Transcript My Ch.4 PP
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