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