1. Describing Motion
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2. This lecture will help you understand:
Aristotle on Motion
Galileo's Concept of Inertia
Mass—A Measure of Inertia
Net Force
The Equilibrium Rule
The Support Force
Equilibrium of Moving Things
The Force of Friction
Speed and Velocity
Acceleration
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3. Aristotle on Motion Aristotle classified motion into two kinds:
Natural motion: motion that is straight up or straight down
Every object has its place and will strive to get there.
Violent motion: imposed motion resulting from an external push or pull
Pulling or lifting
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4. Aristotle on Motion
Aristotle thought that heavier objects strove hard to return to their place than lighter objects.
Everything was composed of 4 elements
Earth, water, air and fire
In the early 1600’s Galileo demolished Aristotle’s belief that heavy objects fall faster than light ones.
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5. Galileo's Concept of Inertia
In the absence of a force, objects once set in motion tend to continue moving indefinitely without friction.
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6. Galileo's Concept of Inertia
Legend of the Leaning Tower of Pisa:
Galileo showed that dropped objects fall to the ground at the same time when air resistance is negligible.
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7. Galileo's Concept of Inertia
Discovery:
In the absence of friction, no force is necessary to keep a horizontally moving object moving.
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8. Galileo's Concept of Inertia
Experiment:
Balls rolling down inclined planes and then up others tend to roll back up to their original heights.
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9. Galileo's Concept of Inertia
Conclusion:
The tendency of a moving body to keep moving is natural—every material object resists change in its state of motion. This property of things to resist change is called inertia.
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10. Galileo's Concept of Inertia CHECK YOUR NEIGHBOR
The use of inclined planes for Galileo's experiments
helped him to
eliminate the acceleration of free fall.
discover the concept of energy.
discover the property called inertia.
discover the concept of momentum.
Explain your answer to your neighbor.
C. discover the property called inertia.
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11. Mass—A Measure of Inertia
The amount of inertia possessed by an object depends on the amount of matter—the amount of material that composes it, its mass:
Greater mass  greater inertia
Smaller mass  smaller inertia
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12. Mass—A Measure of Inertia
Quantity of matter in an object
Measure of inertia or sluggishness that an object exhibits in response to any effort made to start it, stop it, or change its state of motion in any way
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13. Mass—A Measure of Inertia
Weight: the amount of gravitational pull on an object
Weight and mass are proportional.
Twice the mass  twice the weight
Half the mass  half the weight
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14. Mass—A Measure of Inertia
Mass versus volume
Mass involves how much matter an object contains.
Volume involves how much space an object occupies.
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15. Mass—A Measure of Inertia CHECK YOUR NEIGHBOR
The concept of inertia mostly involves
mass.
weight.
volume.
density.
Explain your answer to your neighbor.
A. mass.
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16. Mass—A Measure of Inertia
Standard unit of measurement for mass: the kilogram
One Kilogram (kg) of any material on Earth's surface weighs 10 newtons.
Away from the Earth, as on the Moon or other small bodies, 1 kg of material weighs less than 10 newtons.
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17. Mass—A Measure of Inertia CHECK YOUR NEIGHBOR
When the string is pulled down slowly, the top
string breaks. This best illustrates the
weight of the ball.
mass of the ball.
volume of the ball.
density of the ball.
Explain your answer to your neighbor.
A. weight of the ball.
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18. Mass—A Measure of Inertia CHECK YOUR NEIGHBOR
When the string is pulled down quickly, the bottom
string breaks. This best illustrates the
weight of the ball.
mass of the ball.
volume of the ball.
density of the ball.
Explain your answer to your neighbor.
B. mass of the ball.
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19. Mass—A Measure of Inertia
Measure of compactness
Density is the measure of how much mass occupies a given space.
Equation for density: Density =
in grams per cubic centimeter or kilograms per cubic meter
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20. Mass—A Measure of Inertia CHECK YOUR NEIGHBOR
The density of 1 kilogram of iron is
less on the Moon.
the same on the Moon.
greater on the Moon.
all of the above
Explain your answer to your neighbor.
B. the same on the Moon.
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21. Net Force Force is simply a push or a pull.
Net force is a combination of all the forces that act on an object. It is the net force that changes an object's state of motion.
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22. Net Force Vector quantity
Is both the magnitude (how much) and direction (which way).
The length of the arrow and direction can determine its magnitude and direction.
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23. Net Force CHECK YOUR NEIGHBOR
A cart is pushed to the right with a force of 15 N
while being pulled to the left with a force of 20 N.
The net force on the cart is
5 N to the left.
5 N to the right.
25 N to the left.
25 N to the right.
Explain your answer to your neighbor.
A. 5 N to the left.
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24. The Equilibrium Rule The equilibrium rule:
The vector sum of forces acting on a nonaccelerating object or system of objects equals zero.
Mathematical notation: F = 0
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25. The Equilibrium Rule
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26. The Equilibrium Rule CHECK YOUR NEIGHBOR
The equilibrium rule, F = 0, applies to
vector quantities.
scalar quantities.
both vector and scalar quantities.
neither vector nor scalar quantities.
Explain your answer to your neighbor.
