Seneca P5 Flashcards
What is speed
Speed is a scalar quantity.
Scalar quantities only have a magnitude (size).
Scalar quantities, like speed, do not have a direction.
What is Velocity
Velocity describes an object’s direction as well as its speed.
Velocity is a vector quantity because it has a magnitude (or size) and a direction.
What is the speed of an object
The speed of an object is the total distance that an object travels divided by the total time it takes.
What is the formula for acceleration
acceleration (m/s2) = Δv (change in velocity) / t (time taken)
Why do we work out acceleration using velocity
Because velocity is a vector quantity, acceleration is also a vector quantity
Uniform acceleration
v^2 - u^2 = 2as
final velocity squared
minus initial velocity squared
is equal to distance x acceleration x 2
What is the difference between a scalar and a vector
Scalar only has magnitude (size)
Vector has magnitude and direction
Distance
Distance is how far an object moves.
Distance is a scalar quantity.
This is because it contains a magnitude (size) but not a direction.
Displacement
Displacement is the distance an object moves in a straight line from a starting point to a finishing point.
Displacement is a vector quantity.
This is because it contains a magnitude (size) and direction.
What is a force
A force is a push or a pull that acts on an object when it interacts with another object
Contact forces
Contact forces happen when two objects are physically touching.
Friction, air resistance, tension and normal contact force are all examples of contact forces.
Non Contact forces
Non-contact forces happen when objects are separated (not touching).
Gravitational force, electrostatic force and magnetic force are all examples of non-contact forces.
What are examples of contact forces
. Tension
. Friction
. Air resistance
. Normal contact force
What is an interaction
An interaction pair is a set of 2 forces that are equal and opposite, acting on 2 interacting objects.
What is mass
An object’s mass is a measure of the amount of matter that it contains.
Inertia
An object’s mass is also a measure of how difficult it is to change the object’s motion.
This is called inertia.
An object with a high mass has more inertia than an object with a lower mass.
It is difficult to move an object with a high mass (and high inertia), and once it is moving, the object’s motion is hard to stop.
Weight
. Is measured in newtons
. Equals mass multiplied by gravitational field strength
. is the force that acts on an object in a gravitational field
When will an object topple?
When it’s center of mass is located outside it’s base
What is the center of mass
A single point where in an object where all the mass appears to be
Resultant force
The resultant force is the sum of all of the forces acting on an object.
The change in an object’s motion is caused by the resultant force.
If the forces acting on an object are unbalanced (not equal), it means that a resultant force is acting on the object.
This is Newtons 2nd law
What is the equation for resultant force
Resultant force (F) = mass (m) x acceleration (a)
What is Newtons 1st Law
Newton’s 1st Law says that the velocity of an object will only change if a resultant force is acting on the object. This applies to a stationary (still) or moving object.
Stationary (still)
If an object is stationary (not moving) and there is no resultant force acting on it, it will stay stationary.
Moving
If an object is moving and there is no resultant force acting on it, the object will continue moving in the same direction at the same speed.
This means that the object will continue moving at the same velocity.
This also means that the velocity of an object will only change if a resultant force is acting on the object.
What is Newtons third law
Newton’s Third Law says that: whenever 2 objects interact, the forces that they exert on (apply to) each other are equal and opposite.
If one object exerts (applies) a force on another object, then the other object must be exerting (applying) a force back.
If a hand pushes on a table, the table will push back on the hand with an equal force, but in the opposite direction.
We can use ____ body diagrams to work out the resultant force when more than one force is acting on an object.
free
On a free body force diagram, forces are shown as ________.
vectors
If the resultant force on an object is zero, we say the object is in…
equilibirum
At least how many forces must be acting on an object to stretch, bend or compress it?
2
What are the 2 ways an object can be deformed
Elastically
Inelastically
Inelastic deformation
An inelastically deformed object will not return to its original shape when the force stops.
A car is an inelastic object.
After a car has crashed into a tree, it will not return to its original shape.
Elastic deformation
An elastically deformed object will return to its original shape when the force stops.
A spring is an elastic object.
Springs return to their original shape when forces stop acting on them.
Extension Load Graph
An extension-load graph has the force acting on a spring plotted on the y-axis, and the extension of the spring on the x-axis.
For low forces, the graph is a straight line which passes through the origin.
When no force acts on the spring, there is no extension.
As the force on the spring increases, the spring reaches its limit of proportionality.
