Unit 5 - Forces Flashcards
What are scalar quantities
Physical quantities that have magnitude but no direction
What are vector quantities
Quantities that have a magnitude and a direction, they must be represented by an arrow with the length of the arrow showing the magnitude, and it’s direction or the arrow shows the direction of the quantity
Example of vector quantities
Force , velocity, displacement, acceleration, momentum
Example of scalar quantities
Speed, distance, mass, temperature, time
What is a force
is a push or pull that acts on an object due to the interaction with another object
What are the 2 types of forces
Contant and non contact
What are the 2 types of quantities
Scalar and vector
What is a contact force and examples
the objects are physically touching
(Friction, air resistance, tension and normal contact force)
What is a non contact force and examples
the objects are physically separated
gravitational force, electrostatic force and magnetic force
What is weight
force acting on an object due to gravity
What causes the force of gravity near the earth
Caused by gravitational field around the earth
What factor affects the weight of an object
depends on its mass and the gravitational field strength at the location of the object, gravitational field strength is stronger for larger masses
What is the unit for gravitational field strength
Newtons per kilogram (N/kg)
What is the unit for weight
Newtons
Where is the weight of an object considered to act
At the objects centre of mass
What is the relationship between weight and mass
Weight and mass are directly proportional (as mass increases weight increases )
Two effects of gravity
- makes everything fall to the ground
- gives everything a weight
What is weight measured with
A newton meter
What is the resultant force
When multiple forces act on an object they can add or subtract from each other until there is just one force acting in a single direction
What does it mean for forces to be balanced
when the resultant force is zero, meaning the object is either at rest or moving with constant velocity
What is the unit for momentum
kg m/s
When is work done on an object
Work is done on an object when a force causes the object to move through a distance (displacement).
What is the unit of work done
Joules
What is one joules of work defined as
One joule of work is done when a force of one newton causes a displacement of one meter.
(1 joule = 1 newton-meter)
What is joules to newton meters
One joule = one newton meter
What happens when work is done against friction
the energy is transferred into heat, causing a rise in the temperature of the object
What is meant by energy transfer when work is done
When work is done, energy is transferred from one store to another
What is equilibrium to do with forces
Is if the forces are balanced (resultant force of 0)
How do you calculate the work done if a force acts at an angle to the direction of movement?
Draw your force onto a grid measuring with a protractor it’s angle of direction , then measure the length of the horizontal and vertical and convert to newton using your scale of 1N= 1cm
How can you tell if a set of forces are balanced using a scale diagram
If all arrows placed tip to tail form a closed loop the forces are balanced
What does elastically deformed mean
-it can go back to its original shape and length after the force is removed
-objects that can be elastically deformed are called elastic objects (spring)
What does inelastically deformed mean
If it doesn’t return to its original shape and length after force has been removed
why to change the shape of an object more than one force has to be applied
Otherwise the object would simply move in the direction of the applied force instead of changing shape
What is the relationship between force and extension
Extension is directly proportional to force
What is spring constant measured in
N/m
describe the difference between a linear and non-linear relationship between force and extension
-In a linear relationship, the force applied to the spring is directly proportional to the extension of the spring
-In a non-linear relationship, the force is not directly proportional to the extension. As the force increases, the extension may increase at a greater or lesser rate. This typically occurs when the spring is stretched beyond its elastic limit
What is a moment
Turning effect of a force
examples in which forces cause rotation
if you apply a force to a spanner it rotates. The pivot is at the bolt
Unit of moment
Newton meters
What is the distance in a moment
the perpendicular distance from the pivot to the line of action of the force, in metres
If an object is balanced the total ____________ moment about a ________ equals the total ____________ moment about that __________, the object won’t turn
Clockwise pivot
Anti-clockwise pivot
What is a lever
Increases the distance from the pivot at which the force is applied
This means less force is needed to get the same momentum
Levers make it easier to do work (lift a load wheelbarrow)
What are gears
-Gears are circular discs with teeth around their edges
-their teeth interlock so that turning one causes another one to turn in the opposite direction
How do different sized gears affect moment and speed
Different sized gears can be used to change the moment of the force
A force transmitted to a larger gear will cause a bigger moment
As the distance to the pivot is greater
The larger gear will turn slower than a smaller gear
What is a fluid
A liquid or a gas
What is pressure and unit
Force per unit area
Pascals
What does the pressure of a fluid do
causes a force to be exerted normal (at right angles) to any surface in contact with the fluid
Unit of density
kg/m3
What does pressure in a liquid depend on
Depth= as depth increases the number of particles above that point increases, the weight of these particles adds to the pressure so liquid pressure increases
density = the more dense a given liquid the more particles it has so more particles that are able to collide so pressure is higher
What is upthrust
A partially (or totally) submerged object in a fluid experiences a greater pressure on the bottom surface than on the top surface. This creates a resultant force upwards. This force is called the upthrust. The upthrust is equal to the weight of fluid displaced by the object
Why do objects float
-If upthrust of an object is equal to the objects weight then the forces balance and it will float
-This shows an object that is less dense than water so only has to displace a small volume of water before the weight of the water = weight of object
Why does an object sink
-object is more dense than water
-object cannot displace a volume of water equal to its own weight
-so the weight of the object is greater than upthrust so it will sink
What is the atmosphere
-Thin layer of air around the earth
