P10 Flashcards
What happens if an object has no force propelling itself along?
It will always slow down and stop because of friction as friction always acts in the opposite direction of movement.
What has to happento travel at a steady speed?
The driving force has to balance the frictional forces
Where does friction occur?
You get it between two surfaces in contact, or when an object passes through a fluid (drag).
What is drag?
Drag is the resistance you get in a fluid (a gas or liquid). Air resistance is type of drag.
How can you reduce drag?
Its keeping the shape of the object streamlined. Its where the object is designed to allow fluid to flow easily across it, reducing drag.
Do frictional forces from fluids always increase with speed? If so, give an example?
A car has much more friction to work agaisnt at 70mph compared to 30mph
Describe an objects journey through a fluid…
1) it sets of and the force of gravity is much more than the frictional force slowing it down, so it accelerates.
2) as speed increases, friction builds up.
3) gradually reducing acceleration untilthe forces balance. (Resultsnt force =0)
3) it will have reached its maximum speed (
Or (terminal velocity)
How is the terminal velocity of any object determined?
By its drag in comparison to its weight.
What does the frictional force depend on?
The objects shape and area
What is the equation for stopping distance?
Stopping distance = thinking + braking distance
What is thinking distance?
It is how far the car travels during the driver’s reaction time (the time between a driver seeing the hazard and applying the brakes).
What is the braking distance?
Is the distance taken to stop under braking forde.
Give two things the thinking distance is affected by:
– Speed - the faster you’re going the further you’ll travel during the time you take to react
– Reaction time- the longer your reaction time the longer your thinking distance
What affects the braking distance:
SPEED: faster vehicle travel, longer takes to stop
WEATHER/ROAD SURFACE: wet or icy or oil on road, less grip. Condition of tyres- if no tread left then they cannot get rid of water in wet conditions. Quality of brakes - worn or faulty so they apply much less force compared to well- maintained brakes.
Describe a factor affecting stopping distance and how this affects safety in an emergency…
Icy conditions increase chance of skidding so driving too close to other cars is unsafe. The longer the stopping distance is the more soace you need to leave infront in order to stoo safely
How does braking rely on friction between the brakes and wheels
When the brake pedal is pushed, this causes brake pads to be pressed onto the wheels.
This contact causes friction, which causes work to be done.
The work done between the brakes and the wheels transfers energy from the kinetic enerpy stores of the wheels to the thermal energy stores of the brakes.
The brakes increase in temperature.
The faster a vehicle is going, the more energy it has in its kinetic stores, so the more work needs to be done to stop it.
This means that a greater braking force is needed to make it stop within a certain distance.
A larger braking force means a larger deceleration.
Very large decelerations can be dangerous because they may cause brakes to overheat (so they don’t work as well) or could cause the vehicle to skid.
Wht happens when a car speeds up in relation to thinking distance and speed?
The thinking didtance increases at the same rate as speed. This is beacuse the thinking time stays relatively the same but as speed increases, more distance is covered in that time.
What happens when a car speeds up in relation to braking distance?
Braking distance increases faster the more you speed up. The work done to stop the car is equal to the energy in the car’s ke store. So as speed doubles, ke energy increases 4 fold, so work done to stop the car also increases 4-fold.
Equation for momentum
Momentum = mass x velocity
How would there be more momentum?
The greater the mass of an object or the greater the velocity , the more momentum
What is momentum
A vector quantity
What is the conservation of momentum?
The total momentum before an event is the same as after the event.
What is momentum?
Momentum is the tendency of the object keep moving in the same direction
Momentum before = momentum after: equation to remember
M1 x v1 = m2 x v2
What would a larger force mean in terms of momentum?
A faster change of momentum
What happens if someones momentum changes very quickly?
The forces on the body ill be very large and more likely to cause injury.
Cars have safety features: what are the 3:
CRUMPLE ZONES: crumple on impact, increasing time taken for the car to stop.
SEAT BELTS: they stretch slightly, increasing the time taken for the wearer to stop.
AIR BAGS: inflate before you hit the dashboard. The Compressing air inside slows you down more gradually than if you hit the hard dashboard.
Explain Bike helmets safety feature:
They contain a crushable layer of foam which helps to lengthen the time taken for your head to stop in a crash. Reduces impact on brain
What do crash mats and cushioned playground flooring do for safety?
They increase the time taken for you to stop if you fall on them. This is because they are made from soft, compressible materials
If the impact time is longer then the
The more the impact force is reduced
What happens when you apply a force on an object?
You may cause ut to stretch, bend or compress
For an object to compress, stretch or bend what has to be there
You need more than one force acting on it otherwise the object would just move in the direction ofnthat one force.
What is elastically deformed?
If it can go back to its original shape and length after the force has been removed.
What is inelastically deformed?
If it doesn’t return to its original shape and length after the force has been removed.
Where is the energy transferred to after work is done to stretch and compress the object?
It is transferred to the elastic potential energy store of the object. If it is elastically deformed, all the energy is transferred to the object’s elastic potential energy store.
What is extension directly proportional to?
The EXTENSION of a stretched spring is directly proportional to the load or FORCE applied.
What does a stiffer spring have?
A greater spring constant
Is there a limit in terms of extension being proportional to the force?
Yes there’s a limit to the amount of force you can apply to an object for the extension to keep on increasing proportionally
What is the limit of proportionality?
It is when the extension is no longer proportionsl to the force. Page 55 CGP picture inserted
