Topic 5: Forces Flashcards
What are the two types a quantity can be?
Scalar & Vector
What are scalars?
Quantities which only have a magnitude
What are vectors?
Quantities which have both a magnitude and a direction
What are some examples of scalars?
-Distance
-Speed
-Mass
-Energy
-Volume
-Density
-Temperature
-Power
What are some examples of vector quantities?
-Displacement
-Velocity
-Weight
-Force
-Acceleration
-Momentum
How are vectors represented?
- Using an arrow
- Length of arrow represents magnitude of vector
- Direction of arrow represents direction of vector
What is a force?
A push or a pull that acts on an object due to the interaction with another object
How can forces affect bodies?
Can change their speed, direction and shape
What is electrostatic force?
Force between two charged objects
What is thrust?
Force causing an object to move
What is air resistance / drag?
Friction of air on moving object
What is upthrust?
Force of a fluid pushing an object upwards
What is compression?
Force that act inward on an object, squeezing it
What is tension?
Force transmitted through a cable or string when pulled on by forces acting on opposite ends
What is reaction force?
Force due to contact between two objects
What is a contact force?
A force which acts between objects that are physically touching
Examples of Contact Forces
-Friction
-Air resistance
-Tension
-Reaction force
Examples of Non-Contact Forces
-Gravitational force
-Electrostatic force
-Magnetic force
What is a non-contact force?
A force which acts at a distance, without any contact between bodies, due to the action of a field
What is a magnitude of a force measured in?
Newtons (N)
What is weight?
Force acting on an object due to gravitational attraction
What is a force pair?
Where there is an interaction between two objects and a force is exerted on each object
Examples of things weight causes
- Objects to stay firmly on ground
- Objects always fall to ground
- Satellites are kept in orbit
What is weight measured in?
Newtons (N)
What is mass?
Amount of matter in an object
What is mass measured in?
Kilograms (kg)
What does the weight an object experiences depend on?
- Object’s mass
- The mass of the planet attracting it
How is mass measured?
Top pan balance
How is weight measured?
Newton-meter, also known as a calibrated spring-balance
What is the centre of mass?
The point through which the weight of an object acts
How can you figure out the centre of mass for a symmetrical object of uniform density?
Centre of mass is located at point of symmetry
What is the formula for calculating weight?
W = mg
What is Earth’s gravitational field strength?
9.81 m/s²
What is the change in mass and weight on different planets?
- Object’s mass will always remain the same
- Weight will differ depending on strength of gravitational field
What is a resultant force?
Single force which describes all forces operating on a body by combing forces applied on an object
What is the resultant force also known as?
Net force
What are balanced forces?
Balanced forces mean that the forces have combined in such a way that they cancel each other out and no resultant force acts on the body
What are unbalanced forces?
Unbalanced forces mean that the forces have combined in such a way that they do not cancel out completely and there is a resultant force on the object
How do you calculate resultant force?
- Resultant forces can be calculated by adding or subtracting all of the forces acting on the object
- Forces working in opposite directions are subtracted from each other
- Forces working in the same direction are added together
What is tension normally labelled as?
Normally labelled as T on free body diagrams
What is the normal contact force?
The force arising when an object rests against another object acting at a 90° angle to the plane of contact
Why are free body diagrams useful?
- Useful for modelling the forces acting on an object
- Can be used:
> To identify which forces act in which plane
> To resolve the net force in a particular direction
What is work done?
Work is done when an object is moved over a distance by a force applied in the direction of its displacement
What is the formula to calculate work done?
W = Fs
(Work = Force x Distance Moved)
What is work measured in?
Nm or J (1J=1Nm)
Describe the relationship between work done and energy transfer (include formula)
- Whenever any work is done, energy is transferred from one store to another
- Mechanical (or electrical) working is an energy transfer pathway
- Energy transferred (J) =Work done (J)
Describe friction in terms of energy change
-When friction is present, energy is transferred by heating
-This raises the temperature (energy is transferred to the thermal store) of the object and its surroundings
-The work done against the frictional forces causes this rise in the temperature
-Imperfections at the interface between the object and the surface bump into and rub up against each other
Describe air resistance in terms of energy change
- Particles bump into the object as it moves through air
- As a result, energy is transferred by heating due to work done against the frictional forces
What is the difference between distance and displacement?
- Distance measure how far an object travels (scalar)
- Displacement measures how far something is from its starting point, along with its direction (vector)
What is the speed of an object?
Distance it travels every second
What is the formula for average speed?
Average Speed (m/s) = Total Distance (m) / Time Taken (s)
What is non-uniform motion?
- Refers to change in motion whether that is in speed, direction or both
What are the typical speeds?
