Download - Review the relationship between force and acceleration

TODAY’S OUTCOMES:
FORCE, MOTION AND ENERGY
- Review the relationship between force
and acceleration
- Investigate how mass relates force and
acceleration
- Study the effects of force acting on
objects
The speed limit in Centerburg is 35 mph (15 m/sec); at the city limit there is a sign saying
"Resume safe speed." Maria starts accelerating at the sign but it is 5 seconds before she
reaches 70 mph (30 m/sec). What is her acceleration?
acceleration = (final speed – initial speed) / time
= (30 m/sec – 15 m/sec) / 5 sec
= 15 m/sec / 5 sec
= 3 m/sec2
Where should the policeman park, to catch Maria just as she gets up to speed?
Andrea says, "75 meters from the city limits." Betsy says, "115 meters from the city
limits." Carla says, "150 meters from the city limits." Who's right, and why?
distance = average speed ✕ time
average speed = ½ (initial speed + final speed)
= ½ (30 m/sec + 15 m/sec)
= 22.5 m/sec
distance = 22.5 m/sec ✕ 5 sec
= 112.5 m
This is close to 115 m, so Betsy is right
Discussing the homework you recently handed in:
How long does it take for a car to get to 22 mph (10 m/sec) starting from rest (assuming
normal driving, like when you leave a stop light)? What is the corresponding
acceleration?
acceleration = (final speed – initial speed) / time
= (10 m/sec – 0)/5 sec.
= 2 m/s2
How long does it take to stop a car, when it is going 22 mph (again assuming normal
driving)? What is the corresponding acceleration?
acceleration = (final speed – initial speed) / time
= (0 – 10/m/sec)/4 sec.
= –2.5 m/s2
“deceleration” is just a
negative acceleration
DISTANCE VERSUS TIME GRAPHS:
SOME RULES FOR INTERPRETING........
green = moments
of zero speed
motion
forward
motion
backward
Rules for speed:
1) Any “flat” region (or the top of a
“hill”, or bottom of a “valley”)
means the speed is zero.
2) An “uphill” slope means motion
forward ; a “downhill” slop means
motion backward.
3) The steeper the slope, the higher
the speed.
slow
fast
slow
fast
slow
DISTANCE VERSUS TIME GRAPHS:
SOME RULES FOR INTERPRETING........
blue = (almost)
constant
speed
Rules for acceleration:
1) If you can draw a straight line on
the plot, speed is (nearly) constant
over that interval.
2) If there’s a curve, there’s an
acceleration.
– accel.
red =
acceleration
+ accel.
3) Curved “valley” is forward
(positive) acceleration, a curved
“hill” is backward (negative)
acceleration.
DISTANCE VERSUS TIME GRAPHS:
SOME RULES FOR INTERPRETING........
Rules for acceleration:
1) If you can draw a straight line on
the plot, speed is (nearly) constant
over that interval.
+ acceleration
+ acceleration
– acceleration
2) If there’s a curve, there’s an
acceleration.
3) Curved “valley” is forward
(positive) acceleration, a curved
“hill” is backward (negative)
acceleration.
Is it possible to have zero speed & velocity, but still
have acceleration? YES!
zero speed,
but negative
acceleration
Velocity is zero
for the instant Ball thrown in the air
at the peak
Velocity changes
direction
Acceleration
is always
nonzero and
downward
Last week’s lab,
balanced forces:
Monday’s lab,
rolling a ball downhill:
Force (Newtons)
acceleration (m/sec2)
HOW ARE FORCE AND ACCELERATION RELATED?
height of hill
Pulling a cart
uphill at a
constant
speed
height of hill
What relation do we see between force and
acceleration?
HOW ARE FORCE AND ACCELERATION RELATED?
Combine the
plots:
Force
acceleration
height of hill
Force is proportional to acceleration.
∝
Force
acceleration
Force = (a constant) ✕ acceleration
WHAT YOU ARE EXPECTED TO KNOW:
- How to interpret distance vs. time plots with
both straight lines and curves
- How to use the relations between initial speed,
final speed, average speed and acceleration in
problem-solving
- The relationship between force and
acceleration
TODAY’S OUTCOMES:
FORCE, MOTION AND ENERGY
- Review the relationship between force
and acceleration✓
- Investigate how mass relates force and
acceleration
- Study the effects of force acting on
objects