Physics 2211: Lecture 30

1
Physics 2211 Lecture 30
Conservation Laws Momentum Impulse
2
Question 1 (do it using 3rd Newton law)

• A box of mass m1 1.5 kg is being pulled by a
  horizontal string having tension T 90 N. It
  slides with friction (mk 0.51) on top of a
  second box having mass m2 3 kg, which in turn
  slides on a frictionless floor.
• What is the acceleration of the second box ?
• (1) a 0 m/s2 (2) a 2.5 m/s2 (correct) (3)
  a 3.0 m/s2 (4) a 3.0 m/s2

slides with friction (mk0.51 )
T
m1

a ?
m2
slides without friction
3
Block 1, of mass m1, is connected over an ideal
(massless and frictionless) pulley to block 2, of
mass m2, as shown. Assume that the blocks
accelerate as shown with a magnitude of
acceleration a and that the coefficient of
kinetic friction between block 2 and the plane is
m.
Express the ratio       in terms of  ,  ,
 , and  .
4
Question 3 (8.43)

• A 74.0 tightrope walker stands at the center of
  a rope. The rope supports are 10 m apart and the
  rope sags 11.0 at each end. The tightrope
  walker crouches down, then leaps straight up with
  an acceleration of 8.40 to catch a passing
  trapeze.
• What is the tension in the rope as he jumps?
• 1) 2221N 2)2752N 3)3089N 4)3254N 5)3530

5
Question 4 (8.31)
You see the boy next door trying to push a crate
down the sidewalk. He can barely keep it moving,
and his feet occasionally slip. You start to
wonder how heavy the crate is. You call to ask
the boy his mass, and he replies "49.0 kg   ."
From your recent physics class you estimate that
the static and kinetic coefficients of friction
are 0.800 and 0.4 for the boy's shoes, and 0.5
and 0.2 for the crate. Estimate the mass of the
crate. 1)196kg 2)252kg 3)286kg 4)312kg 5)350kg
6
QUESTION 5 (8.29)
A rope attached to a 17.0kg    wood sled pulls
the sled up a 19.0deg snow-covered hill. A
10.0kg    wood box rides on top of the sled. If
the tension in the rope steadily increases, at
what value of the tension does the box slip?
1)110N 2)128N 3)155N 4)178N 5)224N
7
Conservation Laws
The most fundamental laws in physics are
about things that dont change!
8
Conservation of Mass
mass is the same (conserved) before and after
any physical process
stoichiometry in chemical reactions
explosion of a firecracker
formation of the planets
9
Conservation of Energy
We will learn that energy comes in various types

• Kinetic Energy
• Potential Energy
• Thermal Energy

Total (sum) energy is conserved in physical
processes.
10
Conservation Laws in Physics
11
MOMENTUM is very useful for analyzing COLLISIONS
Simulation of a collision between two buckyballs
12
definition of momentum
13
From the blue curve
does none of the above.
14
Newtons 2nd Law
Impulse
15
Note double subscript
16
Chalk Time!
17
1
2
3
4
Which blue vector best represents the impulse for
this situation?
18
For each component separately, can use Momentum
Bar Charts
19
A particle is at rest at t0. What is the
direction of the momentum vector when the forces
stop acting?
20
Discussion The impulse required to bring the car
to a stop is the same in the two cases shown
below. Why is the amount of damage to the car so
different?
21
Does the weight of an hourglass change when the
sand is falling?
22
Therefore, the weight of each grain during its
free fall get counted as momentum
lost by some other grain when it hits the
pile.
23
Recap
Conservation Laws Momentum Impulse
Next Lecture
Conservation of Momentum Section 9.3