Forces 2 Flashcards
Forces and Motion
What is distance?
How far an object moves.
What does displacement include?
-The distance an object moves, measured in a straight line from the start point to the finish point
-The direction of that straight line.
What is the difference between distance and displacement?
Distance is a scalar quantity (not involving direction), whereas displacement is a vector quantity (giving both the distance travelled and in what direction).
You should be able to use ratios and proportional reasoning to…
…convert units and to compute rates.
What is speed?
The rate at which an object is moving.
What is the difference between speed and velocity?
Speed is a scalar quantity (not involving direction), whereas velocity is a vector quantity (giving the speed of an object in a given direction).
The speed of a moving object is rarely…
…constant.
When people walk, run or travel in a car their speed is constantly changing.
What does the speed at which people can walk, run, or cycle depend on?
Many factors including: age, terrain, fitness and distance travelled.
What is the typical value of speed for a person walking?
̴ 1.5 m/s
What is the typical value of speed for a person running?
̴ 3 m/s
What is the typical value of speed for a person cycling?
̴ 6 m/s
What are the typical values of speed for different types of transportation systems?
Car ̴ 25 m/s
Train ̴ 55m/s
Aeroplane ̴ 250m/s
It is not only moving objects that have varying speed. The speed of _____ and the speed of the ____ also vary.
sound
wind
What factors can affect the speed of the wind?
-temperature
-atmospheric pressure
-structures
What is a typical value for the speed of sound in air?
330 m/s
For an object moving at constant speed, how can the distance travelled in a specific time be calculated?
distance travelled = speed x time
Speed =?
Distance ÷ time
Velocity = ?
Displacement ÷ time
[v = s ÷ t]
What is the unit for speed and velocity?
m/s
What is the unit for distance and displacement?
metres, m
What is uniform motion?
When an object travels at a constant speed in a straight line.
What is non-uniform motion?
When an object travels with a varying speed (acceleration/deceleration) or changes direction.
How do you calculate average speed for non-uniform motion?
average speed = total distance/displacement ÷ total time
How are you going to explain qualitatively, with examples, that motion in a circle involves constant speed but changing velocity?
By using the example of orbits!
e.g. that the speed of the moon remains constant but its velocity changes due to constant changes in direction
Orbit of the moon example
(to explain qualitatively that motion in a circle involves constant speed but changing velocity)
The mass of Earth exerts a gravitational pull on the moon
This acts perpendicular to its motion
This causes the moon to change direction, which means a change in its velocity
The moon is therefore accelerating (with acceleration defined as a change in velocity).
If an object moves along a straight line, how can the distance travelled be represented?
By a distance- time graph.
How do we find the speed of an object from a distance-time graph?
Find the gradient to give the speed.
(as speed = distance ÷ time = Δy ÷ Δx)
What does a line that’s curved upwards represent on a distance - time graph?
Acceleration
What does a line that’s curved downwards represent on a distance - time graph?
Deceleration
What does a horizontal line represent on a distance-time graph?
A stationary object.
If an object is accelerating, how can we determine its speed at a particular time?
(on a distance-time graph?)
By drawing a tangent and measuring the gradient at that time.
Practice…
Drawing distance–time graphs from measurements.
Extracting and interpreting lines and slopes of distance–time graphs.
Translating information between graphical and numerical form.
Acceleration
The rate at which an object’s velocity changes over time.
How do we calculate the average acceleration of an object?
By using the equation:
a =∆v ÷ t
What does a =∆v ÷ t stand for?
acceleration = change in velocity ÷ time taken
What is the unit for acceleration?
m/s²
What is the unit for both speed and velocity?
m/s
If an object is decelerating, it is doing what?
Slowing down.
This is represented by a negative value for acceleration.
Estimations of the magnitude of everyday accelerations…
Walking ~0.5 m/s²
Running ~ 2 m/s²
Car ~ up to 6 m/s²
Roller Coaster ~ 20 m/s²
If an object is dropped, what will its initial velocity be?
