Chapter 10 Flashcards
Equation for resultant force
Resultant force = mass x acceleration
What does Netwon’s second law say? 2
The acceleration of an object is:
Proportional to the resultant force on the object
Inversely proportional to the mass of the object
What do we need to change the velocity of an object?
Resultant force
What is inertia?
The tendency to continue in the same state of motion
What is the inertial mass of an object?
It is a measure of how difficult it is to change the velocity of the object
Equation for inertial mass
Force / acceleration
What is the acceleration of an object the sane as?
The resultant force
When does the object decelerate?
When the resultant force is in the opposite direction to the velocity of the object
When does an object accelerate?
When the resultant force is in the same direction as the velocity of the object
Why does an object fall?
Its weight
What is your weight caused by?
The forec of attraction between you and the earth
Why do you weigh less at the equator?
Weaker force of attraction
How much is 1 N?
100 g
What is your weight? (N)
The force acting on you due to gravity
What is your mass? (kg)
The quantity of matter inside you
Equation for weight (kg)
weight = mass x gravitational field strength
In which direction will friction always act?
Opposite to the direction of movement
When an object falls in a fluid, why does its acceleration decrease as it falls?
The frictional force increases as it speeds up, so the resultant force on it decreases and therefore its acceleration decreases
When does an object reach terminal velocity?
When the frictional force on it is equal and opposite to its weight and accelerating force
How does streamlining help?
Reduces drag so the fluid flows easily around the object
What happens when an object reaches terminal velocity?
It falls at a constant speed
What causes objects to fall at different speeds on earth?
Air resistance
What does the frictional force of an object depend on?
Its shape and area
What does the braking distance of a vehicle depend on?
Its mass and speed when the brakes are applied
Why is it harder to stop when you are going faster?
The greater the deceleration needed, so the braking force needed is much greater
What is a stopping distance?
The shortest distance a vehicle can safely stop in
How to work out stopping distance?
Thinking distance + braking distance
What is the thinking distance?
The distance travelled by the vehicle in the time it takes the driver to react
What is thinking distance equal to?
Speed x reaction time
Why is thinking distance proportional to speed?
The car travels at a constant speed during the time it takes the driver to react
What is the braking distance?
The distance travelled during the time the braking force acts
Equation for stopping distance?
Thinking distance + braking distance
What two things is thinking distance affected by?
Speed and reaction time
Why does speed increase thinking distance?
The faster you’re travelling, the further you’ll travel in the time it takes you to react
Examples of things increasing thinking distance? 4
Tiredness, alcohol, distractions and drugs
4 things affecting braking distance
Speed, weather, condition of tyres, condition of brakes
What does the deceleration of a vehicle depend on?
The friction between the road surface and the tyres
Why does the condition of the road affect braking distance?
If it is wet or icy, there is less friction between the tyres and road, increasing your braking distance
Why does the condition of the tyres affect braking distance?
If tyres don’t have any tread left, they can’t get rid of water in wet conditions, meaning they skid
Why does the condition of brakes affect braking distance?
Worn brakes won’t be able to apply as much force
Equation for deceleration
a = (v^2 - u^2) / 2S
What happens when the brake pedal is pressed?
The brake pads press onto the wheels
This causes friction, which causes work to be done
The work done transfers energy from the kinetic store of the wheels to the thermal store of the brakes, causing them to heat up
Equation for momentum
momentum (kg m/s) = mass x velocity
Why is momentum a vector?
It has size and direction
What is the conservation of momentum?
In a closed system, the total momentum before an event is equal to the total momentum after an event
What happens when things collide?
They recoil with equal and opposite momentum
When two objects collide and stick together, what happens?
Before: The momentum was equal to the mass x velocity of the moving object
After: The weight has increased, so velocity decreases
What is important to remember in momentum calculations?
One object has positive momentum, the other has negative momentum
Equation for the conservation of momentum
(mass of A x velocity of A) = -(mass of B x velocity of B)
What happens when the impact time is increased?
The impact force reduces
Equation for force
(mass x change in velocity) / time taken
SO
change in momentum / time taken
Why does a car crash injure a person?
Their change in momentum is very big, so the force exerted on their body will be greater
What happens when two cars collide?
They exert equal and opposite impact forces on eachother
Why do we want to increase impact time?
It decreases the force exerted on a person
Why should you wear a cycle helmet?
It contains a crushable layer of foam which helps lengthen the time for your head to hit the ground and stop moving. This reduces the force of the impact
Why do seatbelts keep us safe?
1) The seatbelt is across the chest so spreads the force that would otherwise injure the person’s head out
2) They stretch, which increases the time taken for us to stop moving and so decreases the force
How do airbags keep us safe?
In a crash, they inflate before we hit the dashboard
It spreads the force of the impact across the upper part of a person’s body and increases the impact time
So, the effect of the force is weaker
How do playmats keep us safe?
They are made of foam so increase the time it takes us to stop moving
How do crumple zones keep us safe?
They increase the time taken for a car to stop
What is braking distance proportional to?
Speed squared
What is an elastic object?
One that can go back to its original shape when the forces that are deforming it are removed
What happens when an object is elastically deformed?
Work is done when a force stretches or compresses an object and causes energy to be transferred to its elastic potential store
If it is elastically deformed, all this energy is transferred to the elastic potential store
How to calculate the extension of a spring?
The extended length - the original length
What is the extension of a spring directly proportional to?
The force applied
What happens to the spring constant as the spring gets stiffer?
Increases
What is Hooke’s law?
The extension of a spring is directly proportional to the force applied, as long as the limit of proportionality is not reached
Equation for Hooke’s Law?
Force applied = spring constant x extension
What does the limit of proportionality look like on a graph?
The regular gradient will bend or change
What happens when a spring reaches its limit of proportionality?
It won’t go back to normal
