Transcript Lecture 4, Sept 10, 2008

Phy100: More on Energy conservation
Mechanical energy (review);
Goals:
Work done by external forces;
Understand conservation law for isolated systems.
A pebble tossed into the air
Q1
The pebble is tossed up with speed 10m/s.
Which of the following is correct ?
A) It reaches a maximal height of about 10m when all
kinetic energy is converted into the potential energy.
B) It reaches a maximal height of about 10m when the
speed is maximal.
C) It reaches a maximal height of about 5m when the
speed is maximal.
D) It reaches a maximal height of about 5m when the
speed is zero.
Q2
Your result (i.e. the location of the turning
point or the maximal height above the
ground) depends on
A) Mass and initial velocity;
B) Choice of the zero point for the potential
energy;
C) Mass and gravitational acceleration;
D) Initial velocity.
Simulations of energy conservation--Energy skate park
Work and Energy
Work done by an external force over a
distance d is
W= + F d if the force is along the direction of
motion;
W= -Fd if the force is opposite to the direction
of motion.
Power = W/ T, work done per unit time.
Energy transfer, work and energy conservation
for mechanical systems
W= +Fd or -Fd
Q3
A block of mass 1kg with zero initial velocity is pushed on a
frictionless surface over a distance 1m; the exerted force is 2N.
The final velocity is
A)
B)
C)
D)
1m/s;
4m/s;
2m/s;
None of above.
Q4
A block of 2kg slides on a surface with an initial
velocity 5m/s. The frictional force between the
block and the surface is 5N.
How long can the block travel before coming to a
stop?
A) 1m;
B) 10m;
C) 5m;
D) 4m..