Transcript questions

DAY 19 LETS GO!

THE END OF THE QUARTER

Topic 3 Forces continues

Exam next class
END OF QUARTER EXAM


Purpose: A summary of the class since the last
exam
Major ideas from the ungraded work, HW, labs,
demos

Occurring: next class

Topics: All on website: Videos as well

I LOVE THIS QUESTION (because…)
All that matters is the time in air. Which
direction controls time in air?
 Y!
 So which shot has less Y velocity?
 The one to B, because it did not go as high.

REMEMBER

Remind yourself of each of Newton's Laws AND
what they mean.
LETS MAKE SURE…DEMO IT

INERTIA
MEET THE FORCES
The
force of EARTH pulling:
MEET THE FORCES
The
force of the SURFACE
pushing:
MEET THE FORCES
The
force of the SURFACE
pushing opposite motion:
Coefficents:
STATIC
KINETIC
Would
the normal force change..
-If the mass of the block
changed?
If
you pulled right as well?
ELEVATORS
In
an elevator, you feel
lighter/heavier in your knees at
the start/end of the elevator ride.
And
it depends if you go DOWN
or UP….
LETS TRY
 Elevator
velocity:
going UP at a constant
LETS TRY
 Elevator
going UP and speeding up
LETS TRY
 Elevator
going UP and slowing down
LETS TRY
 Elevator
velocity
going DOWN at constant
LETS TRY
 Elevator
up
going DOWN and speeding
LETS TRY
 Elevator
down
going DOWN and slowing
6) An elevator lifts a panda up at an upwards
acceleration of a.

a) Which is larger then, the up force of the
elevator cable or the weight?

6) An elevator lifts a panda up at an upwards
acceleration of a.

c) Draw a FBD of the panda, call the
elevator force the normal force, the panda's mass
M.

6) An elevator lifts a panda up at an upwards
acceleration of a.

d) Create a net force equation for the panda's
situation.


e) If the elevator had 600N of force up, how
much mass was the panda?
THE EXAMPLE I DO

A force of 20N is needed to overcome the
frictional force of 5N and accelerate a 3kg mass
across the floor. What is its acceleration?
YOU TRY…
A 10kg mass is moving to the right at 6m/s, and
you are pulling it right with a force of 20N.
Friction is present.
 A) If you want the mass to come to a stop in 12
seconds, what acceleration would it have?

YOU TRY…
A 10kg mass is moving to the right at 6m/s, and
you are pulling it right with a force of 20N.
Friction is present.
 B) How much friction is required to stop it after
12 seconds even though you are pulling it.

MY EXAMPLE

A block of 2kg is moving at a velocity of 3m/s. The
coefficient of kinetic friction is .2. If it is moving left,
and you push right with a force of 6N, what is the
acceleration of the object?
EXAMPLE


A 7kg block is on a rough surface, and is being
pulled at an angle.
1) Draw a FBD
with X and Y split
up
EXAMPLE


A 7kg block is on a rough surface, and is being
pulled at an angle.
2) Is the normal
force more, less or
equal to the weight?
EXAMPLE


A 7kg block is on a rough surface, and is being
pulled at an angle.
3) If it was moving
right, could the
net force be left?
FORCES AT ANGLES AGAIN



A force on a surface will mean that the angled
force will have an effect on the normal force.
You use Sin and Cos to find the X and Y parts of
the force
If there are other forces in the Y direction, then
the normal force does NOT equal the weight
LETS DO ONE


You pull a cat again at an angle. There is
friction opposing your pull.
1) Draw a FBD
and then another with
the X and Y components
Pull in
Y
Ff
Fn
Pull Y Force of person on the cat
vertically
Pull
Force
person
the
Pull:X:
Force
of of
person
onon
the
catcat
horizontally
Weight: Force of Earth on the
cat
Fn: Force of ground on cat
Ff: Force of ground on cat
Pull
Cat
Pull in
X
Weight
Pull Y Force of person
on the cat vertically
Pull X: Force of person
on the cat horizontally
QUESTIONS

1) Which is larger, the normal force or the
weight?
Fn
Pull in Y

The weight, since the cat is in
equilibrium in Y. And theFf
weight is balanced by the
normal force AND the Y pull.
Weight: Force of Earth
on the cat
Fn: Force of ground on
cat
Ff: Force of ground
on cat
Cat
Pull in X
Weight
Pull Y Force of person
on the cat vertically
Pull X: Force of person
on the cat horizontally
QUESTIONS

Weight: Force of Earth
on the cat
2) If you pull at a larger angle, will the weight Fn: Force of ground on
cat
or the normal force change?
Fn
Ff: Force of ground
Pull in Y
The weight will NOT, since
it’s the Earths pull on theFf
cat.
The normal force WILL,
since you are still in equilibrium in Y,
your larger pull in Y will decrease the
normal force needed from the table.
 In other words…
on cat

Cat
Pull in X
Weight
Pull Y Force of person
on the cat vertically
Pull X: Force of person
on the cat horizontally
QUESTIONS

Weight: Force of Earth
on the cat
3) If the cat was moving at a constant velocity,Fn: Force of ground on
cat
would increasing the angle change
Fn
Ff: Force of ground
Pull in Y
on cat
the friction?
Ff
YES! Since friction depends
Cat
on normal force (Ff = µFnormal)
then more of a Y pull will decrease
the normal force, resulting in less friction
 In other words…

Weight
Pull in X
Pull Y Force of person
on the cat vertically
Pull X: Force of person
on the cat horizontally
QUESTIONS

4) Say that the coefficient of friction is µk and
pulled at angle θ. Lets create
Fn
2nd Law equations for X and Y Pull in Y
Weight: Force of Earth
on the cat
Fn: Force of ground on
cat
Ff: Force of ground
on cat
Ff
Cat
X: Lets say the acceleration
will be with the pull (my guess) so +. There is an
imbalance then.
 X: F1x – Ff = imbalance
Weight
 Y: No imbalance, call down –
 Y: -Weight + FNormal = equilibrium

Pull in X
QUESTIONS
 X:
F1x – Ff = imbalance
 X : F1Cosθ – µFnormal = ma
 Y: -Weight + FNormal = equilibrium
 Y: -mg + FNormal = 0
SUMMARY




When changing the angle of a force on a surface,
the normal force changes.
Which changes the friction as well.
A downward push results in MORE normal force,
an upward push results in LESS normal force.
Friction is the coefficient of friction times the
normal force. Use static for non-moving, kinetic
for moving