-the atmosphere gets less dense with increasing altitude
Relationship between atmospheric pressure and height
-atmospheric pressure decreases as height increases
-this is because as altitude increases the atmosphere gets less dense so there are fewer air molecules that are able to collide with the surface
-there are also fewer air molecules above a surface as the height increases
-this means that the weight of air above it, which contributes to atmospheric pressure decreases with altitude
What is distance
-How far an object moved
-does not involve direction (scalar)
What is displacement
-included both distance and direction object moves, measured in a straight line from start point to finish point
-vector
What is speed
-How fast you are going
-no regard to direction
-scalar
-speed of a moving object is rarely constant
Factors that affect speed of a person walking cycling or running
-age
-terrain
-fitness
-distance travelled
Typical speed of someone walking
1.5 m/s
Typical speed of someone running
3 m/s
Typical speed of someone cycling
6 m/s
Typical speed of a car
25 m/s
Typical value of a train
55 m/s
Typical speed for plane
250 m/s
Other than moving object what else has varying speed
speed of sound and the speed of the wind
What is the speed of sound
330 m/s
What is velocity
-Speed in a given direction ( 30 mph north )
-vector
What affects speed of wind
Temperature, atmospheric pressure, any large buildings or structures nearby
Why does a circle involve constant speed but changing velocity
An object moving in a circle at a constant speed has constantly changing velocity, as the direction is always changing (car going around roundabout)
What is acceleration
-measured in m/ s squared
-the change in velocity in a certain amount of time
-can be calculated from the gradient of a velocity time graph
What is uniform acceleration
-Contant acceleration
What happens to do with acceleration when an object falls through a fluid
-initially accelerates due to force of gravity
-as speed increases friction builds up
-eventually resultant force will be zero and the object will move at its terminal velocity
If an object is accelerating where can you find its speed at a point
Finding the gradient of a tangent to the curve at this point
How to work out distance travelled of a velocity time graph
Area under a graph
What does the gradient represent on a distance time graph
Speed
(Flat line is when objects stationary)
What does gradient on a velocity time graph represent
Acceleration
(Flat sections represent travelling at a constant speed)
What is drag
-resistance you get in a fluid
-air resistance is a type of drag (frictional force produced by air on moving object)
-to reduce drag keep shape streamlined
-drag increases as speed increases
What is terminal velocity
-the maximum speed of an object reached when forces moving the object are balanced by its frictional forces
What is Newton’s first law
-If the resultant force acting on a stationary object is zero, the object will remain stationary
-if the resultant force on a moving object is zero, it’ll just carry on moving at the same velocity (same speed and direction)
-so when a vehicle travels at a steady speed the resistive forces balance the driving forces
-so the velocity of an object will only change if a resultant force is acting on the object
What is inertia
-The tendency of objects to continue in their state of rest or of uniform motion
-measure of how difficult it is to change the velocity of an object
-defined as the ratio of force over acceleration
What is Newton’s second law
The acceleration of an object is proportional to the resultant force acting on the object and inversely proportional to the mass of the object
What symbol means approximately
~
What is Newton’s third law
Whenever two objects interact, the forces they exert on each other are equal and opposite
Newton’s third law to equilibrium situation
-as man pushes on wall, there is a normal contact force acting back on him
-these two forces are the same size
-as the man applies a force and pushes the wall, the wall pushes back on him with an equal force
What is stopping distance
-stopping distance = thinking distance + breaking distance
What is reaction time and what affects it
-typical values range from 0.2 s to 0.9 s
-vary person to person
-can be affected by tiredness drugs alcohol and distractions
-ruler drop test measures it
Factors that affect thinking distance
-speed of the car (faster you’re going the further you’ll travel during the time you take to react)
-reaction time (longer reaction time the longer your thinking distance)
What is thinking distance
the distance the vehicle travels during the driver’s reaction time ( time between seeing a hazard and applying the brakes )
What is braking distance
-the distance taken to stop under the braking force (one the brakes are applied)
-The greater the braking force the greater the deceleration of the vehicle
-The greater the speed of a vehicle the greater the braking force needed to stop the vehicle in a certain distance
Factors that affect braking distances
-adverse road (wet and icy so less grip and more likely to skid )
-weather conditions
-poor vehicles brakes (if worn or faulty they won’t be able to apply as much force)
-poor tyres (if tyres are bald then they cannot get rid of water this means more likely to skid)
What happens when breaks are applied
-When a force is applied to the brakes of a vehicle, work done by the friction force between the brakes and the wheel
-this reduces the kinetic energy of the vehicle and the temperature of the brakes increases
What can large decelerations cause
may lead to brakes overheating and/or loss of control of car
What energy store transfer occurs when a car brakes
Energy is transferred from kinetic energy store of the wheels to the thermal store of the brakes
What is the conservation of momentum
-In a closed system, the total momentum before an event is equal to the total momentum after the event
When does a change in momentum occur
-When a force acts on an object that is moving, or able to move
-the force causing the change is equal to the rate of change of momentum
-a larger force means a faster change in momentum
How do air bags reduce impact of crash
Inflate before you hit the dashboard of a car, the compressing air inside it slows down more gradually that if you just a hard dashboard
How do seat belts improve safety during a crash
Stretch slightly increasing the time taken for the body’s momentum to reach zero and so reduced the forces on it
How do crash mats and cushioned surfaces for playgrounds reduce danger
Increase the time taken for you to stop if you fall on them, made of soft compressible material
How do cycle helmets reduce danger
Contain crushable layer of foam which helps to lengthen the time taken for your head to stop in a crash, this reduces impact on your brain
What is momentum measured in
Kgm/s
