Grade 8 acceleration
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Transcript of Grade 8 acceleration
RECAP
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?
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.
What is the relationship betweenmass and weight?
WeightMass
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.
Quick Question!
What path will the planets take when the
Sun suddenly disappears?
FORCE AND ACCELERATION• After this activity, you
should be able to describe how the net force acting on an object affects its acceleration.
Activity
FORCE AND ACCELERATION
Activity
F = 1 F=2 F=3 F=4
Activity
1
2
3
4
5
Label strips 1-5
Activity5
Compute for average
velocity per strip
22 cm
velocity = 22 cm / 0.10 s
velocity = 220 cm/s
Strip Distance (cm) Velocity (m/s)
Change in V (m/s)
Acceleration (m/s2)
Activity – per strip (F = 4)
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
Tape Chart No. Rubber BandsAverage
Acceleration (cm/s2)
F = 1 1 163F = 2 2 182F = 3 3 200F = 4 4 213
Activity – per group
Activity – Line Graph
F = 1 F = 2 F = 3 F = 40
50
100
150
200
250
Acceleration
Acceleration
FORCE AND ACCELERATION
• What corresponds to the amount of force in the activity?
Activity
Instantaneous vs. Average
Recap!
VelocityAcceleration
VelocityAcceleration
• Velocity
Formula
• Acceleration
Formula
FORCE AND ACCELERATION• What caused the
acceleration to change?• What caused the velocity
to change?
Activity
How is the acceleration of the cart related
to the amount of force acting on
it?
Activity
Acceleration
Movement
• When we see something move we see– Start– Slow– Curve– Stop
• All these things represent a CHANGE in motion
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
Net Force causes acceleration
• Combination of force yields acceleration
Force
Acceleration
Direct Proportionality
How about Mass?
• Mass resists acceleration!
Inverse Proportionality
Acceleration
Mass
Mass resists acceleration
• Example– Full shopping cart vs. empty shopping cart
The greater the mass the more force it takes to accelerate the
object
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.
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.
Newton’s Second Law
F = ma
Applications of 2nd Law
Using consistent units
• a = F m• a =acceleration
(m/s^2 )• F = force
(newtons)• m = mass (kg)
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
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
Think!
Why do objects of various masses fall withequal accelerations?
Question #1
• What do you call an influence on a body producing a change in its movement?
• What is the general equation for that?
Newton’s Second Law
F = ma
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?
Superman would likely win!
Less force from air molecules!
Question #3
How much force is needed to triple the acceleration of an
object with mass M?
Force
Three times the force!
3
Question #4
•Can you walk on a frictionless floor? Why?
1st Law and 2nd Law of Motion
No. There would be no opposing force. You will slide forever and ever and ever.
Recall Formulas!
•Velocity•Acceleration•Force
Kinematic Equations
Please remember!
Kinematic Equations
Please remember!
Problem 1
• What is the force needed to accelerate an object to 2.92 m/s^2 if the object weighs 411kg?
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.
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?
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?
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.
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.)