Walking - 1.5 m/s
Running - 3 m/s
Cycling - 6 m/s
Car - 10-30 m/s
Passenger Aeroplane - 200-250 m/s
Sound - 330-340 m/s
What factors affect typical speeds of a person?
- Age
- Terrain
- Fitness
- Distance
What factors affect typical speeds of transportation systems?
- Shape
- Design
- Cost
- Purpose
What happens to the velocity when an object travels along a circular path?
- Velocity is always changing
- Speed may be constant but direction of travel is always changing therefore changing velocity (vector quantity)
What does a distance-time graph show?
-Shows how the distance of an object moving in a straight line (from a starting position) varies over time
What can you infer from the line in a distance-time graph?
Straight Line - Constant Speed
Steep Slope - Large Speed
Shallow Slope - Small Speed
Flat, Horizontal Line - Stationary
How can you tell if an object is changing speed in a distance time graph?
- Represented by a curve
- If slope is increasing, accelerating
- If slope is decreasing, decelerating
How can the speed of a moving object be calculated in a distance-time graph?
Speed = Gradient = Δy/Δx
How can you calculate speed at a particular time when an object is accelerating or decelerating on a distance-time graph?
Calculating the gradient of a tangent (straight line which touches the curve at a particular point)
What is acceleration?
- Rate of change of velocity
- How much an object’s velocity changes every second
What is the equation to calculate average acceleration of an object?
a = Δv/t
acceleration = change in velocity / time
How can you find change in velocity?
Change in Velocity = Final - Initial
How would you describe the acceleration of an object speeding up?
Acceleration is positive
How would you describe the acceleration of an object slowing down?
Acceleration is negative (deceleration)
What are the typical acceleration speeds?
Family Car - 2-3 m/s²
Falling Object - 10 m/s²
Rocket - 30 m/s²
Formula 1 Car - 50 m/s²
Fighter Jet - 90-120 m/s²
What information does a line on a velocity-time graph show?
Straight Line - Constant Acceleration
Slope - Magnitude of Acceleration :
> Steep Slope - Large Acceleration
> Gentle Slope - Small Acceleration
> Flat Line - Constant Velocity
How can acceleration be calculated on a velocity time graph?
Acceleration = Gradient = Δy/Δx
What does the area under a velocity-time graph represent?
Represents displacement (distance travelled) by an object
What do enclosed areas
under velocity-time graphs represent?
Total displacement/ distance travelled
List all the SUVAT equations
V = U + AT
S = (U+V) /2 * T
V² = U² + 2AS
S = UT + 1/2 AT²
S = VT - 1/2 AT²
What is the acceleration due to gravity near Earth’s surface in the absence of air resistance?
Approximately 9.8 m/s²
What are the two forces which act on a skydiver when they jump out of a plane?
Weight (Due to gravity)
Air resistance (due to friction)
What happens to the resultant force as the skydiver falls?
Decreases
Describe how a skydiver reaches terminal velocity
- Initially, upwards air resistance is very small because the skydiver isn’t falling very quickly
- As they speed up, the air resistance increases, eventually growing large enough to balance the downwards weight force
- Once air resistance equals weight, there is no resultant force
- Therefore, acceleration is zero - travel at a constant speed (terminal velocity)
What is Newton’s 1st Law of Motion?
Objects will remain at rest, or move with a constant velocity unless acted on by a resultant force
What does the 1st law of motion show?
- The object will remain stationary if it was stationary before
- The object will continue to move at the uniform velocity if it was moving
What is Newton’s 2nd Law of Motion?
The acceleration of an object is proportional to the resultant force acting on it and inversely proportional to the object’s mass
What does the 2nd law of motion show/explain?
- An object will accelerate (change its velocity) in response to a resultant force
- The bigger this resultant force, the larger the acceleration
- For a given force, the greater the object’s mass, the smaller the acceleration experienced
What is the equation which Newton’s 2nd law can be expressed as?
F = ma
Resultant Force (N) = Mass (kg) * Acceleration (m/s²)
What symbol must be used for estimating quantities?
~
e.g adult person has a mass of ~70 kg
What is Newton’s 3rd Law of Motion?
Whenever two bodies interact, the forces they exert on each other are equal and opposite
What does the 3rd law of motion show?
- All forces arise in pairs - if object A exerts a force on object B, then object B exerts an equal and opposite force on object A
- Force pairs are of the same type - for example, if object A exerts a gravitational force on object B, then object B exerts an equal and opposite gravitational force on object A
What is the meaning of inertia?
- The tendency of an object to continue in its state of rest, or in uniform motion unless acted upon by an external force
- Object’s resistance to change in motion
What is inertial mass?
How difficult it is to change its velocity
What is the equation for inertial mass?
inertial mass (kg) = force (N) /acceleration (m/s²)
What is inertial mass inversely proportional to?
