Motion

1
Motion Forces

• II. Describing Motion
• Motion
• Speed Velocity
• Acceleration

2
Newtons First Law

• Newtons First Law of Motion
• An object at rest will remain at rest and an
  object in motion will continue moving at a
  constant velocity unless acted upon by a net
  force.

motion
constant velocity
net force
3
Inertia

• Newtons First Law of Motion proves that objects
  do not move or stop moving unless there are
  greater (unbalanced) forces acting upon them.
  This is what inertia is. Inertia is the objects
  tendency to either keep moving or not moving at
  all. A force that changes this has to be strong
  enough to change an objects inertia.

4
Inertia Demo

• https//www.youtube.com/watch?v6gzCeXDhUAA

5
A. Motion

• Problem
• Is your desk moving?
• We need a reference point...
• nonmoving point from which motion is measured

6
A. Motion

• Motion
• Change in position in relation to a reference
  point.

7
A. Motion

• Problem
• You are a passenger in a car stopped at a stop
  sign. Out of the corner of your eye, you notice
  a tree on the side of the road. Your car begins
  to move forward.
• You have mistakenly set yourself as the reference
  point.

8
Distance and Displacement

• You know the distance from a starting point, but
  what is displacement?
• Displacement is distance plus direction FROM THE
  STARTING POINT.

9
Displacment Examples

• You move 20 meters north from your starting
  point
• Distance 20 meters
• Displacement 20 meters North
• You move 20 meters north, and then 18 meters
  south
• Distance 38 meters
• Displacement 2 meters North
• You move 20 meters north and then 20 meters south
• Distance 40 meters
• Displacement 0! (why?)

10
B. Speed Velocity

• Speed
• rate of motion
• distance traveled per unit time

11
B. Speed Velocity

• Instantaneous Speed
• speed at a given instant
• Average Speed

12
B. Speed Velocity

• Problem
• A storm is 10 km away and is moving at a speed of
  60 km/h. Should you be worried?

• It depends on the storms direction!

13
B. Speed Velocity

• Velocity
• speed in a given direction
• can change even when the speed is constant!

14
Speed vs Velocity

• https//www.youtube.com/watch?v-6lrr6-ADY0

15
C. Acceleration

• Acceleration
• the rate of change of velocity
• change in speed or direction

a acceleration vf final velocity vi initial
velocity t time
16
C. Acceleration

• Positive acceleration
• speeding up

• Negative acceleration
• slowing down

17
D. Calculations

• Your neighbor skates at a speed of 4 m/s. You
  can skate 100 m in 20 s. Who skates faster?

GIVEN d 100 m t 20 s v ?
WORK v d t v (100 m) (20 s) v 5
m/s You skate faster!
18
D. Calculations

• A roller coaster starts down a hill at 10 m/s.
  Three seconds later, its speed is 32 m/s. What
  is the roller coasters acceleration?

GIVEN vi 10 m/s t 3 s vf 32 m/s a ?
WORK a (vf - vi) t a (32m/s - 10m/s)
(3s) a 22 m/s 3 s a 7.3 m/s2
19
D. Calculations

• Sound travels 330 m/s. If a lightning bolt
  strikes the ground 1 km away from you, how long
  will it take for you to hear it?

GIVEN v 330 m/s d 1km 1000m t ?
WORK t d v t (1000 m) (330 m/s) t 3.03
s
20
D. Calculations

• How long will it take a car traveling 30 m/s to
  come to a stop if its acceleration is -3
  m/s2?

GIVEN t ? vi 30 m/s vf 0 m/s a -3 m/s2
WORK t (vf - vi) a t (0m/s-30m/s)(-3m/s2)
t -30 m/s -3m/s2 t 10 s
21
E. Graphing Motion

• slope
• steeper slope
• straight line
• flat line

speed
faster speed
constant speed
no motion
22
E. Graphing Motion

• Who started out faster?
• A (steeper slope)
• Who had a constant speed?
• A
• Describe B from 10-20 min.
• B stopped moving
• Find their average speeds.
• A (2400m) (30min) A 80 m/min
• B (1200m) (30min) B 40 m/min

23
E. Graphing Motion

• Acceleration is indicated by a curve on a
  Distance-Time graph.
• Changing slope changing velocity

24
E. Graphing Motion

• slope
• straight line
• flat line

• acceleration
• ve speeds up
• -ve slows down

constant accel.
no accel. (constant velocity)
25
E. Graphing Motion

• Specify the time period when the object was...
• slowing down
• 5 to 10 seconds
• speeding up
• 0 to 3 seconds
• moving at a constant speed
• 3 to 5 seconds
• not moving
• 0 10 seconds

26
Momentum

• Momentum is really a measure of how difficult it
  is to stop a moving object.
• An object that collides with an object at rest
  (and bounces off )will have the same momentum as
  it had before.
• An object that collides with an object in motion
  will experience a momentum that is a combination
  of both moving objects.

27
Momentum Formulas

• Momentum equals the mass of an object times its
  velocity p (momentum) mv
• So an 20 kg object traveling at 100 m/s W
• P(momentum) 20 kg x 100 m/s west 2000kg
  m/s west
• If p m x v, How would you find either mass or
  velocity (if you had force?) You would divide
  either into the force (P)
• M p/v or V p/m

28
Football!

• http//www.nbclearn.com/portal/site/learn/science-
  of-nfl-football

29
Inertia

• Newtons First Law of Motion proves that objects
  do not move or stop moving unless there are
  greater (unbalanced) forces acting upon them.
  This is what inertia is. Inertia is the objects
  tendency to either keep moving or not moving at
  all. A force that changes this has to be strong
  enough to change an objects inertia.