Mechanics Flashcards
Modelling pin being driven into the ground with hammer, staying in contact and moving together after impact before coming to rest.
In this model, the ground exerts a constant resistive force on the pin.
State one way that this model can be refined to be more realistic.
R could be modelled to be varying w/ speed.
A pile driver drives a pile vertically into the ground. Immediately after impact, it can be assumed that ehy both move with the same velocity.
Comment on this model in relation to the motion of the pile and driver immediately after impact. (1)
The model assumes that the pile driver would not ‘bounce’ upon contact with the pile, i.e. the particles coalesce.
Given that the pile driver is much heavier than the pile, this would be a fair assumption.
A car comes to stop when it reaches a rough surface.
In reality, the care stops in a shorter distance than suggested in the model.
Suggest one reason for this. (1)
The car stops in a shorter distance because there will be additional frictional forces acting on it (e.g. air resistance) which will increase the deceleration.
A car comes to stop when it reaches a rough surface.
In reality, the care stops in a shorter distance than suggested in the model.
Suggest one reason for this. (1)
The car stops in a shorter distance because there will be additional frictional forces acting on it (e.g. air resistance) which will increase the deceleration.
Rev. Ex. 2 q17
A ball is droped onto a smooth horizontal plane. Model the ball as a particle.
Criticise this model with respect to the number of times the ball bounces.
- unrealistic
- the ball is not a particle, –> infinite no. of bounces
- stop once the bounce height is less than its radius
- other factors eg. additional energy losses following each ‘bounce’
A slide is modelled to be a plane 48o inclined to the horizontal. Calculate the potential energy lost by the child after sliding 4m.
State one assumption made in the calculation and comment on the validity.
- assumed x air R
- valid for low speeds, but not for high speeds
The rock is modelled as a particle falling freely under gravity.
In reality, the falling rock will be subjected to air resistance opposing its motion.
State how this will affect the kinetic energy of the rock as it hits the surface of the water.
- decreases
- work is done against air R
In reality, the spring is not light.
State one way in which this would affect your energy equation in part (b). (1)
need to include GPE of spring/ spring would have KE
x just mention weight!
Oblique collisions q w/ 2 spheres of equal radii-
State how you have used the fact that P and Q have equal radii. (1)
The line of centres is parallel to the surface the spheres are moving on, so the impulse acts parallel to the surface.
