P5 - Forces Flashcards
Scalar quantities
Have magnitude only
Vector quantities
Have magnitude and an associated direction
How can a vector quantity be represented (2)
By an arrow:
• The length of the arrow represents the magnitude, the direction of the arrow and the direction of the vector quantity
Force (2)
- Is a push or pull that acts on an object due to the interaction with another object
- Is a vector quantity
Contact forces
The objects are physically touching
Non-contact forces
The objects are physically separated
Examples of contact forces (4)
- Friction
- Air resistance
- Tension
- Normal contact force
Examples of non-contact forces (3)
- Gravitational force
- Electrostatic force
- Magnetic force
Describe the interaction between pairs of objects which produce a force on each object (2)
- When two objects interact, there is a force produced on both objects
- An interaction pair is a pair of forces that are equal and opposite and act on two interacting objects
Weight (2)
- Is the force acting on an object due to gravity
* Depends on the gravitational field strength at the point where the object is
What is the force of gravity close to Earth due to
The gravitational field around the earth
Formula linking weight, gravitational field strength and mass
Weight (N) = mass (kg) x gravitational field strength (N/kg)
Centre of mass
The weight of an object may be considered to act at a single point
What is the relationship between weight of an object and mass of an object
They are directly proportional
How is weight measured
Using a calibrated spring-balance (a newtonmeter)
Resultant force
• A number of forces acting on an object
Requered practical - force and acceleration (3) method
- The force is varied by moving spare 100 g masses from the trolley to the falling mass hanger - the total accelerating mass is kept constant
- The datalogger measures the speed - by timing how long the interrupt card takes to pass through each light gate
- It then divides the change in speed by time taken to calculate the acceleration
Requered practical - force and acceleration - safety
Care should be taken that the falling masses don’t land on anyone and that the speeding trolley doesn’t hit anyone
Requered practical - force and acceleration (7) equipment
- Dynamic trolley
- Datalogger
- Ruler - acts as a runway
- pulley
- string
- Light gates
- Interrupt cards
Requered practical - force and acceleration - diagram
Chegg
Required practical - extension of a spring - method (6)
- Ensure the ruler is vertical and aligned with top of the spring
- Measure and record the length of the natural spring
- Add 1N weights one at a time, up to 10 N measuring the spring’s length each time
- Subtract the original length from the extended length to get the extension
- Plot a graph - Force on y-axis and extension on the x-axis
Required practical - extension of a spring - Safety (3)
- G-clamp the apparatus to the desk to prevent it from falling over
- Keep it from under the mass
- Wear safety glasses to protect eyes in case the spring snaps
How to calculate the resultant of two forces that act in a straight line (5)
- Draw a scale drawing of the forces acting
- Use a sensible scale (e,g 1cm = 1N)
- Draw the resultant from the tail of the first arrow to the tip of the last arrow
- Measure the length of the resultant with a ruler and use the scale to find the force in N
- Use a protractor to measure the direction as a bearing
Examples of forces acting on an isolated object or system (2)
- when a car travels at a constant speed, the driving force from the engine is balanced by resistive forces such as air resistance and friction in the car’s moving parts
- an object falling at terminal velocity experiences the same air resistance as its weight
Equation for work done
Work done (J) = force (N) x distance (m)
What is work done
When a force moves an object through a distance, energy us transferred and work is done on the object
Describe the energy transfer when work is done
The force does ‘work’ to move the object and energy is transferred from one store to another
Convert between newton-metres and joules
1 joule = 1 newton-metre
What happens when work is done against frictional forces
It causes a rise in the temperature of the object
Examples of forces used in stretching, bending or compressing an object (2)
- Applied force
* Another force as well as the applied force
Why does more than one force need to be applied to change the shape of a stationary object by stretching,bending or compressing
Otherwise the object would simply move in the direction of the applied force, instead of changing shape
elastic deformation
An object has been elastically deformed if it can go back to it original shape and length after the force has been removed
Inelastic deformation