0 m/s because it started off stationary.
If an object starts off stationary, what will its initial velocity (u) be?
0 m/s
If an object is dropped (so free falling) what is its acceleration?
9.8 m/s² due to the force of gravity
How can we calculate the acceleration of an object from a velocity - time graph?
Find the gradient to give you the acceleration.
How can we calculate the distance travelled by an object (or DISPLACEMENT) from a velocity-time graph?
Find the area under the graph:
-by counting squares if appropriate
-or by using formulae
Practice…
-Drawing velocity–time graphs from measurements and interpreting lines and slopes to determine acceleration
-Interpreting enclosed areas in velocity–time graphs to determine distance travelled/ displacement (HT only)
What is uniform acceleration?
A constant rate of change of velocity over time.
e.g. an object’s velocity increasing by 2m/s each second.
What is the equation that applies to uniform acceleration?
2 a s = v² - u²
What does 2 a s = v² - u² stand for?
2 x acceleration x distance(/displacement) = (final velocity)² - (initial velocity)²
In the equation:
2 a s = v² - u²
What does the v and u mean?
v = final velocity (m/s)
u = initial velocity (m/s)
How is a uniform ‘constant’ acceleration represented on a velocity-time graph?
A straight, increasing line.
Near the Earth’s surface, any object falling freely under gravity has an acceleration of what?
about 9.8 m/s²
Why does an object falling through a fluid initially accelerate?
Due to the force of gravity.
When would an object move at its ‘terminal velocity’?
When the resultant force on the object is zero.
Draw and interpret velocity–time graphs for objects that reach terminal velocity.
From kilonotes and freesciencelessons x
Interpret the changing motion of an object reaching terminal velocity - in terms of the forces acting.
1 - The only force acting on the object is weight. This causes the object to accelerate towards the ground.
2 - The force of weight (acting downwards) is greater than that of air resistance (acting upwards), so the object continues to accelerate.
3- Velocity increases causing air resistance to increase
4-Eventually, weight and air resistance will balance, leading to a resultant force of zero. The object’s velocity remains constant. = ‘terminal velocity’
Interpret the changing motion of an object reaching terminal velocity - continuation for a skydiver
5- The diver opens a parachute, increasing surface area. This increases air resistance.
6- The air resistance (acting upwards) is greater than the force of weight (acting downwards) so a resultant force acts upwards and the skydiver decelerates
7- Eventually, the forces will balance and the diver will fall at a lower terminal velocity
What causes air resistance (‘drag’)?
Collisions with air particles.
What increases air resistance?
Increasing the number of collisions between object and air particles by…
-Increasing the surface area of the object
-Increasing the velocity at which the object is moving
Why must a skydiver have a low terminal velocity?
So it’s safer to hit the ground.
If terminal velocity is too great, the skydiver would die as they hit the ground.
When an object falls what are the two main forces that act on it?
Weight - Pulls the object downward.
Air Resistance (Drag) - Acts upward, opposing the motion.
Terminal velocity
The constant speed a freely falling object reaches when the force of air resistance is equal to the gravitational force acting on the object.
At this point, the object stops accelerating and continues to fall at a steady speed.
What is newton’s 1st law?
A resultant force is required to change the motion of an object.
According to Newton’s first law, what will happen if the resultant force acting on a STATIONARY object is zero?
If the the object is stationary, the object remains stationary.
According to Newton’s first law, what will happen if the resultant force acting on a MOVING object is zero?
If 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.
An object in motion…
An object at rest…
…stays in motion
…stays at rest
Unless acted on by a resultant force!
When a vehicle travels at a steady speed the _________ forces _______ the driving force.
resistive
balance
According to Newton’s 1st law, the velocity (speed and/or direction) of an object will only change if…
…a resultant force is acting on the object.
What is constant velocity?
Where an object travels at a constant speed in a straight line, without changing direction.
What happens if a resultant force is applied to an object moving in the opposite direction to the force?
You get a decrease in speed.
