Download - Momentum and Energy
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Chapter 4ISCI 2002
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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
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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
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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.
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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 momentumNo change unless affected by unbalance forceWhich Law?
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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)
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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)
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Power(1). Work done/time interval
Measure or rate of how fast work is doneRate at which energy is changed from one form
to anotherUnit: watt
One watt = used when one joule of work is done in one second.
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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
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Work-Energy Theorem(1). Work = ΔKE
Or (F)(d) = ΔKEWork 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)
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KE vs Momentum (1). Properties of moving things but:
Momentum is a vector – movement in a directionKE 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
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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