On the graph shown, this is where the line begins to curve.
What is the limit of proportionality?
The point where hook’s law breaks down
Equation for Hooke’s Law
F = ke
F = Force
k = Constant
e = Extension
Investigating Hook’s Law
Set up the apparatus as above.
First, measure the original length of the spring.
Next, hang different masses on the spring and measure the length of the spring in each case.
Adding masses to the spring increases the downwards force as each mass has weight.
The extension of the spring equals the length with masses minus the original length:
extension of the spring = length of the spring with masses − original length of the spring.
Plot a graph with the extension of the spring on the x-axis and force on the y-axis.
Which law relates the force and extension of a stretched spring?
Hooke’s law
Work done on a spring
When a force stretches a spring or compresses another object, work is done. When this work is done, energy is transferred into an elastic potential energy store.
Compressing a spring
When we compress a spring, elastic potential energy is stored in the spring (so long as a spring isn’t inelastically deformed!).
Elastic potential energy stored in a spring
The elastic potential energy stored in a spring is equal to the work done when stretching it.
Elastic Potential Energy Stored in a Spring equation
elastic potential energy =
1/2 x spring constant x extension^2
force extension graph
The elastic potential energy stored in a stretched spring equals the area under the force-extension graph.
Free fall
If an object is in free fall, then the object’s weight is the only force acting on it.
The weight of an object is the force that acts downwards on an object due to gravity.
Acceleration due to gravity
An object in free fall will accelerate at a constant rate. This constant rate is called the acceleration due to gravity (g).
The average value for acceleration on Earth due to gravity is 9.81 m/s2, but we round it up to 10 m/s2 in most calculations.
What is the definition of weight?
Force acting downwards on an object due to gravity
Falling with Air resistance
Air resistance slows down a falling object.
The force due to air resistance increases as the speed of a falling object increases.
What is air resistance
Air resistance is a frictional force that opposes the motion of objects moving quickly through the air.
How does the acceleration due to gravity, g, change with time?
It stays constant
What is momentum
The momentum of an object is its mass multiplied by its velocity (p = mv).
Change in momentum
Change in momentum = force x time
Δp = F x t
Conservation of momentum
The law of conservation of momentum says that momentum cannot be created or destroyed.
So, if two objects collide, the sum of momentum before collision = sum of momentum after the collision.
Change in momentum = mv - mu
Change in momentum = mv - mu, where m is mass, u is the initial velocity of an object and v is the final velocity of an object.
Force and Momentum Change
When a force acts on an object that is moving, or able to move, a change in momentum happens. Force equals the rate of change of momentum.
Acceleration and momentum
acceleration = change in velocity ÷ time taken.
a = Δv ÷ t.
Force and acceleration
Force = mass × acceleration.
F = m x a.
Force = rate of change of momentum
Combining the 2 equations we get F = m x Δv ÷ t.
F = m Δv ÷ t.
F = Δp ÷ t.
Force = change in momentum ÷ change in time.
In other words, force is equal to the rate of change of momentum.
What is the rate of change of momentum equal to?
Force
Safety features
Cars have safety features such as seat belts, air bags and crumple zones that absorb the kinetic energy transferred by collisions.
These features reduce injuries to the people in the car by absorbing energy when they change shape.
They increase the time taken for the change in momentum to happen, reducing the forces involved.
What is a moment
A moment is the turning effect of a force around a fixed point.
See-saw
Balancing on a see-saw is a good example for demonstrating how turning effects work.
To balance the weight of a heavier person, a lighter person must sit further away from the pivot.
Where is a turning effect larger?
As far from the pivot as possible
Equation for moment
Moment=F×d
For an object to be in equilibrium in circular motion
The sum of the clockwise moments must be equal to the sum of the anti-clockwise moments acting on the object.
This means that the object is not rotating or rotates at a constant speed.
For an object to be in equilibrium in linear motion
There must be no resultant force acting on the object.
It must be stationary or travelling at a constant speed in a straight line.
For an object to be in equilibrium, it must be either __________ or moving at a constant speed in a straight line.
stationary
Which of the following is NOT true of circular motion:
It never accelerates
Circular motion
An object moving in a circle at a constant speed (circular motion) is constantly changing direction.
A change in direction gives a change in velocity because velocity is a vector quantity.
Acceleration equals the change in velocity per unit time, so an object travelling in a circular motion at a constant speed is accelerating.
The resultant force always acts towards the centre of the circle and the object is always accelerating.
