Transcript Physics 101 Fall 02 - University at Buffalo

Lecture 8, Pre-Flight Questions 1&2
Consider the following situation: You are driving a car with constant
speed around a horizontal circular track. On a piece of paper, draw a
Free Body Diagram (FBD) for the car. How many forces are acting on
the car?
1
2
3
4
5
correct
FN
f
W
HW: car
Lecture 8, Pre-Flight Question 3 & 4
The net force on the car is
1. Zero
2. Pointing radially inward
3. Pointing radially outward
FN
correct
f
R
W
SF = ma = mv2/R
If the car has constant velocity, there is no
acceleration, no acceleration=no net force
For the car to stay traveling around a horizontal
circular track, there must be a net force pointing
radially inward, toward the center of the circle. If
there wasn't, the car would drive in a straight line.
the car is akin to a ball on a string that is moving in a
circle. If you let the ball go it will fly about a 90
degree angle from where you let it go away from the
circle.
Lecture 8, Pre-Flight Questions 5 & 6
Suppose you are driving through a valley whose bottom has a circular
shape. If your mass is m, what is the magnitude of the normal force FN
exerted on you by the car seat as you drive past the bottom of the hill
1. FN < mg
2. FN = mg
3. FN > mg
correct
a=v2/R
R
FN
v
mg
There will be a centripetal force pointing up,
reducing N=mg.
SF = ma = mv2/R
when level your normal force = your weight
FN - mg = mv2/R
Since there is centripetal acceleration, the normal
force is greater than simply Mg
FN = mg + mv2/R
I don't know the real reason but I was thinking that when you are skating
in a half pipe and you reach the very bottom it feels like you are being
pulled down by more then just your weight.
From personal experiences with hills, as you pass the bottom of the hill
your body kind of scrunches down towards the seat. This extra force that
you feel has to be exerted back on you according to newton's 3rd law,
therefore the normal force must be greater than the gravitational force.
If you are at the exact bottom of the hill, the ground is "flat". The car isn't being
affected by acceleration due to gravity as a result of driving downhill or uphill so
it's just like you're driving on regular land. The car isn't going up or down so the
normal force and the force of gravity acting upon it (mg) are the same.
According to newtons third law, forces are always equal and opposite
Because when you go on a rollercoaster, and you go down a huge drop, your body
feels really really heavy once you get to the bottom of the hill. It's like your body
is being pushed into the seat. Therefore, your normal force is less then your
weight.