If very slow moving to start, the object might completely stop moving.
If a resultant force is applied to a stationary object, what happens?
You get movement in the direction of the resultant force.
If a resultant force is applied to an object moving in the same direction as the force, what happens?
You get an increase in speed.
What can the acceleration of an object (caused by a resultant force) lead to?
A change in direction.
This is because acceleration is a change in velocity, and velocity includes direction.
Inertia
The tendency of objects to continue in their state of rest or of uniform motion.
What is Newton’s second law?
If a non-zero resultant force acts on an object, it will cause the object to accelerate.
According to Newton’s second law, what is the acceleration of an object related to?
-Acceleration is proportional to the resultant force acting on the object
-Acceleration is inversely proportional to the mass of the object.
What is Newton’s second law as an equation?
resultant force = mass x acceleration
F = m a
What is the unit for force?
Newtons, N
What is the unit for mass?
kg
What is the unit for acceleration, a?
m/s²
What is inertial mass?
A measure of how difficult it is to change the velocity of an object.
It’s defined as the ratio of force over acceleration.
An object with a large mass (e.g. a moon) has a lot of inertia. This means…
…it requires a really big force to change its velocity.
(so acceleration would be small - F=m/a).
You should be able to estimate the speed, accelerations and forces involved in…
… large accelerations for everyday road transport.
What is Newton’s third law?
Whenever two objects interact, the forces they exert on each other are equal and opposite.
Why does your foot hurt when you kick a door?
The door exerts an equal and opposite force on your foot.
Applying Newton’s Third Law to an equilibrium situation example - Earth vs Satellite
Earth exerting a gravitational force on a satellite, pulling it toward the centre of the Earth.
The satellite exerting an equal and opposite gravitational force on the Earth, keeping it in orbit.
Applying Newton’s Third Law to an equilibrium situation example - Book s Table
A book at rest exerting a downward force on a table - equal to its weight.
The table exerting an upward
that is equal in magnitude and opposite in direction to the book’s weight.
What does an object need too be accelerated?
Either a large force or a small mass.
What is the stopping distance of a vehicle?
The minimum distance required to stop a vehicle in an emergency.
How do we calculate the stopping distance of a vehicle?
Thinking distance + Braking distance
Thinking distance
The distance a vehicle travels during the driver’s reaction time.
Braking distance
The distance a vehicle travels under the braking force.
For a given braking force: the greater the _____ of the vehicle, the greater the stopping distance.
speed
How does thinking distance increase in relation to speed?
Thinking distance increases proportionally with speed.
How does braking distance increase in relation to speed?
Breaking distance ∝ (velocity)²
Assuming other factors remain constant.
So, more quickly than you’d expect!
If speed is doubled, breaking distance…
is increased by 4x.
If speed is quintupled , breaking distance…
increases by 25x.
If travelling in a car, it will take longer to slow down…
…than you expect. Stay safe queen x
You will be required to interpret graphs relating speed to stopping distance for a range of vehicles.
So, practice them cutesy car diagrams x
What is the typical range of reaction time?
- 2 to 0.9 seconds
Reaction time varies from person to person.
What are the factors affecting a driver’s reaction time?
-Tiredness
-Drugs
-Alcohol
-Distractions
explain methods used to measure human reaction times and recall typical results
fill in from textbook. x
interpret and evaluate measurements from simple methods to measure the different reaction times of students
fill in from textbook x.
You need to evaluate the effect of various factors on thinking distance based on…
…given data.
What factors can affect the braking distance of a vehicle?
-Adverse road conditions
-Adverse weather conditions
-Poor condition of the vehicle
Explain the factors which affect the distance required for road transport vehicles to come to rest in emergencies, and the implications for safety
Condition of brakes - if worn or faulty, then the vehicle won’t slow as quickly, will travel further, and hence won’t stop quickly enough in an emergency.
Condition of Tires -If bald (no tread) the car won’t grip the road = no traction.
What is poor condition of a vehicle limited to for gcse?
