Motion Chapter 11

Distance and Displacement To describe motion accurately and completely a frame of reference is necessary. There can be numerous frames of reference and it depends on the one you choose to measure motion. Example: One frame of reference on earth- the sun sets. If you were on the sun though the earth would be rising Relative motion- movement in relation to a frame of reference. 2

Distance and Displacement Movement can be measured only with reference to something that is assumed to be in a fixed place. The most common frame of reference is the earth 3

Distance and Displacement cont. Distance is the length of a path between two points. When an object moves in a straight path distance is the length between the objects starting point and ending point The unit is the meter Distance is a scalar quantity which means it has only magnitude. Ex. 12 m scalar quantityscalar quantity 4

Distance and Displacement cont. Displacement is the direction from the starting point and length of a straight line from the starting point to the ending point. Displacement is a vector quantity which means it has both magnitude AND direction. Ex. 12 m North vector quantityvector quantity 5

Speed and Velocity Motion – a change of position relative to a frame of reference. Motion is measured by: –1. distance: length between two points in meters –2. time measured in seconds 6

Speed and Velocity Speed is the distance traveled by a moving object per unit of time Speed is a Scalar: has only magnitude Speed = distance time time 7

Speed and Velocity Problem 1 – What is the speed of a jet plane that flies 7200 km in 9 hr. Answer – 800 km/hr Problem 2 – The speed of a cruise ship is 50 km/hr. How far will the ship travel in 14 hours? Answer – 700 km 8

Speed and Velocity cont. Velocity is speed in a given direction. Velocity is a Vector Velocity = displacement time time Example: A car is traveling 60miles/hr and moving east 9

Constant Speed - does not change. When graphed on a distance-time graph constant speed is always a straight line. The speed of a moving object is not always constant. Dividing total distance by total time gives average speed. Instantaneous Speed – speed at a given moment in time (speedometers, radar guns etc.) 10

Using Graphs Which runner won the race? Which runner stopped for a rest? How long did he stop for? How long did Bob take to complete the race? Calculate Albert's average speed. 11

Acceleration Acceleration- the rate of change in velocity Acceleration = final velocity –initial velocity change in time change in time * units = m/s 2 = m/sec/sec = m/sec – m/sec sec sec *Deceleration : a decrease in velocity, or negative acceleration *A curved line on a distance / time graph shows acceleration. 12

Free Fall acceleration of an object toward Earth due to gravity Gravity is 9.80 m/s 2 on Earth 13

Using Graphs When is the bus: –decelerating? –steady speed? –Stopped? During which part of the journey was the bus moving fastest? During which part of the journey did the bus have greatest acceleration? Calculate the acceleration between D and E. 14

Circular Motion Acceleration is a change in velocity. In circular motion the velocity is continuously changing because direction continuously changes. An object in circular motion is always accelerating even though speed may be constant. Centripetal acceleration – acceleration that is directed toward the center of a circular path. 15

Centripetal Acceleration 16

Momentum All moving objects have momentum, abbreviated “p” Momentum is equal to the mass of an object multiplied by its velocity. p = mv The law of conservation of momentum states that total momentum of any group of objects remains the same unless outside forces act on the objects. Ex. The momentum of the base ball bat is transferred to the ball when the ball and the bat meet 17