Acceleration
What will large inertial masses experience?
Small accelerations
What will small inertial masses experience?
Large accelerations
What is the stopping distance?
The total distance travelled during the time it takes for a car to stop in response to some emergency
What is the equation for stopping distance?
Stopping distance = Thinking distance + Braking distance
What is thinking distance?
The distance travelled by a car from when a driver realises they need to brake to when they apply the brakes
What is braking distance?
Distance travelled by a car under the braking force - i.e. whilst it is slowing down
What happens to stopping distance in terms of speed?
The greater the speed of the vehicle, the greater the stopping distance
What happens in a velocity time graph during the thinking distance?
- While the driver reacts (the time taken to press the brakes is called the reaction time), the vehicle continues moving at a constant velocity
- The area underneath represents the thinking distance
What happens in a velocity time graph during the braking distance?
- As soon as the brakes are applied, the vehicle decelerates to a halt
- The area underneath represents the braking distance
What is the reaction time?
A measure of how much time passes between seeing something and reacting to it
What is the average reaction time for someone who is alert?
0.2 - 0.9 seconds
What is the equation for reaction distance?
Reaction Distance = Speed of the car × Driver’s reaction time
What factors affect thinking distance?
- Tiredness
- Distractions
- Intoxication (consumption of alcohol and drugs)
What is the main factor which affects both thinking and braking distance?
Speed
What factors affect braking distance?
- Vehicle Condition
- Road Condition
- Vehicle Mass
What can large decelerations lead to in a car?
- Brakes overheating and/or loss of control of vehicle
How is energy transferred during braking?
- When brakes are applied, friction force between brakes and wheels
- Frictional force does work on brakes, transfers energy from car to brakes
- Kinetic energy decreases, thermal energy increases
- Brakes heat up
- Car decelerates
What is the equation to figure out work done by brakes?
Work Done = Braking Force * Braking Distance= 1/2 * Mass * Velocity²
What is work done in the brakes in terms of energy?
Transfer of kinetic energy
What is braking distance directly proportional to?
Speed²
What is the equation for momentum?
p=mv
The momentum of an object will change if…
-The object accelerates (speeds up) or decelerates (slows down)
- Changes direction
- Its mass changes
What is the principle of conservation of momentum?
In a closed system, the total momentum before an event is equal to the total momentum after the event
What is a closed system?
The energy within the system is constant and the absence of external forces (e.g. friction)
What are collisions?
Examples of momentum in an event
What happens in an elastic collision?
- Objects collide and move in opposite directions
- Each object will have a different velocity
What happens in an inelastic collision?
- Objects collide and move in the same direction
- Objects have a combined mass and velocity
What collision do objects move in opposite directions?
Elastic Collision
What collision do objects move in the same direction?
Inelastic Collision
What happens to momentum in a collision?
It is always conserved
What is force in terms of momentum?
- Rate of change in momentum
Force = Change in Momentum / Change in Time
What is the equation for change in momentum?
Change in Momentum = Final Momentum - Initial Momentum
What is the relationship between force and momentum in a collision?
Force is equal to the rate of change in momentum. Increasing the contact time during a collision decreases the force of impact.
How do vehicle safety features reduce the force of impact in a collision?
They absorb energy by changing shape (e.g., crumple zones, seat belts, airbags) and increase the time over which the force is applied, reducing the force on passengers.
What is the purpose of crumple zones in vehicles?
Crumple zones are designed to crush or crumple in a controlled way during a collision, increasing the time over which the vehicle comes to rest and reducing the impact force on passengers.
How do seat belts reduce injury in a collision?
Seat belts keep passengers fixed to their seats, stretch slightly to increase the time for momentum to reach zero, and reduce the force on the passenger.
What is the role of airbags in vehicle safety?
Airbags act as a soft cushion to prevent injury by reducing the force on passengers when they are thrown forward during a collision.
How do crash mats in gymnasiums reduce injury?
Crash mats are thick and soft, increasing the contact time over which a person’s momentum is reduced, thereby decreasing the impact force and risk of injury.
Why are cushioned surfaces used in playgrounds?
Cushioned surfaces increase the contact time during a fall, reducing the impact force and the risk of severe injury for children.
Why can children use thinner crash mats than adults in gymnasiums?
Children have a lower mass, so they exert less force during a fall, making thinner crash mats suitable for their activities.
What factors determine the effectiveness of safety equipment?
The mass and velocity of the object or person involved in the impact. Larger mass and higher velocity require longer contact times to reduce the force effectively.
Do safety features completely prevent injuries in all cases?
No, safety features are designed to reduce the chance of serious injury but cannot completely prevent injuries in all cases.