The vehicle’s brakes or tyres.
For estimating how the distance required for road vehicles to stop in an emergency varies over a range of typical speeds…
Use the cute car diagrams.
Why does a vehicle slow when a force is applied to the brakes?
-Work is done by the friction force between the brakes and the wheel
-The work done reduces the kinetic energy of the vehicle, so it slows
-(The temperature of the brakes increases)
When a force is applied to the brakes of a vehicle, what energy store decreases?
Kinetic
When a force is applied to the brakes of a vehicle, what energy store increases?
Thermal, because the temperature of the breaks increases.
The greater the speed of a vehicle the greater the _______ _____ needed to stop the vehicle in a certain distance.
braking force
F=m a
The greater the braking force…
…the greater the deceleration of the vehicle.
Explain the dangers caused by large decelerations
Large decelerations may lead to brakes overheating and/or loss of control.
You need to be able to estimate the forces involved in the deceleration of road vehicles
(in typical situations on a public road)
How do we about this?
Use F = m a
Because the vehicle is decelerating, the acceleration will be negative.
What is momentum?
The product of an object’s mass and velocity.
It is a property of all moving objects.
How do we calculate momentum?
Momentum = mass x velocity
p = mv
Symbol for momentum?
p
Unit for momentum?
kilograms metre per second (kg m/s)
What is conservation of momentum?
In a closed system, the total momentum before an event is equal to the total momentum after the event.
Describe and explain examples of momentum in an event, such as a collision
When bodies collide, whatever momentum is lost by one body, the other gains in equal amounts.
E.g. if one momentum was 0.6 the other would be -0.6.
How would we find the total momentum before a collision?
Add both momentums.
Subtract the momentum of an object travelling in the negative direction!
How would we find shared velocity from a collision?
sum of momentums ÷ sum of masses
give the directionnn
What makes momentum a vector quantity?
It is affected by velocity, a vector quantity.
Closed system
A group of objects whose total mass remains constant and that are not subject to any external forces.
If an _____ acts on a system, energy will be _____ to or from it and momentum will not be _____.
external force
transferred
conserved
In an ‘explosion’, such as firing a gun, the total momentum before is….
zero (because the system is stationary)
This means that the momentum of the bullet and the momentum of the recoiling gun must equate to zero.
An object moving backwards has a _________ velocity.
negative
Why does a change in momentum occur when a force acts on an object?
Applying a force causes an object to accelerate (speed up).
This increases the velocity of the object, which increases its momentum (p = mv).
How do we calculate a change in momentum?
mv - mu
(mass x final velocity) - (mass x initial velocity)
m ∆ v can also be written as what?
mv - mu
where v is the final velocity of an object, u is the initial velocity, and m is the mass
What is the equation linking force, momentum, and time?
Force = change in momentum ÷ time
Force = change in momentum ÷ time
Can be written how?
F = ∆p ÷ t
F = mv - mu ÷ t
What do all safety features do?
Increase the time it takes for momentum to be lost.
This decreases the force acting on a person, which decreases the risk of injury.
Name the safety features from the spec.
Air bags, seat belts, gymnasium crash mats, cycle helmets and cushioned surfaces for playgrounds.
When talking about safety features in an exam, you must…
Reference the rate of change of momentum over time.
According to Newton’s 2nd law of motion, this is equal to the force acting on an object.
Because 𝐹 = Δp ÷ t.
What safety features of a car increase the amount of time it takes for momentum to be lost?
-crumple zone gives a small amount of extra time for the car to lose momentum
-seat belts stretch so you slow down more slowly (and prevent you flying out)
-air bags compress
Students should be able to apply equations relating force, mass, velocity and acceleration to explain how the changes involved are inter-related.
Practice some of these!.
Do practice questions explaining safety features…
..referencing the rate of change of momentum (Force!)
Why are car crashes bad?
-They involve a massive change in momentum
-Which causes a huge force to be experienced by those in the car
-This can lead to injuries such as head trauma or spinal damage
