Chapter 9 momentum

steady stream of water strikes a wall horizontally without rebounding

• its horizontal velocity changes, causing its horizontal momentum to decrease to 0

• wall exerts a force on water hence by newton’s third law, the water exerts an equal and opposite force on the wall

• steady - the water flows at a constant rate

• the same mass per second collides with wall at the same velocity and experiences the same change in momentum

• the force is constant

steady stream of water strikes a wall horizontally and does rebound

• the change in momentum would be greater in the same time

• the magnitude of the force on the wall and the force on the water would be greater

state in words how the force acting on a body is related to the change in momentum of the body

• force = the rate of change of momentum

ball moves horizontally towards a foot which kicks it. what would happen if the ball had approached the foot at. higher speed but still received the same impulse

• its initial momentum would be greater

• impulse = change in momentum so this ball would experience the same change in momentum

• the ball will usually rebound so the final and initial velocity will be in opposite directions

• the final speed to the opposite direction would be lower

• if the initial velocity was high enough, the final and initial momentum may be in the same direction

reducing impact force

• material compresses, increasing distance taken to stop, increasing impact time

• the same momentum change occurs over a greater time

• a smaller force acts on the foot from the ground and on the ground from the foot

• the impact time can be increased if the material can compress, reducing impact force

elastic collision and inelastic collision

• momentum is always conserved in a system with no external forces acting

• kinetic energy is only conserved in elastic collisions

• in inelastic collisions, some of the kinetic energy is transferred into other energy stores - thermal usually

object falling onto top pan balance (without rebounding)

• before collision, resultant forces can stop downwards due to weight

• must have a resultant force upwards to decelerate - this is provided by pan exerting a force greater than object’s weight upwards

• by newton’s third law, the object exerts an equal force, greater than mass, on the pan

• the balance gives a reading of this force / 9.81

collision between two balls of same mass, A collides with B which is at rest - elastic

• during impact, A exerts a force on B, accelerating it from rest

• B exerts an equal and opposite force on A, decelerating it

• momentum before MV1

• momentum must be conserved after collision

• mV1 = mV2

• elastic so ke conserved

• no energy transferred to surroundings or internal - B must receive all of A’s kinetic energy