#### unit 6 physics

what is momentum

the product of an object's mass and its velocity

is momentum a vector or scalar quantity

vector

what are the units of momentum

kg m/s

p→ =

mv

momentum equals

mass times velocity

F=

ma (or m v/t)

∆p =

F∆t

what is impulse

F∆t

closed system

no objects enter or leave

isolated system

no net external force on a system

internal forces are

between objects within a system

can internal forces change the total momentum of a system?

no

external forces are

exerted by objects outside of a system

do external forces change the total momentum of a system

yes

law of conservation of momentum

the total momentum of any closed, isolated system is a constant

completely inelastic collison

objects stick together after the collision (so have same final velocity)

youre driving on the highway and a bug splatters on your windshield. which undergoes the greater change in momentum?

both are the same (∆p=F∆t, both force and time are the same in this case)

the greatest change in momentum of an object will be produced by

a large force acting over a long time (think big x big = bigger)

when you are in the way of a fast moving object and cant get out of its way, you will suffer a smaller force of impact if you

decrease its momentum over a long time

tow objects, A and B, have the same size and shape, but A is twice as heavy as B. When the two are dropped simultaneously from a tower, they reach the ground at the same time but A has a greater

momentum (p→ = mv, larger mass with same velocity makes bigger momentum)

what has the smallest magnitude of momentum

an object that is at rest (bc 0)

a freight train rolls along a track with considerable momentum. if it rolls at the same speed, but has twice as much mass, its momentum is

doubled

if both the force that acts on an object and the time of impact are doubled, by what factor is the impulse multiplied

4

what is true if the momentum of a system changes

there is an external force on the system

a ball with an initial momentum of 4 kg m/s x hits a wall and bounces back with the same momentum as it was before the collision. what is ∆p

-8 kg m/s x (final - initial =∆)

the impulse experienced by a body is equivalent to the bodys change in

momentum

padded dashboards in cars are safer in an accident than non-padded ones because an occupant hitting the dash has

both increased time of impact and decreased impact force

if there is no net external force on a system, can the total linear momentum change

no

is momentum a scalar or vector quantity

vector

is impulse a scalar or vector quantity

no

two trucks that look the same collide. truck B was originally at rest. friction is negligible. the trucks stick together and move off at *more than half* the original speed of the moving truck, A. compare the masses of the contents of truck A and truck B.

mass A > mass B

two trucks that look the same collide. truck B was originally at rest. friction is negligible. the trucks stick together and move off at *exactly halff* the original speed of the moving truck, A. compare the masses of the contents of truck A and truck B.

mass A = mass

two trucks that look the same collide. truck B was originally at rest. friction is negligible. the trucks stick together and move off at *less than half* the original speed of the moving truck, A. compare the masses of the contents of truck A and truck B.

mass A < mass B

an objects momentum is defined as

its mass times its velocity

it is harder to stop or turn a moving supertanker than a small speedboat because

the supertanker has a larger momentum

the impulse-momentum relationship is a direct result of

Newtons 2nd law

a rifle recoils while firing a bullet. the speed of the rifles recoil is small because

the rifle has much more mass than the bullet

to catch a ball, a baseball player extends the hand forward before impact with the ball and then lets it ride backward in the direction of the balls motion. doing this reduces the force of impact on the players hand principally because

the time of impact is increased

a rubber ball and a lump of clay have equal mass. they are thrown with equal speed against a wall. the ball bounces back with nearly the same speed which hit it. the clay sticks to the wall. which one of these objects experiences the greater momentum change?

the rubber ball

a 4 kg ball has a momentum of 12 kg m/s x. what is the balls speed?

3 m/s (∆p = mv)

an astronaut, floating alone in outer space, throws a baseball. if the ball floats away at a speed of 20 m/s, the astronaut will

move in the opposite direction, but at a lower speed than 20 m/s

suppose a ping pong ball and a bowling ball are rolling toward you. both have the same momentum and you exert the same force to stop each. how do the time intervals to stop them compare.

both take the same time

a person attempts to knock down a large wooden bowling pin by throwing a ball at it. the person has two balls of equal size and mass, one made of rubber and the other of putty. the rubber ball bounces back, while the putty sticks to the pin. which ball is most likely to topple the bowling pin

the rubber ball (greater ∆p)

cart a is at rest. an identical cart b, moving to the right, collides with cart a. they stick together. after the collision, what is true of the combined speed

carts a and b move to the right with a speed less than cart b's original speed

two ice skaters, lily and john, face each other while stationary and push against each other's hands. john's mass is twice that of lily. how do their speeds compare after the push off

lily's speed is twice that of johns

a truck traveling at 50 km/h is brought to rest by colliding with a haystack. the same truck could also have been brought from 50 km/h to rest by colliding with a brick wall. *compare the momentum of the truck before*

haystack = brick wall

a truck traveling at 50 km/h is brought to rest by colliding with a haystack. the same truck could also have been brought from 50 km/h to rest by colliding with a brick wall. *compare the momentum of the truck after.*

haystack = brick wall

a truck traveling at 50 km/h is brought to rest by colliding with a haystack. the same truck could also have been brought from 50 km/h to rest by colliding with a brick wall. *change in momentum of the truck*

haystack = brick wall

a truck traveling at 50 km/h is brought to rest by colliding with a haystack. the same truck could also have been brought from 50 km/h to rest by colliding with a brick wall. *compare the time of impact for the truck*

haystack > brick wall

a truck traveling at 50 km/h is brought to rest by colliding with a haystack. the same truck could also have been brought from 50 km/h to rest by colliding with a brick wall. *compare the force of impact of the truck*

haystack < brick wall

a truck traveling at 50 km/h is brought to rest by colliding with a haystack. the same truck could also have been brought from 50 km/h to rest by colliding with a brick wall. *compare the impulse delivered to the truck*

haystack = brick wall

you are standing perfectly still and then take a step forward. before the step, your total momentum was zero, but afterwards you have some momentum. does this violate conservation of momentum?

no

a small beanbag and a bouncy rubber ball are dropped from the same height above the floor. they both have the same mass. which one will impart the greater impulse to the floor when it hits

the bouncy ball (bc changes direction by bouncing, therefore increasing momentum)

consider two carts, of masses 2 and 4 kg, at rest on an air track. if you push both carts for the same amount of time, exerting equal force on each, the momentum of the light cart is __________ the momentum of the heavy cart.

equal to