Chapter 4 Linear motion

1) You can describe the motion of an object by its position, speed,

direction and acceleration

2) Distance is a scalar.

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

3) Displacement is a vector

Displacement is DX “final position – initial position”

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

5) 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.

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

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

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

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

9) Instantaneous speed is the speed at any instant.

10) A speedometer shows instantaneous speed.

11) Average speed = total distance time interval

12) Velocity = speed in a given direction. Velocity is a vector quantity.

13) Constant velocity = means same speed

in same straight-line direction.

14) Changing velocity : if either the speed changes

or the direction changesor both change

15) Acceleration is the rate of change of velocity

acceleration is a vector.

16) acceleration = change of velocity time interval

17) Deceleration = decrease in acceleration

18)Acceleration is due to change in direction of moving object

19) Accelaration for object in a straight line

= change in speedtime interval

Free fall

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

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

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

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

24) 6 kinematic variables in y and x

y xyo xo

vy vx

voy Vox

ay ax

t t

25)For object moving only in y direction,

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

26) What is meant by “-g”?

The object is accelerating in the negative direction

27) For object moving only in x direction

x = final position in x directionxo = initial position in x directionVox = initial velocity in x directionVfx = final velocity in x directionay = acceleration in x directiont = time elapsed

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

V = vo + at

x = xo + vot + ½ a t2

V = vo + at

28) For An object dropped from height h in free fall, equations simplify

y = h - ½ g t2

V = -gt

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

down, stop and fall back down

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

28) Projectile motions