Exam 1--141 Flashcards
x_final = x_initial + (average velocity)*(delta t)
Explain why x_final is made up of these two parts
multiplying average velocity by delta t should give how far you went in the total time due to your velocity
however, this is not necessarily where you end up. the starting position matters
you need to add x_initial to how far you moved to get where you end up (x_final)
Why can you not just multiple acceleration by time to get velocity and then velocity by time to get distance traveled? The units work out?
This implies that the car is going for one velocity the entire time
The work needs to show that velocity is changing by adding 1/2at^2. Or the student needs to use the average velocity
Why can’t you throw a bowling ball as far as a baseball? Mass doesn’t come into play in kinematic questions?
It is hard to make the bowling ball have the same initial velocity as the baseball. Need to overcome more mass.
The reason kinematic equations don’t include mass is because we are only looking at the object when it is in the air, not before throwing it. We don’t care how hard it was to get to the initial velocity
Will a ball launched at 50m/s spend more time in the air on Earth or on Jupiter? On Jupiter gravity is 22.9 m/s there
The ball spends less time in the air on Jupiter
Once in the air, the ball has to slow to 0m/s and return to the ground. On Jupiter, you lose 22.9m/s of velocity every second where on Earth you only lose 9.8m/s every second. Therefore, the ball will reach 0 and turn around faster on Jupiter than on Earth
How can net force be zero and the object not be at rest?
If the object has no net force, then acceleration equals 0. However, you can still have a 0 acceleration when velocity is constant. This would mean there is no net force on the object (no change in velocity) but the object is not at rest either
How does a=-9.8 m/s2 fit in with Newton’s second law?
Since the only force acting on the object is mg (in the downwards direction) F=-mg=ma. If we divide by the m, we get that a=-9.8. This is in the y-direction.
There is no x component of acceleration in free fall because there is no force acting upon the object in the x and the horiztonal velocity remains constant during the trip
How is velocity encoded on a position versus time graph?
as slope
Why does a bowling ball continue straight after hitting a pin but the pin is knocked down? Does Newton’s 3rd law apply here?
Newton’s 3rd law does apply
The bowling ball does not feel as great of an acceleration because it has a larger mass to mediate the force (F/m=a)
Direction also works out because the pin experiences a positive acceleration to the right as it speeds up from rest
The bowling ball experiences a negative acceleration as its speed is decreased a little
What does the sign of velocity encode?
direction
Can velocity be positive if position graph is always negative?
yes
If accleration is positive does this mean you are speeding up?
No
If acceleration is positive but velocity is negative, you are actually slowing down. Going from bigger negative to zero
If accleration is negative does this mean you are slowing down?
No
If acceleration is negative but velocity is also negative, you are going faster in the other direction. Getting bigger negatives
Why does x_final = x_initial + Vit + 1/2at^2 work?
x_initial is the initial position
Vit is the distance traveled if always stayed at Vi for all the motion
1/2at^2 is the extra distance you go since velocity is changing
So how do a golf ball and lead ball fall at the same rate?
Earth pulls more on the lead ball than on the golf ball
Inertia mediates the motion
Inertia
quantifies how hard it is to change an object’s motion
Why does a ball land right back in a horizontally moving canon that it was launched vertically up from?
although the canon adds y-velocity, the x-velocity stays the same as the cart
x and y velocities are independent. the x velocity was given by the cart
x and y physics evolve simultaneously but separately
How is g a vector?
in the x direction there is 0
in the y direction, the entire force is downwards so the y component equals 9.8
Why does air resistance have a bigger effect on faster moving objects?
because air resistance comes from hitting air molecules and you hit many more when going fast
Why is drag more noticeable for larger surface area objects?
hit more air molecules
Why is drag more noticeable for small mass versus large mass objects?
inertia in larger mass objects lessens the effect of drag
What happens if you have more drag?
less range in trajectory
What is the Coriolis effect caused by?
the Earth rotating at different speeds under the launcher and the receiver (in a m/s sense)
Is speed larger or smaller at the equator?
speed is larger
at the equator, you have to rotate around more meters in a single day
What is the overall result of the Coriolis effect?
projectiles deflect to the launcher’s right in the North hemisphere
projectiles deflect to the launcher’s left in the souther hemisphere
*launcher is always tossing ball due North to reciever
Throwing a ball in northern hemisphere
receiver is not going as fast as the launcher near the equator
the ball will out pass the receiver and deflect right
Throwing a ball in southern hemisphere
receiver at equator is going faster than the launcher
the receiver will out pass the ball and the ball deflects to the left
Natural state of an object
an object on which no net force is currently acting
Newton’s first law
an object on which no net force is acting (natural state) will remain at rest or in motion at a constant velocity
What happens when you kick a bowling ball that is rolling on a frictionless surface?
the bowling ball starts in its natural state (constant velocity + direction)
after you apply a contact force, the ball returns to it’s natural state and moves in a straight line again
What causes acclerations?
unbalanced forces
mass
how much matter is in something
property of the object, independent of location
weight
a force
the gravitational pull as one object attracts another object
location dependent, not an inherent property of an object
Why can you not convert kilograms to pounds?
kilogram is a mass and pound is a measure of force (like a newton)
Is friction a contact force?
yes
Newton’s second law
F=ma
acceleration is caused by unbalanced force
Newton’s third law
if object A exerts a force on object B, then object B will exert an equal sized, but oppositely directed force on object B
What is crucial to understand about third law pairs?
they act on different objects!!
they cannot directly cancel each other out because they act on different objects
What does the friction equation account for?
material with the coefficient
direction (friction is always opposite to motion)
mashedness (press two surfaces together=more mashed. this is encoded in the normal force)
size of applied force (friction can adjust to a LIMIT to remain static)
How to determine if an object moves?
use the static friction formula and see if applied force is greater than the static friction force
If a person pushes two back-to-back boxes what is the accleration of each?
accleration is the same in each
If a person pushes two back-to-back boxes how much of the push force transmits to the second box?
None
push force is a CONTACT force and there is no contact
