* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download No Slide Title
Survey
Document related concepts
Centrifugal force wikipedia , lookup
Relativistic quantum mechanics wikipedia , lookup
Weightlessness wikipedia , lookup
Electromagnetism wikipedia , lookup
Fictitious force wikipedia , lookup
Lorentz force wikipedia , lookup
Newton's law of universal gravitation wikipedia , lookup
Matter wave wikipedia , lookup
Woodward effect wikipedia , lookup
Centripetal force wikipedia , lookup
Transcript
PHYSICS 231 Lecture 13: Keeping momentum Remco Zegers Walk-in hour: Tue 4-5 pm helproom BPS1248 PHY 231 1 Chapter 6 Momentum & Collisions When a bullet hits the wall, its velocity is very much reduced. The wall does not move, although the force on the ball is the same as the force on the wall (Newton’s 3rd law: Fwall-bullet=-Fbullet-wall). Fwall-bullet=mbulletabullet Fbullet-wall=mwallawall Mbullet << Mwall |abullet|>> awall PHY 231 2 Is it only the mass??? Vbullet=100 m/s Vbullet=200 m/s When the bullet gets stopped in the wall, it deaccelerates from its initial velocity to 0. So, its acceleration is vbullet/t, with t some small time (independent of v). Second law: Fwall-bullet=mbulletabullet=mbulletvbullet/t The force also depends on the velocity of the bullet! PHY 231 3 More general…and formal. F=ma Newton’s 2nd law F=mv/t a=v/t F=m(vfinal-vinital)/t Define p=mv p: momentum (kgm/s) F=(pfinal-pinitial)/t F=p/t The net force acting on an object equals the change in momentum (p) in a certain time period (t). Since velocity is a vector, momentum is also a vector, pointing in the same direction as v. PHY 231 4 Impulse F=p/t p=Ft Definition: F2 F1 Force=change in (mv) per time period (t). The change in momentum equals the force acting on the object times how long you apply the force. p=Impulse What if the force is not constant within the time period t? p=Ft=(F1s+F2s+F3s)= = t(F1s+F2s+F3s)/t = tFaverage F3 s s s t p=Faverage t PHY 231 5 car hitting haystack car hitting wall The change in momentum (impulse) is the same, but the force reaches a much higher value when the car hits a wall! 6 PHY 231 Some examples A tennis player receives a shot approaching him (horizontally) with 50m/s and returns the ball in the opposite direction with 40m/s. The mass of the ball is 0.060 kg. A) What is the impulse delivered by the ball to the racket? B) What is the work done by the racket on the ball? PHY 231 7 Child safety A friend claims that it is safe to go on a car trip with your child without a child seat since he can hold onto your 12kg child even if the car makes a frontal collision (lasting 0.05s and causing the vehicle to stop completely) at v=50 km/h (about 30 miles/h). Is he to be trusted? PHY 231 8 question The velocity change is largest in case: A B The acceleration is largest in case: A B The momentum change is largest in case: A B The impulse is largest in case: A B PHY 231 9 accident Consider the cases A and B in which a car crashes into a wall. In which case is the likelihood that the passengers are severely hurt greater? PHY 231 10 Momentum p=mv F=p/t Impulse (the change in momentum) p= Ft Demo: the ‘safe’ bungee jumper PHY 231 11 Conservation of Momentum F21t = m1v1f-m1v1i F12t = m2v2f-m2v2i Newton’s 3rd law: F12=-F21 (m1v1f-m1v1i)=-(m2v2f-m2v2i) Rewrite: m1v1i+m2v2i=m1v1f+m2v2f p1i+p2i=p1f+p2f CONSERVATION OF MOMENTUM PHY 231 CLOSED SYSTEM! 12 momentum conservation PHY 231 13 Moving in space An astronaut (100 kg) is drifting away from the spaceship with v=0.2 m/s. To get back he throws a wrench (2 kg) in the direction away from the ship. With what velocity does he need to throw the wrench to move with v=0.1 m/s towards the ship? a) 0.1 m/s b) 0.2 m/s c) 5 m/s d) 16 m/s e) this will never work? PHY 231 14 Types of collisions Inelastic collisions Elastic collisions •Momentum is conserved •Momentum is conserved •Some energy is lost in the •No energy is lost in the collision: KE not conserved collision: KE conserved •Perfectly inelastic: the objects stick together. next quiz about this PHY 231 15