Download Speed

MOTION NOTES
Motion: When an object changes position over time when compared with a reference point. You can
describe the direction of its motion with a reference direction.
Direction: N, S, E, W, up and down.
Point of Reference: buildings, mountains, etc.
Speed: Speed (s) depends on distance (d) and time (t).
Formulas
s=d
t
a=v
t
Symbols and units
s = speed (m/s) d = distance (m) t = time (s)
v = velocity (m/s) a = acceleration (m/s/s)
Units: speed: m/s (meter per second)
distance: m (meter)
time: s (second)
acceleration: m/s/s or m/s2
Speed is the rate at which an object moves.
Average speed = total distance
total time
Velocity: The speed of an object in a particular direction.
The rate of change of an object’s position.
Constant velocity: an object’s velocity is constant only if its speed and direction don’t change.
Acceleration: The rate at which velocity changes
(speed up, slow down, change in direction or any combination).
Acceleration tells you how fast velocity changes. To accelerate means to change velocity.
a = final velocity – starting (initial) velocity
time it takes to change velocity
OR
a = vf – vi
tf – ti
f = final
i = initial
Momentum
The property of a moving object that depends on the object’s mass and velocity. The more momentum
an object has, the harder it is to stop the object or change its direction.
Law of conservation of momentum: two or more objects interact, they may exchange momentum but
the total momentum remains the same.
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Why don’t planets fall?
They move around so fast that their speed gives them momentum. Planets don’t fall in toward the sun
because they are speeding around their orbits. The sun’s gravity stops them flying off into space. The
closer a planet is to the sun the faster it orbits. They orbit in an elliptical orbital because of the sun’s
gravity that pulls them towards it. Otherwise they would be moving in a straight line as Isaac Newton’s
first law indicates that an object in motion will stay in motion in a straight line unless a force is applied
to it.
Speed
Cockroach
1.25 m /s
Kangaroo
15 m/s
Cheetah
27 m/s
Sound
330 m/s
Space shuttle
Light
10,000 m/s
300,000,000 m/s
Force: a push or a pull
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F=mxa
Unit: the Newton
Forces in combination
Forces in the same direction
Forces in Different Directions
Unbalanced Forces set an object in motion
Balanced Forces keep an object static (not moving)
Unbalanced Forces produce a change in motion
Balanced Forces produce no change in motion
If the net is 0, the forces are balanced.
Friction: A force that opposes motion (rough surface with microscopic valleys and hills).
Types of friction:
 Sliding ex. eraser
 Rolling friction ex. wheels
 Fluid friction ex. wet floor
 Static friction ex. force applied but does not cause it to move (see p. 122).
Friction – harmful or helpful
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Ways to reduce friction – lubricants: wax; motor oil; grease. Also by switching from sliding to
rolling friction.
Ways to increase friction: make surfaces rougher. Ex. Textured shoe soles for sports.
Gravity: A force of attraction between two objects.
Gravity depends on mass (m) and the distance (r) between the objects.
F = G x m1 x m2
r
2
G = constant of universal gravity G = 6.673 x 10 –11 N. m2
kg2
2