P10 - Forces and motion Flashcards
State Newton’s second law of motion:
the acceleration of an object is proportional to the resultant force acting on it, and inversely proportional to its mass
F = ma
What is inertia?
tendency for an object to stay in a state of rest/uniform motion
uniform motion means a constant velocity
What is inertial mass?
-a measure of how difficult it is to accelerate an object
-defined as m = F/a (Newton’s 2nd law of motion)
What is terminal velocity? Describe the forces acting on a body at this velocity:
-the maximum velocity an object can reach while in freefall in a fluid
-the weight of the object would be equal to the frictional/drag force on the object (resultant force is 0, no further acceleration)
Define stopping distance:
sum of the distance travelled while the driver reacts (reaction distance) and the distance travelled under the braking force (braking distance)
What factors can affect a driver’s reaction time?
-tiredness
-drugs/alcohol
-distractions (eg mobile phone, conversations)
Give some factors that can affect braking distance:
-wet/icy road/weather conditions or poor vehicle condition (brakes/tires)
-downhill slope
-mass of vehicle
-speed of the vehicle
Why might having poor brakes or wet roads increase braking distance?
decreased friction on the road
Explain how having a car with more mass increases braking distance:
-car has more kinetic energy
-this means more work needs to be done to stop the car
Explain why a driver’s reaction time affects the thinking distance:
distance = speed x time
-increasing the time taken to react will increase the distance travelled whilst the driver reacts to the situation
Why does having a steeper downhill slope increase braking distance?
-a larger component of the car’s weight force acts against the braking force
-resultant braking force on the vehicle is reduced
-takes longer to decelerate the car (Newton’s 2nd Law)
How do brakes work?
-work is done between the brakes and the tires, due to the added frictional force applied over a certain amount of time
-kinetic energy transferred to thermal energy
-speed decreases, but temperature of brakes increases
What is momentum? Give its equation and unit:
the quanitity of motion of a moving body, taking into account both its mass and velocity
p = mv
kg m s⁻¹
ie mass in motion
How does inertia differ from momentum?
when an object is not moving, it has no momentum but it still has inertia
What is the principle of conservation of momentum?
the total momentum before an event is equal to the total momentum after the event in a closed system
What happens to an object’s momentum when an external force is applied to it?
This is not a question about the conservation of momentum
-it changes by a certain amount depending on how long the force is applied for
F = ∆p/t (rearranging gives ∆p = Ft, so if the force is applied over a longer time then the momentum changes more)
In an impact, what effect does increasing the time of the impact have?
F = ∆p/t
-impact force is inversely proportional to time
-increasing impact time (time to reduce velocity to 0) means the impact force is reduced
This is why falling on the floor hurts more than falling on a bed - the bed takes longer to reduce your velocity to 0, so impact time is increased, and thus decreases impact force
Why do airbags work?
-airbags are compressible, increases impact time
-rate of change of momentum is decreased, so force is also decreased
-force is distributed across a large area of the body
-reduces pressure on person
-reduces the chance of an injury
Seat belts work in the same way as they are slightly stretchy
What is an elastic object?
an object that returns to its original shape when the forces deforming it are removed
Describe Hooke’s law:
the extension of an elastic object is directly proportional to the force applied, provided that the limit of proportionality is not exceeded
F = ke
What is the limit of proportionality?
the extension of a spring where going past it means Hooke’s law is no longer followed, meaning the relationship between extension and force is non-linear
What is the difference between elastic and inelastic deformation? Give an example for each:
-elastic deformation is when an object returns to its original shape after being stretched/bent/compressed by 2 external forces (eg rubber band)
-inelastic is where the object does not return to its original shape (eg crushing a can)
When can an elastic object undergo inelastic deformation?
when it goes past its elastic limit (the furthest point it can be stretched while still being able to return to its original shape)
This could happen when a metal spring is stretched too far and isn’t springy anymore
RP6 - How can you determine the spring constant of an elastic object?
-set up the equipment as shown in the diagram (without the masses), using another clamp stand to hold the ruler
-measure the spring’s initial length
-add a mass to the base of the spring, calculate its extension, and repeat for at least 5 added masses
-calculate the mass’s weight force
-plot force vs extension, calculate gradient of line of best fit, which will be the spring constant
RP6 - How could you improve the accuracy of measuring the spring’s extension?
-use set square to line up bottom of the spring with ruler
or
-attach horizontal pointer to bottom of spring so the pointer goes across the ruler scale
Why might a spring be useful in a mass balance?
-deforms elastically so it can return to its original shape
-compression of spring is directly proportional to the weight force acting on it, so it gives a linear relationship making it easy to use it as a scale
RP7 - How would you set up the equipment required to investigate Newton’s 2nd Law?
-put a trolley on rails
-attach a string from the trolley to a pulley and add a mass on the other end to supply a force
-put 2 light gates near the end of the rails (measures Δv, then a)
RP7 - How would you change the trolley on rails experiment to investigate how the mass of an object affects its acceleration?
-put a constant mass on the pulley’s end of the string
-pull trolley up to a fixed point on the rails and release it
-take reading of acceleration
-add masses to the trolley and repeat
-plot graph of acceleration vs mass added
RP7 - How would you change the trolley on rails experiment to investigate how the force on an object affects its acceleration?
-don’t change the mass of the trolley
-pull trolley up to a fixed point on the rails and release it
-take reading of acceleration
-add masses to the string to increase the force exerted on the trolley, and repeat
-plot graph of acceleration vs force applied
Remember to convert the mass to weight
Why do cars have a top speed?
-maximum driving force provided by car motor
-as speed increases, frictional force increases too
-eventually, magnitude of air resistance force = driving force
-no more acceleration as resultant force = 0 (by Newton’s first law of motion)
Why does paper fall slower than an object like a coin?
air resistance has the greatest effect on paper due to its increased surface area
They would fall at the same rate in a vacuum though
What happens to the braking distance of a car if the speed is doubled and why?
-doubling speed will quadruple Ek (Ek∝v²)
-work done by brakes to stop car must also quadruple
-Work done = force x distance, assuming brake force is constant, then braking distance must quadruple to make work done quadrupled
