7.1 - 7.5 Flashcards
what is Newton’s third law
two objects interacting exert equal and opposite forces on each other
what is Newton’s first law
an object will maintain its current velocity unless acted on by a resultant force
what’s the change in momentum of a ball bouncing off a wall
double the initial momentum of the ball (2mv). this is because its direction changes so it starts at mv then changes to -mv. the difference is mv–mv=2mv
what is the principle of conservation of momentum
the total momentum of a system remains the same before and after a collision
what’s the difference between perfectly elastic and inelastic collisions
for a perfectly elastic collision: momentum, total energy and kinetic energy are conserved
for inelastic collisions: momentum and total energy are conserved but not kinetic energy
how do airbags reduce injury in a crash
they reduce your momentum more gradually. by increasing the time taken the force is reduced because Force= change in momentum/time (Newton’s second law)
what is Newton’s second law
the resultant force is directly proportional to the rate of change of momentum in the same direction
whats the special case of Newton’s second law
Newton’s second law: F= (mv-mu)/t. if mass is constant then F= m (v-u)/t= ma. the special case is F=ma when an object has constant mass
why is momentum conserved in collisions
It’s a consequence of Newton’s laws. according to the third law colliding objects exert equal and opposite force on each other which makes the net force of the system 0. since F= Δp/t then Δp =0. therefore the total momentum of the system does not change
define impulse
the product of the force and the time for which that force is applied to the object. therefore impulse=change in momentum
how can the impulse be found from a force-time graph
the area under the curve
how can vectors be used to solve momentum problems
since p =p1 + p2 then the victory p1 and p2 must add to equal the vector p. this can be shown by drawing a victory triangle.