An object has been inelastically deformed if it doesn’t return to its original shape and length after the force has been removed
What is the relationship between an elastic object and the force applied
Directly proportional - provided that the limit of proportionality is not exceeded
Equation linking force, spring constant and extension
Force (N) = spring constant (N/m) x extension (m)
What is the linear relationship between force and extension
Directly proportional - so the gradient is equal to the spring constant
What is the non-linear relationship between force and extension
The spring stretches more for each unit increase in force
How to calculate work done in stretching a spring
Elastic potential energy = 0.5 x spring constant x (extension)2
Distance (2)
- Is how far an object moves
* Scalar quantity
Displacement (2)
- Includes both the distance an object moves, measured in a straight line from the start point to the finish point and the direction of the straight line
- Vector quantity
Speed
- Does not involve direction
* Scalar quantity
Typical values of speed for a person walking, running and cycling
Walking - 1.5 m/s
Running - 3 m/s
Cycling - 6 m/s
Typical values of speed for cars, trains and planes
Car - 25m/s
Train - 30 m/s
Plane - 250 m/s
Equation to calculate the distance travelled by an object moving at a constant speed
Distance travelled (m) = speed (m/s) x time (s)
Velocity (2)
- Is an objects speed in a given direction
* Vector quantity
Objects moving in a circle
At a constant speed an object has a constant changing velocity, as the direction is always changing e.g a car at a roundabout
Distance-time graphs (6)
- Gradient = speed
- Flat sections are where it’s stationary
- Straight uphill sections mean its travelling at a steady speed
- Curves represent acceleration or deceleration
- A steepening curve means it speeding up (Increased gradient)
- A levelling off curve means it’s slowing down
Equation for acceleration
Acceleration (m/s2) = change in velocity (m/s)/time taken (s)
Velocity-time graph (7)
- Gradient = acceleration
- Flat sections - travelling at steady speed
- The steeper the graph, the greater the acceleration or deceleration
- Uphill sections are acceleration
- Downhill sections are deceleration
- curves mean changing acceleration
- The area under any section of the graph is equal to the distance travelled in that time
What is the acceleration of an object falling freely near the earth’s atmosphere
9.8 m/s2
An object falling through a fluid (2)
- Initially accelerates due to the force of gravity
* Eventually the resultant force will be zero and the object will move at its terminal velocity
Newton’s first law (3)
If the resultant force acting on an object is zero and:
• the object is stationary, the object remains stationary
• The object is moving, the object continues to move at the same speed and in the same direction, so the object continues to move at the same velocity
Apply Newton’s First Law to explain the motion of objects with a uniform velocity
• when a vehicle travels at a steady speed the resistive force balance the driving force
What is inertia?
Is the tendency of objects to continue n their state of rest or of uniform motion
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
Equation linking resultant force, mass and acceleration
Resultant force (N) = mass(kg) x acceleration (m/s2)
What is inertial mass (2)
- Is a measure of how difficult it is to change the velocity of an object
- Is defined as the ration of force over acceleration (m=f/a)
Newtons third law
Whenever two objects interact, the forces they exert on each other are equal and opposite
Apply newton’s third law to examples of equilibrium situations (3)
- A man pushing a wall
- As the man pushes the wall there is a normal contact force acting back on him - these two forces are the same
- As the man applies a force and pushes the wall, the wall ‘pushes back’ on him with equal force
What is stopping distance
Stooping distance = thinking distance + braking distance
Typical ranges of reaction times
0.2s to 0.9s (vary from person to person)
How can a driver’s reaction time be affected (4)
- Tiredness
- Drugs
- Alcohol
- Distractions
Methods used to measure human reaction times (2)
- Computer-based test - clicking a mouse when the screen changes colour
- ruler drop test
How can braking distance of a vehicle be affected (2)
- By adverse road and weather conditions
* Poor condition of the vehicle
Factors which affect the distance required for road transport vehicles to come to rest in emergencies (2)
- thinking distance - (the time between the driver seeing a hazard and applying the brakes)
- Braking distance - (is the distance taken to stop under the braking force)
Typical braking distances (3)
- 14m at 30mph
- 55m at 60mph
- 75m 70mph
