Download Lecture 4

8/29/2011
Acceleration and forces
Physics 1010: Dr. Eleanor Hodby
Find your clicker!
Last time:
Motion at constant velocity:
• x = x0 +vt
• Velocity is gradient of position versus time graph
Motion at constant acceleration:
• v = v0 + at
• x = x0 + v0t + ½ at2
• Acceleration is the gradient of a velocity versus time graph
But what causes acceleration?  Forces
Today:
Day 4:
What causes acceleration?
Gravity
Reminders:
HW 2 due Monday night
Reading for Tues: 3.1, 2.2 Bloomfield
Help sessions today
Motion down a ramp 1 (with forces!)
Motion with constant force and acceleration
• Newton’s second law: Fnet = ma
• Force and acceleration due to gravity
Motion down a ramp (with forces!)
Sketch Velocity vs. time and Acceleration vs. time graphs for the car moving
away from the motion detector and speeding up at a steady rate.
+
0
time
-
Force
Force is a VECTOR
Units: Newton (N)
1N = 1kg * 1m/s2
Net force on object = Vector sum of all
forces acting on object
• N2: Fnet = ma
Car accelerates in same direction as net force:
Fnet = m a
(Newton’s second Law of motion)
Force and Acceleration are vectors, mass is a scalar
Force is pointing in positive direction. So acceleration is also positive
Motion down a ramp 2 (with forces!)
Sketch Velocity vs. time and Acceleration vs. time graphs for the car moving
away from the motion detector and slowing down at a steady rate.
+
Velocity
time
0
time
Acceleration
Velocity
0
Acceleration
Clearly identify the
POSITIVE
direction of motion
What causes the acceleration down the track?
There is a net force on the car.
Force of gravity pulls cart down the track
+
•
•
•
•
+
+
0
time
-
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8/29/2011
Motion down a ramp 3 (with forces!)
Motion down a ramp 4 (with forces!)
v start
v start
+
0
time
-
+
0
time
Gravity results in a constant force on
the car in the direction away from the
motion detector.
Useful hints:
- moves toward the motion detector,
slowing down at a steady rate
- stops instantaneously
- reverses direction and speeds up
(Start your graph after the push that
gets the cart moving.)
0
time
Acceleration
Acceleration
Velocity
Sketch Velocity vs. time and Acceleration vs. time graphs for the
following situation: Start car up ramp towards detector with a push
Velocity
+
+
0
time
-
The car is released from rest and exhibits
the velocity and acceleration graphs
given by the dashed lines.
Now, add weights to the car and redo
experiment.
Sketch the new velocity and acceleration
graphs on the same axes.
-
Gravity
Force and acceleration due to gravity
Dropping stuff
I drop heavy metal ball and a hacky sack from lecture hall ceiling
a. the light ball will fall fastest and hit the ground first
b. they will fall at the same speed and hit the ground together
c. the heavy ball will fall fastest and hit the ground first.
d. neither will fall, they will stay suspended in mid air
e. they will both fall up and hit the ceiling.
Metal
Hacky sack
Question: Relating position, velocity and acceleration
Toss a basketball straight up in air with initial velocity v0
Plot position, velocity and acceleration vs time.
position
Dropping stuff
I drop heavy metal ball and a hacky sack from lecture hall ceiling
b. they will fall at the same speed and hit the ground together
+
0
time
Fgravity on
hacky sack
Velocity
+
0
time
Fgravity on
metal ball
Acceleration
+
0
time
-
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8/29/2011
If I drop the basketball off a tall building, what will the ball’s
the velocity be after 0.5 s:
a. 1 m/s
b. 9.8 m/s
c. -9.8 m/s
d. Not enough information given to determine
e. -4.9 m/s
0
time
Acceleration Velocity
+
0
time
+
0
+100
+
0 meters
position
b.
time
Would these plots for a, v and x change if I used a medicine ball instead of a
basketball?
a. No
b. Yes
Common confusion: Weight and Mass…….
Mass is a measure of how much stuff an object has
Units: kg (old fashioned units = lb)
Forcenet = mass X acceleration
Units: Newton (N).
1 kg X 1 m/s2 = 1N
Darn, I’ve gained
10N this week
Weight = force of gravity on an object of mass m
Measured in N NOT kg or lb
kg and lb are units of mass
-100
-
The acceleration due to gravity on the Moon is less than it is on Earth.
Suppose
mE = your mass on Earth
mM = your mass on the Moon
wE = your weight on Earth
wM = your weight on the Moon.
Which statement is correct?
a)
b)
c)
d)
mE > mM,
mE = mM,
mE = mM,
mE > mM,
wE > wM
wE > wM
wE = wM
wE = wM
e) None of these
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