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Chapter 3: Reading/Review Guide
Key Term
Definition
Example
(Not just the equation!)
Acceleration
Any change in velocity (speed or direction)
Terminal Velocity
Speeding up, slowing down
Falling with no more acceleration; Fgrav = Fair
Friction
The force between two contacting surfaces – it
resists motion
Free Fall
Falling free of air resistance – only fgrav
Air Drag
The resistance that air provides as you move
through it – faster = more
Directly
Proportional
Two variables behave in the same way
Fnet goes up, acceleration goes
up
Inversely
(Indirectly)
Proportional
Two variables behave in opposite ways
Mass goes up, acceleration
goes down
Mass
How much stuff is in stuff
Weight
The force of gravity acting on an object
Newton’s 2nd
Law of Motion
Acceleration is proportional to the net force
and inverse to the mass
Galileo Developed the Concept of Acceleration
1. Distinguish between velocity and acceleration.
Velocity is speed and direction (a vector) while acceleration is just any change in velocity
2. When are you most aware of motion in a moving vehicle – when is it moving at constant velocity
of accelerating? Explain!
You are most aware of motion in an accelerating vehicle. Since acceleration is a change in velocity,
when a car accelerates it pushes you in different directions. If the car speeds up, then the seat pushes
you forward. However, if there is no acceleration, the car does not push you.
3. What is the acceleration of free fall? Why? Explain!
Free fall acceleration on earth is 10 m/s2. Acceleration in free fall is due only to gravity and on earth,
gravity pulls on everything the same amount.
Force Causes Acceleration
4. Is acceleration proportional to net force or does it equal net force? Explain!
Newton’s 2nd law states that acceleration is proportional to the net force. When using the same mass,
changing the Fnet changes the acceleration. More Fnet = more acceleration while less Fnet = less
acceleration.
Mass is a Measure of Inertia
5. What relationship does mass have with inertia? Explain!
Mass is a measure of inertia. Since inertia is the “ability” of an object to remain in equilibrium, the more
mass the more difficult (more force required) it is to move it. Heavier, more massive objects require
more force to remove from equilibrium.
6. What relationship does mass have with weight? Explain!
Mass is a measure of how much stuff is in stuff – it depends on the type and amount of atoms in an
object. Weight is the force of gravity pulling on you. Mass and weight have a directly proportional
relationship – the more mass you have the more weight you have.
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7. What is the weight of a 1kg brick?
Show the DA
Mass Resists Acceleration
8. Is acceleration directly proportional to mass, or is acceleration inversely proportional to mass?
Give and explain an example.
Newton’s 2nd law states that acceleration is inversely proportional to mass. Applying 100N to a 10 kg
mass gives an acceleration of 10 N/kg. However, applying the same force to a mass of 20kg yields an
acceleration of 5 N/kg. Twice the mass = half the acceleration.
Newton’s 2nd Law Links Force, Mass and Acceleration
9. If the net force acting on a sliding block is somehow tripled, then by how much does the
acceleration increase? Show me two maths!
Before
After
100N/10kg = 10 N/kg
300N/10kg = 30 N/kg
10. If the mass of a sliding block is somehow tripled at the same time the net force acting on it is
tripled, then how does the resulting acceleration compare to the original acceleration?
Before
After
100N/10kg = 10 N/kg
300N/30kg = 10N/kg
Friction is a Force That Effects Motion
11. Suppose you exert a horizontal push of a crate that rests on a level floor, and it doesn’t move.
How much friction acts on the crate compared to your push? Explain!
Since the crate is NOT moving, it is in static equilibrium with a Fnet of 0. If the Fnet = 0, then all the forces
balance or cancel out. A pull of 100N on an object that does not move is balanced or cancelled by 100N
of Ffrict
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12. As you increase your push, and the crate doesn’t move, will friction on the crate increase also?
Explain.
Since the crate is NOT moving, it is in static equilibrium with a Fnet of 0. If the Fnet = 0, then all the forces
balance or cancel out. A pull of 500N on an object that does not move is balanced or cancelled by 500N
of Ffrict
13. Once the crate is sliding, how hard do you push to keep it moving at a constant velocity? Explain!
Constant velocity indicates that speed and direction are constant (not changing). This is dynamic
equilibrium with a Fnet of zero. The pushing force is then equal to the Ffrict.
Newton’s 2nd Law Explains Why Objects in Free Fall have Equal Acceleration
14. Why doesn’t a heavy object accelerate more that a light object when both are freely falling?
Explain!
Free fall only takes into account the effect of Fgrav. Without air resistance to change the acceleration,
everything falls at the same rate since the Earth pulls on everything with the same amount of force.
Acceleration of Free Fall is less When Air Drag Acts
15. If two objects of the same size and shape fall through the air at different speeds, then which one
encounters more air resistance? Explain!
Air resistance builds as an object moves through air; the faster it moves the more air resistance builds.
Since the objects are the same size and shape, the faster falling object could have more mass and
would encounter more air resistance. It’s greater mass enables it to “plow” though the air resistance
better than the lesser mass object.
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