Mechanics and Materials (4): Force and momentum Flashcards
What is the equation for momentum?
p = mv
What needs to be applied to an object in order to change its velocity?
A force
Force is proportional to the…?
Change in momentum per second
What is the detailed equation for the force applied to an object (in terms of changing momentum)?
F = mv - mu/t
= m(v-u)/t
ma = m(v-u)/t
What is the unit of momentum?
kgms^-1
What is the general equation for force applied to an object (in terms of changing momentum)?
F = (delta)mv/ (delta)t
OR (if mass is constant)
F = m(delta)v/ (delta)t
OR (if velocity is constant)
F = v(delta)m/ (delta)t
What is the equation for Force if the mass changes at a constant rate?
F = v(delta)m/ (delta)t
If a rocket ejects burnt fuel from its engine at a speed v, what is the equation for the force exerted by the engine to eject the hot gas?
F = v(delta)m/ (delta)t
How does ejecting hot gas from a rocket cause it to propel forwards?
Force exerted by the engine to eject the hot gas is given by:
F = v(delta)m/ (delta)t
Due to Newtons third law of motion - The hot gas ejected exerts an equal and opposite force on the jet engine, propelling the rocket forwards
What is the definition of the impulse of a force?
The force x the time for which the force acts
What is the equation for the impulse of a force?
F(delta)t = (delta)mv
- either m or v can be constantly changing or both
What is the impulse of force acting on an object equal to?
The change in momentum of that object
F - mv - mu/t when…?
An object of a constant mass or velocity is acted on by a constant force, F which changes its mass or velocity
What does the area under a force-time graph represent?
The change in momentum or the impulse of the force
given by:
Ft = mv - mu
What are the units for momentum (in terms of impulse)?
Ns
What is the equation for the impact force experienced by an object in a collision?
F = mv - mu/t
For a rebound impact what is the equation for the impact force, F?
F = (-mv) - (mu)/t
This is because when a ball hits a wall it rebounds off with a different same speed, but also different direction. This means its velocity has changed and therefore, we need to account for the direction of velocity in the equation
If there is no loss of speed in an impact then what is the equation for the impact force, F?
F = -2mu/t
If the initial direction of an object that collides with a wall is not perpendicular to it, then what do you have to do with its velocity?
You have to find the component of its velocity that is perpendicular to the wall
State the principle of conservation of momentum
For a system of interacting objects the total momentum remains constant providing no external resultant force acts on the system
What is the equation that shows the principle of conservation of momentum?
Total momentum before the collision = total momentum after the collision
If colliding objects stick together then what value do they share that is the same?
Same final velocity
For a head on collision (two objects moving towards each other) which direction will the momentum be acting?
In the direction of the object with more momentum
For a head on collision what do you need to consider?
The direction of the objects colliding as one will have a greater momentum than the other
If the individual momentums of two objects that collide in a head on collision are equal, what will the total momentum before and after equal?
0kgms^-1
What is the definition of an elastic collision?
One where both kinetic energy and momentum are conserved (no loss of kinetic energy)
What is the definition of an inelastic collision?
One where momentum is conserved but kinetic energy is not/ where the colliding objects have less kinetic energy after the collision than before the collision
How do you work out whether a collision is elastic or inelastic?
Calculate the kinetic energies of each object before and after the collision
Objects that collide and couple together undergo what type of collision?
Inelastic collision
If you want to calculate the kinetic energy of a ball immediately after impact with the ground what do you have to do?
Calculate the GPE gained after its first bounce by using GPE = mgh (GPE gained = KE lost)
An explosion is an example of what type of collision?
Inelastic collision
Why is an explosion an inelastic collision?
As the kinetic energy of the objects involved are greater after the collision comparative to before
What is the total momentum before and total momentum after an explosion? Why?
0kgms^-1
Before: the objects are stationary therefore have 0 velocity, therefore 0 momentum
After: the objects fly off in opposing directions, the momentum in opposite directions sums to 0
How is energy transferred to the kinetic energy store of objects in an explosion?
Chemical -> Kinetic + heat, light and sound