Levers
We use levers and gears to transmit (send) the rotational effect of a force from one place to another.
We can use levers to increase the distance between the pivot and where we’re applying the force.
Moment = force x distance, so using a lever means we need to use less force to get the same moment.
Gears
Gears are circular discs with ‘teeth’ around their edges.
Their teeth interlock. So if we turn one gear, another turns in the opposite direction.
Transmit rotational effect
A set of gears can transmit (send) the rotational effect of a force from one place to another.
Changing moment in gears
We can use different sized gears to change the moment of the force.
For example, if we send a force to a larger gear, there will be a bigger moment.
This is because the distance to the pivot is greater.
We can use levers to increase the distance between the _____ and where we’re applying the force.
pivot
What is the calculation for stopping distance
Stopping distance = thinking distance + braking distance.
Thinking distance
The time it takes for a driver to react to a situation is their reaction time.
During this reaction time, the car carries on moving.
The thinking distance is the distance travelled between when the driver realizes they need to brake and when they apply the brakes.
Braking distance
The distance the car travels between the driver applying the brakes and the car coming to a stop.
What are the factors that affect thinking distance
. Distractions
. Drugs or alcohol
. Tiredness
Factors affecting breaking distance
. Road conditions
. Condition of the car
. Initial car speed
Work done when breaking
When we push the brake pedal, brake pads are pressed onto the wheels.
This contact causes friction.
This causes work to be done.
The work done between the brakes and the wheels converts (changes) energy from kinetic energy in the wheels to thermal energy in the brakes.
The temperature of the brakes then increases.
Higher speed
The greater the speed of a vehicle, the greater the braking force needed to stop the vehicle before a certain distance.
This means that more work needs to be done on the brakes to stop the car.
Higher mass
The greater the mass of the vehicle, the greater the braking force needed to stop the vehicle. This means that more work needs to be done on the brakes to stop the car.
Higher grip
For the same work done, the stopping distance will decrease if the force (grip) between the road and the vehicle increases.
Estimating forces
When a car comes to a stop, the work done by the brakes must equal the initial kinetic energy of the car.
Work done = initial kinetic energy.
F d = 1⁄2 m v2.
We can use this equation to estimate the force applied by the brakes.
Dangers of Large Decelerations
The greater the braking force, the greater the deceleration of the vehicle. Large decelerations can cause brakes to overheat and/or the car to skid. A larger deceleration will transfer more stopping force to passengers. This harms passengers
What is pressure
Pressure is the force per unit of area. This relationship can be rearranged to calculate force by multiplying the pressure by the area
Pressure equation
Pressure = force / area
What is atmospheric pressure
Atmospheric pressure is the force per unit area created by the weight of the air (particles) in the atmosphere.
Aeroplanes
When we move quickly up into the Earth’s atmosphere on a plane, your ears pop because of the drop in atmospheric (air) pressure.
Mountains
At the top of mountains, atmospheric pressure is lower because there is less air (fewer particles) pressing down on the mountain.
Air is lighter than water, but it still exerts pressure on the things beneath it.
Liquid pressure
As you dive deeper into a swimming pool, there is more water (and weight) on top of you.
This extra weight exerts a larger force (and higher pressure) on your body.
The deeper down you swim, the more pressure you feel
Liquid pressure equation
Liquid pressure = density x gravitational field strength x depth
The pressure beneath a liquid’s surface = density of fluid x gravitational field strength x depth.
Pressure is measured in pascals (Pa).
Density of fluid is measured in g/cm3.
Depth is also equal to the height of the column of water above you.
Upthrust
A partially submerged object (an object that’s not fully in a liquid) will experience greater pressure on the bottom surface than on the top surface. This creates a resultant force upwards. We call this force upthrust
What is upthrust
Upthrust = weight of liquid displaced
The upthrust that acts on an object is equal to the weight of the liquid that has been forced away (displaced) by that object.
Floating
If the object’s weight is equal to the upthrust, then the forces balance and the object will float in the liquid.
Sinking
If the object’s weight is greater than the upthrust, then the object will sink.
Submarine - Rising
When a submarine wants to come up to the surface, it fills its tanks with compressed air to reduce its weight.
Weight becomes less than upthrust so the submarine rises.
Submarine - Sinking
When a submarine wants to sink, it fills its tanks with water to increase its weight.
This means the submarine’s weight is greater than the upthrust and it sinks.
