:acceleration and force Physics :acceleration and force NB p 47 NOTES.
-
Upload
ralph-greene -
Category
Documents
-
view
235 -
download
1
Transcript of :acceleration and force Physics :acceleration and force NB p 47 NOTES.
:acc
ele
ratio
n an
d fo
rce
Phy
sics
Free Body Diagram HO 3.4
A free body diagram is a pictorial representation of all forces acting on an object.
We consider the object to be isolated from the physical system and choose to examine only the forces directly acting ON the object, not forces applied BY the object.
Explanation
:acc
ele
ratio
n an
d fo
rce
Phy
sics
Free Body Diagram Example
Explanation
Label the forces acting on the block.
:acc
ele
ratio
n an
d fo
rce
Phy
sics
Free Body Diagram Example
Explanation
Force on the block by the ground: FNormal
Force on the block by the earth: Fgravity
Force on the block by Antonio
Force on the block by the rope.
Force on the block by the ground: Ffriction
:acc
ele
ratio
n an
d fo
rce
Phy
sics
Galileo’s Investigation
Engagement
What are the similarities and differences between a basketball and a tennis ball?
:acc
ele
ratio
n an
d fo
rce
Phy
sics
Galileo’s Investigation
Engagement
What will happen if the balls are released from the same height above the floor?
Try to use Physics vocabulary in your response.
:acc
ele
ratio
n an
d fo
rce
Phy
sics
Galileo’s Investigation
Engagement
What’s going on here, why did they move?
How would you describe the motion of the balls?
– Path
– Speed
Again, try to use Physics vocabulary in your response.
:acc
ele
ratio
n an
d fo
rce
Phy
sics
Force of Gravity: Pull or Push?
Force can be described as a push or pull one object exerts on another.
One specific force is weight. Weight = the force of gravity on an object.
But what causes gravity?
– The presence of mass
Explanation
:acc
ele
ratio
n an
d fo
rce
Phy
sics
Newton’s Universal Law of Gravitation HO 3.2
Masses attract one another about their centers with a force proportional to the product of their masses and inversely proportional to the square of the distance between them (whew!).
Explanation
F =G m1 m2 ÷ d2
Fweight = mg
:acc
ele
ratio
n an
d fo
rce
Phy
sics
Newton’s First Law
Objects require a force to change their velocity.
– Another term for this property of matter is inertia
What is the principle property of an object that determines its inertia?
– mass
Explanation
:acc
ele
ratio
n an
d fo
rce
Phy
sics
Newton’s First Law Example
If we apply the same force (push) to a bus and a bicycle, which object will experience a greater change in velocity?
Explanation
:acc
ele
ratio
n an
d fo
rce
Phy
sics
Newton’s Second Law
Force causes a change in motion
– Change in motion means change in velocity
– Change in velocity means???
A change in velocity occurs in a certain amount of time.
AHA! That’s a rate.
Explanation
F = ma
:acc
ele
ratio
n an
d fo
rce
Phy
sics
Acceleration is a Rate
The speedometer in your car indicates what property of motion?
What happens to the needle if you speed up or slow down?
The speed of the needle is a measure of your acceleration (in a straight line).
Acceleration is the rate of change in velocity.
Explanation
:acc
ele
ratio
n an
d fo
rce
Phy
sics
Newton’s Second Law Example #1
Important note: the acceleration vector is always in the same direction as the force vector.
If we apply a 100N force to a wagon full of rocks initially at rest with a mass of 50 kg, what is its acceleration?
What does this answer tell us?
What is the velocity of the object after 1 second?
– 2 seconds?
– 5 seconds?
Explanation
F = ma
100 N2 m/s2
:acc
ele
ratio
n an
d fo
rce
Phy
sics
Newton’s Second Law Example #2
What happens when the acceleration vector points opposite to the velocity vector?
Explanation
30 m/s
F -5m/s2
How long will it take Mighty Mikey to stop the train?
:acc
ele
ratio
n an
d fo
rce
Phy
sics
Newton’s Third Law
No single isolated force can exist
Equal and Opposite forces exist only between two objects
Explanation
Push on water
WRONG!
:acc
ele
ratio
n an
d fo
rce
Phy
sics
Newton’s Third Law Example
You push on the water The water pushes back on you with an equal force in the
opposite direction So why do you move?
Explanation
Push on water
CORRECT!
Push on swimmer
:acc
ele
ratio
n an
d fo
rce
Phy
sics
Acceleration Due to Gravity
The Tennis Ball and Basketball hit the ground at the same time because they accelerated at the same rate (~10 m/s2).
What force changed their velocities?...Weight.
Weight = mass X gravity (FW=mg).
Explanation
Wbb>Wtb
:acc
ele
ratio
n an
d fo
rce
Phy
sics
Acceleration Due to Gravity
Explanation
F = weightF = weight
m = massm= mass
F/m=a F/m = a
a = gregardless of mass!
:acc
ele
ratio
n an
d fo
rce
Phy
sics
Free Body Diagram Example 2
Forces on coffee filter before release (balanced forces)
Explanation
Weight
Support from my
hand
:acc
ele
ratio
n an
d fo
rce
Phy
sics
Free Body Diagram Example 2
Forces on coffee filter immediately after release (unbalanced force acceleration)
Explanation
Weight
:acc
ele
ratio
n an
d fo
rce
Phy
sics
Free Body Diagram Example 2
Forces on coffee filter while falling
Explanation
Weight
Air resistance
What factors influence the magnitude of
the force of air resistance?
:acc
ele
ratio
n an
d fo
rce
Phy
sics
So Why Different Times?
Force due to gravity is different for the two objects
Their masses are different and therefore they have different inertias
They do not accelerate at the same rate …WHY???
– We must examine ALL the forces acting on the objects in order to determine the Net Force.
Explanation
:acc
ele
ratio
n an
d fo
rce
Phy
sics
Net Force
If you earn $2000/mo but taxes, “free” health care, social security, and other reductions total $1350/mo then your:
– Gross Pay = $2000/mo
– Net Pay = $650/mo
Explanation
Net Force
F
:acc
ele
ratio
n an
d fo
rce
Phy
sics
Net Force
The force of air resistance is greater for the basketball than the coffee filter.
However, the force of air resistance is negligible compared to the weight of the basketball, therefore the Net Force is toward the floor.
Explanation
Air resistance
Weight
:acc
ele
ratio
n an
d fo
rce
Phy
sics
So Why Different Times?
The force of air resistance is also acting on the coffee filter and is comparable in size to the weight.
In fact, at some point, the force of air resistance is equal to the weight of the coffee filter.
When this is the case:
– There is NO net force
– Therefore there is no acceleration
– Therefore the object moves at a constant speed
This equilibrium condition is called terminal velocity
Explanation
:acc
ele
ratio
n an
d fo
rce
Phy
sics
Evaluation
Sky Diver Physics HO 3.5
A skydiver jumped out of a plane 1 sec ago.
– Label the forces on the skydiver
After some time, the skydiver has zero acceleration
– What does this indicate about her speed?
– What is the name for this equilibrium condition of a falling object?
– Label the forces on the skydiver
What factors influence the force of air resistance on the skydiver?
BONUS: What is the terminal velocity of a human?