Transcript Momentum and Energy PP

Momentum and Energy
ISCI 2002
Objects in Motion - Momentum
1. “Inertia” in motion
2. Momentum = (mass) x (velocity)
3. Greater the mass > Inertia > momentum of the
object
Changes in Momentum
1. If the mass or velocity of an object changes – momentum
changes
2. Acceleration is the result
3. __________ produces acceleration. Which of Newton’s Laws
apply?
4. What else affects acceleration?
•
Time – how long the force is applied
5. Impulse = force x time interval
•
Impulse = change in momentum or Ft = Δmv
Conservation of Momentum
1. If a physical quantity remains unchanged –
‘conserved’
2. In the absence of an external force, the momentum
of a system remains unchanged.
Momentum Conservation
1.
In the exploding cannon demonstration, total system momentum is
conserved. The system consists of two objects - a cannon and a tennis
ball. Before the explosion, the total momentum of the system is zero
since the cannon and the tennis ball located inside of it are both at rest.
After the explosion, the total momentum of the system must still be zero.
If the ball acquires 50 units of forward momentum, then the cannon
acquires 50 units of backwards momentum. The vector sum of the
individual momenta of the two objects is 0. Total system momentum is
conserved
on in mind. Eight-ball is one of them! Go to a pool table, and the object is to hit the correct balls in chosen pockets using the momentum of th
Conservation of Momentum
When you lift a sphere on one side and let
it go (Figure 1), its energy gets transferred
from the original metal sphere, through
the ones in the middle, all the way to the
furthest sphere from the original (Figure
2). The last sphere to receive the energy
continues with the momentum of the
original sphere, until gravity overcomes its
inertia and pulls it down (Figure 3). The
energy is then transferred back through
the spheres in the middle to the first
sphere (Figure 4), where the cycle starts all
over again
Collisions
1. Collisions
•
Elastic vs inelastic
2. Net momentum before = net momentum after
3. Case of the ‘coupled’ train cars.
M = 1.0 kg
V = 2.0 m/s
M = 1.5 kg
V = 0 m/s
V = ??
Elastic Collision
The gray rubber ball collides with the
static red rubber ball.
KE
Here momentum and kinetic energy is
conserved.
Almost no energy is lost to sound, heat
or deformation.
The KE from the first ball is transferred
to the second ball
Work and Power
1. Work = Force x Distance
2. Force is exerted, and something is moved!
3. Unit = N x m or Joule
4. Power = work done / time interval
5. Unit = Watt
Energy
Previously studied ‘heat’ energy.
• Different forms of energy
• Sound, electricity, radiation, etc.
• Objects may:
1. Store Energy (PE) or possess movement (KE)
2. Potential Energy – Energy of Position
•
Gravitation Potential Energy (mgh)
3. Kinetic Energy – Energy of Motion
•
•
Transfer; transformation
KE = ½ mv2
Energy, Work and Simple Machines
• Machines
• Multiply forces; change direction of a force
• Conservation of Energy is the key
• Work Input = Work Output
• (Force x distance) = (Force x Distance)
• A car jack
• A girl pushes the handle with a force of 50 N (input force) a
distance of (input distance) 0.25 cm. The car (output
force) weighs 5000 N rises a distance of 0.25 cm (output
distance).
• Car rises with 100 times the force!
• Multiplies the initial force.
Mechanical Advantage
Efficiency
• Simple machines
• Same energy input, but increases work
• More efficient
• Efficiency = work done/energy used