A. vector quantities.
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27. The Support Force
The force that supports an object on a surface against gravity is called the support force, often called the normal force = weight of the object.
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28. The Support Force CHECK YOUR NEIGHBOR
When you stand on two bathroom scales, with one foot on
each scale and your weight evenly distributed, each scale
will read
your weight.
half your weight.
zero.
actually more than your weight.
Explain your answer to your neighbor.
B. half your weight.
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29. Equilibrium of Moving Things
An object that moves at constant velocity is in equilibrium.
When two or more forces cancel to zero on a moving object, the object is in equilibrium.
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30. Equilibrium of Moving Things CHECK YOUR NEIGHBOR
A bowling ball is in equilibrium when it
is at rest.
moves steadily in a straight-line path.
both of the above
none of the above
Explain your answer to your neighbor.
C. both of the above.
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31. The Force of Friction Friction
is the resistive force that opposes the motion or attempted motion of an object through a fluid or past another object with which it is in contact.
always acts in a direction to oppose motion.
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32. The Force of Friction
Between two surfaces, the amount of friction depends on the kinds of material and how much they are pressed together.
Friction is due to surface bumps and also to the stickiness of atoms on the surfaces of the two materials.
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33. The Force of Friction CHECK YOUR NEIGHBOR
The force of friction can occur
with sliding objects.
in water.
in air.
all of the above
Explain your answer to your neighbor.
D. all of the above.
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34. The Force of Friction CHECK YOUR NEIGHBOR
When Nellie pushes a crate across a factory floor at
constant speed, the force of friction between the crate and
the floor is
less than Nellie's push.
equal to Nellie's push.
equal and opposite to Nellie's push.
greater than Nellie's push.
Explain your answer to your neighbor.
C. equal and opposite to Nellie's push.
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35. The Force of Friction CHECK YOUR NEIGHBOR
When Nellie pushes a crate across a factory floor at an
increasing speed, the amount of friction between the crate
and the floor is
less than Nellie's push.
equal to Nellie's push.
equal and opposite to Nellie's push.
greater than Nellie's push.
Explain your answer to your neighbor.
A. less than Nellie's push.
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36. Speed and Velocity
Speed is the distance covered per amount of travel time.
Equation for speed:
Speed =
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37. Speed and Velocity
Average speed is the total distance traveled divided by the time of travel.
Equation for average speed:
Average speed =
Instantaneous speed is the speed at any instant of time.
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38. Speed and Velocity
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39. Speed and Velocity Velocity
When we know both the speed and direction of an object.
Motion is Relative
Everything is always moving
You are moving at 100,000 km/h when compared to the Sun.
Velocity or Speed of an object are compared to something else.
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40. Speed and Velocity CHECK YOUR NEIGHBOR
The average speed in driving 30 km in 1 hour is
the same as the average speed in driving
30 km in 1/2 hour.
30 km in 2 hours.
60 km in 1/2 hour.
60 km in 2 hours.
Explain your answer to your neighbor.
D. 60 km in two hours.
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41. Acceleration
Acceleration is the rate at which velocity changes with time. The change in velocity may be in magnitude, in direction, or both.
Equation for acceleration:
Acceleration =
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42. Acceleration
Accelerating, braking (deceleration), or changing directions are all acceleration because the velocity of the object are changing.
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43. Acceleration CHECK YOUR NEIGHBOR
An automobile cannot maintain a constant speed
when
accelerating.
rounding a curve.
both of the above
none of the above
Explain your answer to your neighbor.
C. both of the above.
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44. Acceleration CHECK YOUR NEIGHBOR
Acceleration and velocity are actually
much the same.
rates, but for different quantities.
the same when direction is not a factor.
the same for free-fall situations.
Explain your answer to your neighbor.
B. rates, but for different quantities.
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45. Acceleration Free fall When the only force acting on a
falling object is gravity (with
negligible air resistance), the
object is in a state of free fall.
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46. Acceleration Acceleration of free fall is 9.8 m/s2
This is also the pull of gravity on any
object
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47. Acceleration Hangtime How high can you jump
It is almost always less than 1 second when jumping straight up.
d = ½ g t2
d = distance in meters
g = gravity
t = time
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48. Acceleration A vertical jump of .6 meters or 2 feet is exceptional.
World record is 1.25 m
Their hang time was .5 seconds each way for a total of 1 s
Jumping ability increases with the length of your legs and strength of your muscles
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49. Acceleration
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50. Acceleration CHECK YOUR NEIGHBOR
If a falling object gains 10 m/s each second it falls,
its acceleration is
10 m/s.
10 m/s per second.
both of the above
neither of the above
Explain your answer to your neighbor.
B. 10 m/s per second.
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51. Acceleration CHECK YOUR NEIGHBOR
A free-falling object has a speed of 30 m/s at one
instant. Exactly one second later its speed will be
the same.
35 m/s.
greater than 35 m/s.
60 m/s.
Explain your answer to your neighbor.
C. more than 35 m/s.
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52. Acceleration CHECK YOUR NEIGHBOR
The distance fallen by a free-falling body
remains constant each second of fall.
increases each second when falling.
decreases each second when falling.
none of the above
Explain your answer to your neighbor.
B. increases each second when falling.
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