Forces - More on speed, Newton Laws and Momentum Flashcards
what is terminal velocity
when an objects velocity remains constant and they are no longer accelerating or decelerating
what factors can affect how many collisions there are between a person and air particles
- Surface area - the larger the surface area, the larger the area over which collisions can take place
- Velocity - the faster the person is moving, the more particles they can collide with
what happens when a person falls off an airplane (with parachute)
- The skydiver accelerates as they begin to fall due to the force of gravity acting on it
- As the skydiver falls, he experiences an upward force of friction with air particles called air resistance
- At terminal velocity, the air resistance force and weight are equal so speed is constant
- The parachute opens which increases the air resistance and causes diver to slow down
- The skydiver continues to slow down until the new air resistance force and weight are equal again
what does the size of air resistance depend on when a person is falling
how many collisions there are between the person and air particles
what is air resistance due to
the collisions between the person and all the tiny air particles that make up the air
why would a diver accelerate downwards immediately after leaving an aircraft
because there is a resultant force acting downwards
what happens as a skydiver gains speed
their weight stays the same but the air resistance increases
what is Newton’s first law
- an object will stay stationary or continue moving at the same speed or direction, unless a resultant force is applied
what happens if the resultant force of a stationary object is 0
it remains stationary
what happens if the resultant force on a moving object is 0
it will carry on moving at the same velocity
what is Newton’s second law of motion
- the size of the resultant force is proportional to the acceleration it causes and inversely proportional to the mass of the object
what are the 5 things that could happen to an object if a resultant force is applied
- it may start moving
- it might speed up
- it might slow down
- it might stop moving
- it might change directions
why does a change in direction cause a change in acceleration
because acceleration is defined as the change in velocity/change in time, and since velocity is determined by speed and direction, any change in direction changes the velocity
why do we say the moon is accelerating around the earth
although speed of the moon remains constant, its direction is always changing, as the Earths mass exerts a gravitational pull on the moon, which acts perpendicular to the moons motion
what would happen to the acceleration if we doubled the resultant force
the acceleration will double
what is Newton’s second law as an equation
F=ma - mass x acceleration
what is inertia
the tendency for an object to remain unchanged (basically just newtons first law)
what is an objects inertial mass
how difficult it is to change an objects velocity
how to find an objects inertial mass
by dividing force by acceleration (newton second law)
what is Newton’s third law
that when two objects interact, the forces they exert on each other are equal and opposite
what do we call the force that an object is exerting
the normal contact force
in order for an object to be accelerated and moved, what needs to happen?
you need either a high force or smaller mass
what is stopping distance
the minimum distance required to stop a vehicle in an emergency
stopping distance equation
stopping distance = thinking distance + braking distance
what is thinking distance
how far the car travels during the drivers reaction time
what things affect thinking distance
- speed of vehicle
- reaction time
- anything that decreases alertness (being tired, drunk, high, or just distracted)
what is braking distance
the distance taken to stop under the braking force
what affects braking distance
- the speed of vehicle
- the mass of vehicle
- conditions and quality of the brakes
- conditions of the road (eg whether or not the road is wet, icy) as they reduce friction betweeen the tyres and the road
- conditions of tires, bald tires unable to grip road as well
why might the quality of a cars brakes affect braking distance
if they are worn or faulty, they wont be able to apply as much pressure against the wheel, meaning they wont slow the car down as quickly
why does the speed an d the mass of a vehicle affect braking distance
because they both affect kinetic energy, which will have to be reduced to 0 in order for the vehicle to stop
what is momentum equation
mass x velocity
what is the conservation of momentum
in a closed system, the total momentum before an event = the total momentum after an event
when do objects have no momentum
when it is stationary
change in momentum equation
force = change in momentum (kgm/s)/ time
what would happen in a car crash in terms of momentum
- momentum will decrease from a large amount to 0 almost instantly
- this massive change in monentum means that the people inside the car will experience a huge force which may cause injury like head trauma
how to reduce the chance of damage in a car crash
spread the change in momentum over a longer period of time, as this will reduce the force they experience
- in order to help them acheive this, cars have to have safety features such as crumple zones, seatbelts, air bags
what do seat belts do
- stop you flying out of the car
- slightly stretchy so you slow down a bit more slowly
what are forces like friction called
resistive forces
Forces acting on a skydiver
- as soon as the skydiver jumps out, the only force acting is weight. this is due to gravity
- because of weight, the skydiver experiences a resultant force acting downward, so they accelerate towards the ground
- as he falls, he experiences friction with air molecules, air resistance increases
- weight is still greater than air resistance, so the skydiver continues to accelerate towards the ground
- as the skydivers velocity increases, air resistance also increases. at a certain point the air resistance balances the weight
- this means there is no resultant force and velocity stays constant. the skydiver reaches terminal velocity
what happens when a skydiver opens their parachute
- surface area now increases, causing air resistance to massively increase
- at this point, air resistance is greater than the weight, so there’s a resultant force acting upwards
- this causes the river to decelerate (velocity decreases)
- because velocity has decreased, air resistance also decreases
- at some point, the air resistance will balance the weight and the resultant force will be 0, so velocity will stay constant
- skydiver falls at a lower terminal velocity, safe for them to hit the ground
what is reaction time
the time taken for the driver to spot the obstruction, make a decision, then move their foot to the brake
the greater the speed of the vehicle…
the greater the stopping distance (assuming that the same braking force is applied)
typical ranges of reaction times
0.2 to 0.9
what happens in terms of energy stores, when a car brakes
- during braking, the brake presses against the wheel
- the force of friction now acts between the brake and the wheel
- the kinetic energy of the car is now converted into thermal energy in the brakes
- this causes the temperature of the brakes to increase
- at the same time, the car slows down as it loses kinetic energy
if the car is travelling at a high speed, what can we say about the size of the braking force they have to apply to stop
it has to be large
how can brakes overheat
- a large braking force will cause a car to decelerate very quickly
- at the same time, a large amount of kinetic energy is transferred to the thermal energy in the brakes
- this can cause the brakes to overheat and can also cause the driver to lose control of the vehicle
momentum in a canon example
- before a canon fires, both the cannon and the cannonball are not moving, so the total momentum before firing equals 0
- when a cannon fires, the cannonball is moving at a very large velocity, so the cannonball has momentum in the forward direction
- at the same time, the cannon recoils (moves backwards), meaning the canon has momentum acting in the backwards direction
- the backwards momentum of the canon equals the forward momentum of the cannonball, meaning the total momentum after firing is 0
what are crash tests used for
to design safety features such as seat belts
examples of safety devices that slow down momentum change
- airbags
- seatbelts
- crash mat
- bike helmet
- cushioned surface
