Download Stacey Carpenter - University of Hawaii

PPT
Equilibrium
Developer Notes
 Need more exercises where ∑F ≠ 0.
Version
05
Date
2003/12/02
Who
dk
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Revisions
Added revision table and re-formatted
Expanded goals
Added student reading
Added more exercises
Expanded teacher section
Goals
 Students should understand the sum of forces, ∑F and Fnet.
 Students should understand that the sum of forces can be zero or non-zero.
 Students should understand that if the sum of forces is zero, the object is in equilibrium.
 Students should understand that if an object is not moving, the sum of forces is zero, and the
object is in equilibrium.
 Students should understand that if the sum of forces is non-zero, the object will accelerate.
 Students should understand that an object can be moving and still be in equilibrium. This is
true if the object is moving at a constant velocity (constant speed and direction).
Concepts & Skills Introduced
Area
physics
Concept
equilibrium, Fnet = 0, ∑F=0
Time Required
Warm-up Question
Place a bowling ball on the floor: Write about the forces on the ball and its motion.
Presentation
Discuss the warm-up question. The point is that there are forces on the ball, but the ball isn't
moving because all the forces equal out. Gravity is pulling down on the ball, but the floor is
pushing up. Air might be blowing on the ball, but friction is pushing back. The ball is in
equilibrium. In mathematical notation:
Fnet = 0, the net force is zero, or
∑F = 0, the sum of forces is zero.
If the forces on the ball didn't equal out, then the ball would start moving; it would accelerate.
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Equilibrium
For moving objects, the same holds true; if the forces on the object don't equal out, the object
will accelerate - it will speed up, slow down, or turn.
It is very important here that the students understand the equivalence of non-zero net force and
acceleration (or ∆v.):
 If ∑F = 0, there will be no acceleration. If there's no acceleration, then ∑F = 0.
 If ∑F = 0, then a = 0. If a = 0, then ∑F = 0.
 If ∑F = 0, then ∆v = 0. If ∆v = 0, then ∑F = 0.
 If ∑F  0, there is acceleration. If there's acceleration, then ∑F  0.
 If ∑F  0, a  0. If a  0, then ∑F  0.
 If ∑F  0, ∆v  0. If ∆v  0, then ∑F  0.
Assessment
Writing Prompts
1.
Relevance
Summary
Exercises
Challenge/ extension
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Equilibrium
Background
Newton’s 1st Law says that an object at rest will stay at rest, and an object in motion will
continue at a constant speed in a straight line unless a force makes it change.
Problem
If an object is not moving, does that mean there are no forces acting on it? If an object is moving,
does that mean there is an overall force acting on it?
Materials
2
spring scales, calibrated
1
paper clip
1
mass cart
1
electric car
Procedure
1. Hook both spring scales to the paper clip.
a. Pull the two scales a little way apart and record the readings on the scales.
b. Pull the scales a little further apart and record the readings on the scales.
2. Roll the cart across the floor.
a. Does the cart speed up, slow down, or go at a steady speed?
b. What are the forces on the cart as it rolls across the floor?
3. Run the electric car across the floor.
a. Does the car speed up, slow down, or go at a steady speed?
b. What are the forces on the car as it rolls across the floor?
Summary
1. What is the sum of forces on the paper clip?
2. Do you think the total force on the electric car is zero or non-zero?
3. Do you think the total force on the cart is zero or non-zero?
4. Look at your data from the friction activity, where you were pulling the backpack at steady
speed. Do you think the total force on the backpack was zero or non-zero?
5. Can you make a general rule from the data you've found here? (Think about inertia.)
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Equilibrium
Reading
What is the sum of all the forces on a moving object?
An object sitting still (relative to what?) has forces acting on it, like gravity and the normal force,
but it isn't moving. All the forces on it equal out. Gravity may be pulling it down, but the floor is
pushing it up an equal amount. Wind may be blowing on it, but there's an equal amount of
friction. It is in equilibrium. Equilibrium means that the total (sum, net) of all the forces acting
on it is zero. If the total wasn't zero, it would start moving; it would accelerate. That's what
Newton's 1st Law says.
How about an object that is moving? Can it be in equilibrium? By Newton's 1st Law, if the object
is going in a straight line at a constant speed, there must be no net force on it, or else it would
speed up, slow down, or turn. For a car going at a steady speed on a straight highway, the force
of the tires pushing it forward is exactly matched by friction and air resistance, so it doesn't
speed up or slow down. The car is in equilibrium.
Imagine a hockey puck sliding across frictionless ice in a straight line at a steady speed. There is
no force pushing it, and there is no friction resisting it. The hockey puck is in equilibrium.
Both the car and the hockey puck have constant velocities. They are not accelerating. The sum of
forces acting on them is zero.
A projectile on Earth is never in equilibrium because the forces of air resistance and gravity are
always acting on it. Projectiles slow down, speed up, and turn - they accelerate.
The total force on an object can be zero or non-zero. If it is zero, the object is in equilibrium. If it
is non-zero, the object will accelerate (change velocity). Likewise, if an object goes at a constant
velocity, the sum of forces on it is zero. If it changes velocity, the sum of forces is non-zero.
For an object that's in equilibrium, the sum of forces is zero. This can be written in symbols:
∑F = 0. ∑ means "sum of," so this means the sum of forces is zero, or
Fnet = 0. The net force is zero,
Exercises
1. A box is sitting on a floor, not moving.
a. What is the net force on the box?
b. You push on the box as hard as you can, but it doesn’t move. What is the net force
on the box?
c. You climb on top of the box in frustration and sit down. What is the net force on
the box?
d. You get a friend to help you push the box, and you finally get it moving, slowly at
first, then a little faster. What is the net force on the box while it is speeding up?
e. Eventually you reach a constant velocity. What is the net force on the box?
f. You reach your destination and quit pushing. The box slides to a stop. What is the
net force on the box while it is slowing down?
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2. When a rocket is taking off, is the net force on it zero or non-zero? What is the net force on a
rocket coasting through deep space with its engines off? (Deep space means far away from
any planets or stars, where the universe is essentially empty. Ignore gravity.)
3. When an airplane is taking off, is the net force on it zero or non-zero? If the airplane is flying
at a constant speed of 200 kph, what is the net force on it?
4. In football, coaches make the players run into a blocking sled and push it. When the players
are pushing the sled as fast as they can, what is the net force on the sled?
5. If an object is moving, does that mean it's not in equilibrium?
6. When something is balanced, like a Chinese acrobat on a stack of chairs, could we say they
are in equilibrium?
7. If a car is speeding up from a stop light, what can you say about the net force on it?
8. What happens when the net force on an object is not zero?
9. If a car is turning a corner, what can you say about the net force on it?
10. If an object has a constant velocity, what can you say about the net force on it?
11. If an object is accelerating, what can you say about the net force on it?
12. If the net force on an object is zero, what can you say about its acceleration?
13. If the net force on an object is not zero, what can you say about its velocity?
14. If an object does not have a constant velocity, what can you say about the net force on it?
Challenge/ extension
1. Imagine a rowboat pulling an ocean liner. If the rowboat could keep a constant force on the
ocean liner (ignore wind, waves, current, etc.), how fast would it eventually go?
2. Politically, what does it mean when we are in equilibrium?
Vocabulary
Equilibrium – a state of balance, when the total force on an object is zero. This can be written as
Fnet = 0 or ∑F = 0.
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