Download 7.3-7.4

Suppose a cannon is propped against a
massive tree to reduce recoil when it fires.
Then the range of the cannonball will be
1. increased.
2. decreased.
3. unchanged.
Collisions (7-3, 7-4)
Elastic Collisions
• If we choose our system so there is no net
external force on the system, then the
momentum of the system will be conserved
during the collision.
A particle of mass m moving with speed v
collides head-on with a second particle of the
same mass at rest. What are the speeds of the
two particles after the collision, if it is elastic?
• Kinetic energy may or may not be conserved
What if the second particle has a mass of 100m?
• Three kinds of collisions:
– Elastic: total kinetic energy is conserved
– Inelastic: total kinetic energy not conserved
– Completely inelastic: if the objects stick
together after colliding
Newton's Cradle
How does it work? i.e. why do equal numbers
of balls bounce on each side?
Inelastic Collisions
A cart moving at speed v collides with an
identical stationary cart on an airtrack, and the
two stick together after the collision. What is
their velocity after colliding?
A. v
B. ½ v
C. zero
D. -½ v
E. -v
Ballistic Pendulum
A ballistic pendulum can be used to
measure the speed of a projectile. It
consists of a large block of wood
hanging from two long cords. The
projectile is fired into the block, quickly
coming to rest. The block + projectile
swing upward and their maximum
vertical displacement is measured.
If MA = 100 g, MB = 2.733 kg and
h=7.9 mm, find the initial speed of
the arrow
Practice: Compare the initial KE of the
bullet and the final KE of the
block+bullet. Where did the energy go?
Collisions in Two Dimensions (7-4)
A marble moving with speed 3 m/s in the +x
direction hits an equal-mass marble initially at
rest. The two balls move off at 45º, marble 1
above the x axis, and marble 2 below. What are
the speeds of the two marbles after the collision?