Download Overheads - Physics 420 UBC Physics Demonstrations

Demonstration Design
Light Sensor
Light
Truck
Zero Line
As the truck passes the light, the light sensor turns off the power to the truck,
ensuring that the truck begins stopping at the same point on each run.
What do we need to know?
• Velocity of the truck on low speed:
d=
t=
v=
• Velocity of the truck on high speed:
d=
t=
v=
Derivation of Stopping Distance
• Equations you are
familiar with:
F=ma
W=Fd
W=ΔK
K=mv2/2
Derivation of Stopping Distance
Fd=ΔK
mad=mvf2/2 – mvo2/2
We see that the stopping distance is mass
independent.
ad=vf2/2 – vo2/2
In our case, vf=0. The equation becomes:
d= -vo2/2a
Can you explain the negative sign?
d = -vo2/2a
• Remember we are working with vector values.
v
a
d
While the value of v is positive, the value of a
is negative. Negative signs will cancel to give
a positive distance.
d= -vo2/2a = cvo2 where c is a constant
volow=
volow2=
dlow=
c=
Now predict stopping distance on high
speed
vohigh2=
dhigh=
A Note About Error
• Data varies over a range depending on the
equipment and the person making the
measurements. This is not uncommon.
• The more measurements used to find an
average value, the more accurate that
value will be.
• If the experimental value is equal to the
expected value within error, the
experimental value is correct.
• What are some sources of error in the
demonstration?
So what does this mean?
• When cars hit objects, the car is compressed,
not the object. The faster the car is going, the
more it will compress, and the greater the
likelihood of injury.
Review of Friction
Sliding friction:
• Acts between two surfaces
• Opposes the direction of
motion.
• Is proportional to the normal
force.
• Varies with the slipperiness
of the surfaces.
F = μN
Friction
• The friction between the
road and a car’s wheels
is called traction.
• Traction allows cars to
accelerate and to
change direction.
• What happens when the
surface the wheels
contact changes (the
coefficient of friction is
changed)?
Accelerating rolling object
How does friction act?
Applied force
acceleration
velocity
Normal
weight
force
(a)
friction
(b)
(c) No friction is acting
Force diagram of a accelerating rolling object
Applied force
acceleration
Normal
weight
force
friction
The answer is (b)
velocity
Why does rolling friction act this
way?
• Imagine that no friction exists between the
tires and the road. When the wheels are
rotated, they will simply spin in place (think
of peeling out)
• In order to make the tire travel forward, the
tire must “bite into” the road at the point of
contact - i.e., friction must exist in the
direction opposite the motion.
• Imagine the spinning wheel again. The
motion at the point of contact is towards
the left.
• The friction must act to the right in order to
make the wheel roll!
• Hence, the road pushes the tire.
Decelerating rolling object
How does friction act in?
Applied force
velocity
acceleration
Normal
weight
force
(a)
friction
(b)
(c) There is no friction force acting
Force diagram of a decelerating rolling object
Applied force
velocity
acceleration
Normal
weight
force
friction
The answer is (a)
Why does friction act this way?
• Imagine a tire rolling along. Now imagine
that the rotation of this tire is slowed by
some internal force (e.g. brakes).
• Remember Newton’s first law: an object in
motion will stay in motion unless an
external force acts upon it.
• The tire’s inertia will carry it on at the same
velocity (though slipping) unless rolling
friction slows the speed of the tire.
• This is what happens when your car skids.
Friction
• Applied force speeds up or slows down
rotation; friction force is what actually
accelerates or decelerates the object.
• Friction force varies with amount of
applied force.
• The maximum friction force is determined
by the coefficient of friction between the
wheel and surface. If the applied force
exceeds the maximum friction force,
slipping occurs.
What force is stopping the
truck?
FRICTION
Internal friction acting on the wheel axis is slowing
the rotation of the wheels.
However, according to Newton’s first law, an
external force must act upon an object to change
its inertia.
Rolling friction is what slows the moving body.
ABS
• Anti-lock breaks work by keeping the
breaking force on the tires equal to or less
than the friction force.
• The rotation then slows at the same rate
the car decelerates, so the car does not
skid.