Flashcards in Unit 4: Force and momentum Deck (60):
What is needed to change the velocity of an object?
What is momentum equal to?
mass x velocity
What does Newton's second law simplify to?
force = mass x acceleration
What are the two different units for momentum?
kgm/s or Ns
Is momentum a scalar or vector quantity?
In which direction does the momentum of an object act?
It is in the same direction as the object's velocity
What is Newton's first law of motion?
An object remains at rest or in uniform motion unless acted on by a force
What is the resultant force on an object equal to when the momentum of an object is constant?
What would have to change in order for an object to keep constant momentum when its gaining or losing mass?
What is Newton's second law of motion?
The rate of change of momentum of an object is proportional to the resultant force on it. In other words, the resultant force is proportional to the change of momentum per second.
What is the Newton?
The amount of force that gives an object of mass 1kg an acceleration of 1m/s^2
When does F = ma apply?
When the mass is constant
How is the constant of proportionality in Newton's second law determined and what is it?
The constant is 1 and it is determined through defining the unit of force.
What version of Newton's second law is used when an object gains or loses mass continuously?
F = v x (change in mass/change in time)
What is the impulse of an object?
It is defined as the force x the time for which the force acts
What is the formula for impulse?
Ft = mv-mu
What does the area under a force-time graph represent?
The change of momentum or the impulse of the force
What is the unit of impulse?
What are some safety features of a vehicle that are designed to lessen the effect of an impact?
Crumple zones, seatbelts, collapsible steering wheels, airbags and side-impact bars.
How do the safety features in a car reduce the effect of an impact?
They increase the time taken by the impact so the acceleration or deceleration is less and therefore the impact force is less.
What is the momentum argument for the safety features of a vehicle reducing the effect of an impact?
For a given momentum, the force is reduced if the impact time is increased.
What equipment is used to measure the variation of an impact force with time on a ball?
This can be recorded using a force sensor connected using suitably long wires or a radio link to a computer. The force sensor is attached to the object that causes the impact.
Why can the force sensor be attached to the object causing the impact rather than the ball?
As equal and opposite forces act on the ball, the force on the ball due to the object varies in exactly the same way as the force on the object due to the ball.
What does a typical force-time graph look like for an impact?
The force increases and then remains constant for a while and then it decreases again.
What happens to the momentum of a ball if it hits a wall normally?
It rebounds normally so the direction of momentum is reversed.
What is the formula for the force of an impact when a ball hits a wall and rebounds normally?
F = (-mv - mu)/t
How does the formula for the force of a rebound impact change if there is no loss of speed?
F = -2mu/t
What is an oblique collision?
The initial direction of the ball is not perpendicular to the wall
How does the formula for the force of a rebound impact change if the collision is an oblique one?
F = -2mucosθ/t where θ is the angle taken from the normal
What components of velocity must be used when the collision is an oblique one?
The normal components of velocity
What is Newton's third law of motion?
When two objects interact, they exert equal and opposite forces on each other.
What is an example of Newton's third law that involves the Earth?
The Earth exerts a force due to gravity on an object which exerts an equal and opposite force on the Earth
What is an example of Newton's third law that involves a jet engine?
A jet engine exerts a force on hot gas in the engine to expel the gas; the gas being expelled exerts an equal and opposite force on the engine
What is the principle of the conservation of momentum?
It states that for a system of interacting objects, the total momentum remains constant, provided no external resultant force acts on the system
What is the resultant force acting on an object if there is no change in momentum?
The resultant force is zero
How does the total momentum change if objects transfer momentum between each other in a system?
There is no change in total momentum
What happens when two objects collide and separate?
They exert equal and opposite forces on each other when they are in contact so the change of momentum of one object is equal and opposite to the change of momentum of the other object
What is the formula for the principle of conservation of momentum?
m1u1 + m2u2 = m1v1 + m2v2
How does the principle of conservation of momentum formula change if the objects collide and stick together?
(m1 + m2)V = m1u1 + m2u2
What equipment is required to test the conservation of momentum?
Two trolleys that are placed on a horizontal runway with a motion sensor that is linked to a computer.
What must be measured before the test on the conservation of momentum is carried out?
The mass of each trolley
How is the test on the conservation of momentum carried out?
With trolley 2 at rest, trolley 1 is given a push so that it moves towards trolley 2 with a constant velocity, the two trolleys then stick together on impact, the computer records and displays the velocity of trolley 1 throughout this time
What is the relationship that should be discovered from the test on the conservation of momentum?
(m1 + m2)V = m1u1
In which case would a collision cause both objects to stop?
If the initial momentum of one object was exactly equal and opposite to that of the other object
What type of collision occurs when a ball bounces back to exactly the same height?
What is an elastic collision?
One where there is no loss of kinetic energy i.e. momentum and kinetic energy are conserved
What is a totally inelastic collision?
One where the colliding objects stick together
What happens to the kinetic energy in an inelastic collision?
It converts into other forms of energy such as heat or sound or it may go into damaging the vehicles involved in a crash
What is a partially inelastic collision?
One where the colliding objects move apart and have less kinetic energy after the collision than before but momentum is always conserved
What is always conserved in collisions?
Momentum and energy but kinetic energy may be converted into other forms
What happens when two objects fly apart after being initially at rest?
They recoil from each other with equal and opposite amounts of momentum so they move away from each other in opposite directions
What setup is required to test a model explosion?
There are two trolleys which are placed next to each other and the spring is released from one of the trolleys which leads to the two trolleys pushing each other apart, there are also two blocks on either side of the trolleys and they are positioned so that the trolleys hit the blocks at the same moment
What is equal to the distance travelled by the two trolleys to the point of impact with the blocks in the test for the model explosion?
Their speed x the time taken to travel the distance
What allows you to conclude that the ratio of the speeds of the trolleys is the inverse of their mass ratios in the test for the model explosion?
The time taken for the two trolleys to hit the blocks is the same so the distance ratio is the same as the speed ratio and also the trolleys have equal and opposite amounts of momentum
What is equal to the energy stored in the spring when it was originally compressed in one of the trolleys?
The kinetic energy of the two trolleys immediately after they separate from each other
What happens in terms of energy when two or more objects fly apart due to an explosion?
Their total kinetic energy immediately after the explosion is less than the total chemical energy released in the explosion because heat, light and sound all carry away energy
What is linear momentum?
Momentum in a straight line
What is the resultant force?
The vector sum of all the forces
In which direction does acceleration act in terms of the resultant force?
In the same direction as the the resultant force