Chapter 4 Linear motion

1) Linear means in a straight line

2) Describe the motion of an object by its position,

speed, direction

and acceleration

3) Distance is a scalar.

Distance is “how much ground is covered” during motion.

4) Displacement is a vector

Displacement is DX “final position – initial position”

5) Example: distance is the path length

from o to c

Example: distance is the path length

from a to c

But displacement is the vector from a to c = final position – initial position

6) Motion is relative.An object is moving if its position relative to a fixed point is changing.

7) Example: A crate on a train traveling at 20 m/sec.

The crate is at rest wrt the train.

But the crate is moving 20 m/sec relative to the ground.

8) Unless stated otherwise, we measure speed relative to the Earth.

7) Speed is how fast an object is moving. Speed is a scalar quantity.

10) From motion experiments, Galileo defined Speed = distance/time

11) Before Galileo, motion was simply described as “fast” or “slow”

12) Instantaneous speed is the speed at any instant.

13) A speedometer shows instantaneous speed.

11) Average speed = total distance time interval

15) Velocity = speed in a given direction.

Velocity is a vector quantity.

16) Constant velocity = means same speed

in same straight-line direction.

17) Changing velocity : if either the speed changes

or the direction changesor both change

18) Acceleration is the rate of change of velocity

acceleration is a vector.

19) acceleration = change of velocity time interval

20) Deceleration = decrease in acceleration

21) Accelaration for object traveling in a straight line

= change in speedtime interval

Free fall

22) projectile: an object that moves only under the influence of gravity.

23) Free fall = object falling under influence of gravity.

24) Trajectory = path of a projectile

25) When we talk about free fall, we ignore air resistance.

26) Acceleration of object in free fall is about g= 10 meters/sec2

27) Kinematics is the study of motionGalileo discovered these relationships.

28) There are 6 kinematics variables and 2 kinematics equations

29)Kinematics variables for free falling object,

Y = final position in y directionYo = initial position in y directionVoy = initial velocity in y directionVfy = final velocity in y direction

ay = -gt = time elapsed

30) What is meant by “-g”?

The object is accelerating in the negative direction

31) Kinematic Equations y = yo + vot + ½ a t2

V = vo + at

x = xo + vot + ½ a t2

V = vo + at

32) For freefall, equations simplify to

y = - ½ g t2

V = - gt

33) Rising objects: an object thrown straight up will slow

down, stop and fall back down

34) At the highest point, the vertical velocity is zero.

28) Projectile motions

1-D 1-D 2-D