Chapter 4
ISCI 2002
Momentum and Inertia
• (1). Momentum is inertia in motion
– Momentum = mv
• (2). A heavy object in motion will have a great deal of
momentum
• (3). Mass is constant; momentum changes
– Change in velocity
•
Acceleration occurs
Impulse
• (1). Force produces acceleration which changes velocity
(changes the momentum)
– Time of force
•
•
Longer force applied, more change in momentum
Impulse = (F)(t)
• (2). Impulse-Momentum Relationship
– Ft = (mv)
•
Net force applied over a period of time changes momentum
Examples
• (1). Why are cannons barrels so long?
– Applied force on ball is extended
•
Increased impulse, increased momentum ,
• (2). Your Choice: Hitting a wall or soft surface when
driving a car?
– Of course the soft surface
•
Relate momentum , impulse and impact
•
When you hit a soft surface you extend time your momentum is brought to zero, extend impact which
reduces force that will be incurred.
Conservation of Momentum
 (1). Mirrors Newton’s Third Law
 As the cannonball gains momentum the cannon also
gains momentum by recoiling

Net momentum = zero
 (2). If an object has momentum
 No change unless affected by unbalance force
 Which Law?
Collisions and Momentum
• (1). One object in motion – with a specific momentum hits second
object at rest
–
–
–
–
–
First object collides and stops
Second object moves with Vi of first ball
“elastic collision”
Momentum is transferred from one ball to another. “rebound”
Conservation of momentum
• (2). Object one collides with static object two
– No rebound (freight cars) “inelastic”
– Object one (moves 10 m/s) so
•
•
•
(net mv)before = (net mv)after
(m x 10)b = (2m x v)a *2m refers to twice mass or two cars moving
V = 5 m/s (using the formula, velocity of object hit can be predicted)
Energy
• (1). Impulse = (F)(t)
– (F)(d)? Force over a specific distance
– Refers to energy – work
• (2). Energy
– Sun, food, heat, etc.
• (3). Work = (F)(d)
– When work is done energy is transferred to an object
– Unit of work = Nm or the joule (J)
Power
 (1). Work done/time interval
 Measure or rate of how fast work is done
 Rate at which energy is changed from one form to
another
 Unit: watt

One watt = used when one joule of work is done in one
second.
PE vs KE
• (1). PE
– Stored energy
– Chemical energy (fuels, glucose)
•
Chemical changes occurs makes energy available
–
•
Can do work
Gravitational PE
–
–
–
Work to elevate objects against gravity
GPE of an object raised = work done lifting against gravity
PE = mgh
• (2). KE
– Object in motion or moving and energy involved
•
•
•
Object hits another transfers energy
GPE of object transforms into KE when object is dropped
KE = ½ mv2
Work-Energy Theorem
 (1). Work = ΔKE
 Or (F)(d) = ΔKE
 Work done on an object basically equals gain in KE by
that object
 Push on a box


it slides (you are doing work)
It gains KE (moving)
KE vs Momentum
 (1). Properties of moving things but:
 Momentum is a vector – movement in a direction
 KE is scalar – can never be cancelled (like mass)



Momentum depends on velocity (mv)
KE depends on the square of velocity (1/2 mv2)
Two objects with same mass
 Object 1 moves with 2x velocity as object 2
 Object 1 has 2x the momentum and 4x the KE
 If a car travelling 2x velocity crashes, will crash with 4x more
energy
Machines
• (1). Machines
– Change direction of force
•
•
Lever – push on one end does work on load end (4.24)
Work input = work output
– Multiply forces
• Fulcrum is close to load, small force input produces large output force
• (2). Examples of machines
– Car jack – push it down 25 cm lifts car 0.25 cm
•
Applies 100x the force
– Pulley
• Pull on it (a little) multiplies force applied
• Remember fD input = Fd output !
• Car example
– She applies 50N x 25cm = 5000N x 0.25
• (3). Efficiency
– Work done / energy used
– 100J work done and 98J of productive work = 98% efficiency