 p.m.) t= 45 min ∆t= 60-45=15 min t=60 min ∆t= 105-0=105 min t ime (min)

11. 14.5. Position vector The position vector has the following elements: Point of application: Line of action: Direction: Magnitude:   O M x   O The x-axis From O to M =|x|  

12. Position vector (example) O   T R F 2.5m 3 m 1 m Body rabbit tiger Flower Expression of Position vector       Characteristics Point of application       Line of action       direction       magnitude             O O O X-axis X-axis X-axis From O to F From O to T From O to R =|2.5|= 2.5m   =|-1|= 1m   =|5.5|= 5.5m  

13. 14.6. Distance and Displacement O M 3 M 1   2. Total distance covered = the length of the whole followed path = Displacement vector: (Between )   Displacement vector is the vector joining the initial and final position of the object between two instants.   7 m M 2 Magnitude = = |3| = 3 m   The magnitude of displacement vector is always less than or equal the total distance covered  

14. 14.7. Speed and velocity Average speed “ V av ”: The average speed of a moving particle between two instants is the quotient of the distance covered by the particle between these two instants by the time interval (duration) between them. SI-Units: V(m/s) d(m) t(s)   Another unit: Km/ hr Km/ hr m/s x 3.6 /3.6

15. Overall Application:

16. Overall Application :             The car starts from rest. If the time interval between two consecutive recordings is τ =0.5 sec: a) determine the average speed of the car between: t 3 and t 5 t 1 and t 4 t and t 5 t 2 and t 4 t 1 and t 5 t and t 3   = s   s       s   1.   2.   3.   exercise       1 cm

17. 2. Instantaneous speed “V i ”: The instantaneous speed “ V i ” at an instant t is approximately equal to the average speed between two instants t 1 and t 2 very close to t , with t lying between t 1 and t 2 .                  

18. Overall Application:             b) Determine , , and                           1 cm    

19. 3 . Instantaneous velocity “   The velocity vector “ has the following elements: Point of application: Line of action: Direction: Magnitude:   The chosen scale determines the length of the velocity vector Representation:   X-axis direction of the motion at the considered instant the speed at that instant (instantaneous speed) O M i x    

20. Overall Application: c) Represent to the scale: 1 cm  1 cm/s the velocities                   1 cm 1 cm 1 cm/s ?? 2.5 cm/s Length of 2.5 cm   1 cm 1 cm/s ?? 4.5 cm/s Length of .5 cm                  

21. 14.8. Acceleration Def.: is a quantity used to describe the variations of the velocity with respect to time. Average acceleration: is the ratio of the variation of the speed to the elapsed duration: Instantaneous acceleration: is approximately equal to the average acceleration between two instants t 1 and t 2 very close to t so that t 1 < t < t 2 .   SI-unit:                  

22. Overall Application: d) Determine the average acceleration between 1. t 1 and t 3 2. t 1 and t 4 e) Determine a 1 , a 2 and a 3 at t 1 , t 2 and t 3 respectively               1 cm         e)                      

23. 4. Acceleration vector “   The acceleration vector “ has the following elements: Point of application: Line of action: Direction: Magnitude:   The chosen scale determines the length of the acceleration vector Representation:   X-axis With direction of motion if motion is accelerating ( ) Opposite to direction of motion if motion is decelerating ( )   the absolute value of at that instant  

24. Overall Application: f ) Represent to the scale: 1 cm  2 cm/s 2 :     2               1 cm 1 cm 2 cm/s 2 ?? 4.5 cm/s 2 Length of 2.25 cm   1 cm 2 cm/s 2 ?? 2 cm/s 2 Length of cm