grade 8 acceleration
DESCRIPTION
TRANSCRIPT
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RECAP
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Inertia: Mass and Weight
Think!Does a 2-kilogram bunch of bananas
have twice as much inertia as a 1-kilogram loaf of bread? Twice as much mass? Twice as much volume? Twice
as much weight, when weighed in the same location?
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Mass—A Measure of Inertia
Mass and weight are proportional to each other in a given place:–In the same location, twice
the mass weighs twice as much.
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What is the relationship betweenmass and weight?
WeightMass
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Newton’s 1st Law of Motion
An object at rest will stay at rest or an object in motion
will stay in motion and travel in a straight line, as
long as no external net force acts on it.
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Quick Question!
What path will the planets take when the
Sun suddenly disappears?
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FORCE AND ACCELERATION• After this activity, you
should be able to describe how the net force acting on an object affects its acceleration.
Activity
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FORCE AND ACCELERATION
Activity
F = 1 F=2 F=3 F=4
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Activity
1
2
3
4
5
Label strips 1-5
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Activity5
Compute for average
velocity per strip
22 cm
velocity = 22 cm / 0.10 s
velocity = 220 cm/s
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Strip Distance (cm) Velocity (m/s)
Change in V (m/s)
Acceleration (m/s2)
Activity – per strip (F = 4)
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Strip Distance (cm) Velocity (cm/s)
Change in V (cm/s)
Acceleration (cm/s2)
1 12 14516 160
2 14 161
27 2703 16 188
22 2204 18 210
20 2005 20 230
AVERAGE21.3 cm/s
AVERAGE213 cm/s2
Activity – per strip
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Tape Chart No. Rubber BandsAverage
Acceleration (cm/s2)
F = 1 1 163F = 2 2 182F = 3 3 200F = 4 4 213
Activity – per group
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Activity – Line Graph
F = 1 F = 2 F = 3 F = 40
50
100
150
200
250
Acceleration
Acceleration
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FORCE AND ACCELERATION
• What corresponds to the amount of force in the activity?
Activity
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Instantaneous vs. Average
Recap!
VelocityAcceleration
VelocityAcceleration
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• Velocity
Formula
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• Acceleration
Formula
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FORCE AND ACCELERATION• What caused the
acceleration to change?• What caused the velocity
to change?
Activity
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How is the acceleration of the cart related
to the amount of force acting on
it?
Activity
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Acceleration
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Movement
• When we see something move we see– Start– Slow– Curve– Stop
• All these things represent a CHANGE in motion
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What is the cause of acceleration?
• FORCE causes ACCELERATION• Example – ball rolling on the floor
– Still until force is placed on it– Stays moving in a straight path until another force
causes it to accelerate • Change direction• Speed up• Slow down
– Change in velocity acceleration
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Net Force causes acceleration
• Combination of force yields acceleration
Force
Acceleration
Direct Proportionality
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How about Mass?
• Mass resists acceleration!
Inverse Proportionality
Acceleration
Mass
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Mass resists acceleration
• Example– Full shopping cart vs. empty shopping cart
The greater the mass the more force it takes to accelerate the
object
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Mass resists Acceleration
The amount of acceleration
depends not only on the force, but also on the mass being
pushed.
F
F
Force of hand accelerates the brick
The same force accelerates 2 bricks ½ as much.
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Newton’s Second Law
• If you combine these two relationships, you would come up with this relationship:
The acceleration of an object is directly proportional to the magnitude of the net force acting on it and is inversely
proportional to its mass.
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Newton’s Second Law
F = ma
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Applications of 2nd Law
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Using consistent units
• a = F m• a =acceleration
(m/s^2 )• F = force
(newtons)• m = mass (kg)
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Problem 1
• How much force, or thrust, must a 30,000-kg jet plane develop to achieve an acceleration of 1.5 m/sec^2
• F = ma
= (30,000 kg)(1.5 m/sec^2) = 45,000 kg m/sec^2
= 45,000 N
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Problem 2
• What acceleration is produced by a force of 2000 N applied to a 1000-kg car?
• a = F/m = 2000 N/ 1000 kg = 2000 kg m/sec^2/1000 kg = 2 m/sec^2
• If the force is 4000 N, the acceleration doubles – 4000N/1000 kg = 4 m/sec^2
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Think!
Why do objects of various masses fall withequal accelerations?
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Question #1
• What do you call an influence on a body producing a change in its movement?
• What is the general equation for that?
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Newton’s Second Law
F = ma
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Question #2
Superman and Aquaman joined a competition called, “Around the World in a Day”. Both heroes had different modes of transport – Superman flies in the sky while Aquaman swims in the ocean. If
they have the same mass, acceleration, and route, who would win the
competition? Why?
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Superman would likely win!
Less force from air molecules!
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Question #3
How much force is needed to triple the acceleration of an
object with mass M?
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Force
Three times the force!
3
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Question #4
•Can you walk on a frictionless floor? Why?
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1st Law and 2nd Law of Motion
No. There would be no opposing force. You will slide forever and ever and ever.
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Recall Formulas!
•Velocity•Acceleration•Force
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Kinematic Equations
Please remember!
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Kinematic Equations
Please remember!
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Problem 1
• What is the force needed to accelerate an object to 2.92 m/s^2 if the object weighs 411kg?
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Problem 2
• Suppose we have two different masses. One has thrice the mass of the other. If you apply the same amount of force, describe the acceleration of both masses.
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Problem 3
• A toy car having a mass of 0.2kg is accelerated so that its velocity changes from 1.5m/s to 4m/s in 4 seconds. What amount of force caused this change in motion?
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Problem 4
• A delivery van (8000kg) changes its velocity from 20m/s to 80m/s while covering a distance of 300m. What is the van’s acceleration? What is the force on the van?
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Problem 5
• A 910-kg car is speeding at 97 km/h on a level road. Determine the horizontal braking force applied to stop it in 67m.
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Problem 6
• A 1,500-kg car moving at a velocity of 25 km/h along EDSA is accelerated by a force of 1,800 N. What will be its velocity after 11 seconds? (Neglect friction.)