Momentum& Work Force and Power&Circular Motion

Collisions between balls - explain using Newton Laws

• ball A applies a force to ball B

• ball B must apply an equal and opposite force to ball A

• This force opposes motion of ball A

• ball A decelerates due to N2

• there is then a resultant force on B accelerate B

• the force act on balls are equal, so the change in momentum must be equal for both spheres

parachute and terminal velocity six markers

• as parachute opens, upward force increases

• velocity first decreases at a constant rate as drag is greater than weight

• drag is decreasing, and eventually drag balances the weight

• so no acceleration, resultant force is zero, terminal velocity is reached

why does a man in a train with support not lean back

• man pulls back on the support

• N3, support exerts an equal but opposite force on the man

• the force is a resultant force on man

• due to N1 man accelerates

• with the same acceleration as the train

why the man lean back in a train ?

• N1 resultant force = 0

• friction between the floor and feet accelerates with the train

• but the man continues travelling at the original speed so lean backward

Describe an experimental technique by which you could determine accurately the speeds of the trolleys after they separate

• use 2 light gates, which display the time for trolley to pass the one gate to the other

• measure the length between the two light gates using a meter rule

• speed = length between the two light gates/time taken to pass

A person is standing on a ladder. Use the principle of moments to explain why the system is stable in the position shown but will topple over if theta becomes too large.

• the ladder is stable because forces/moments are in equilibrium

• when in equilibrium the clockwise momentum of the normal contact force at the wall is equal to the moment of the weight

• taking moment about bottom of ladder

• if theta is large enough, center of gravity is to the right of the bottom of the ladder

• moment of weight is now clockwise

• no anticlockwise moments, so total clockwise moments bigger than total anticlockwise moments

State and explain which position of the kite would supply the most power to the surfer.

• horizontal component of the tension in the line produces the forward force acting on the surfer

• as the angle to horizontal decreases work done increases

• power transferred to surfer = work done/time has increased so the power increases

Explain why there is a maximum safe speed for a train travelling on a curved track

• For train to travel on curved track, this requires centripetal force acting towards the centre

• the centripetal force is provided by the reaction/friction force

• F = mv²/r

• when speed is at maximum, exceeds the maximum value of reaction force then train leaves the track