Topic 2- Forces And Motion Flashcards
What is distance ?
Distance is how far an object moves.
What is displacement ?
Displacement is the distance an object moves in a straight line from a starting point to a finishing point.
What type of quantity is distance ?
scalar quantity
What type of quantity is diplacement ?
Vector quantity
Why is distance a scalar quantity ?
This is because it contains a magnitude (size) but not a direction
Why is displacment a vector quantity ?
This is because it contains a magnitude (size) and direction.
What type of quantity is speed ?
Speed is a scalar quantity.
Scalar quantities only have a magnitude (size).
Scalar quantities, like speed, do not have a direction
What type of quantity 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.
Fact
To add vectors, draw each vector as an arrow one after the other.
The length of the arrow represents the magnitude (size) of a quantity.
The direction of the arrow represents the direction of the vector quantity.
If an object travels 4 metres forwards then 2 metres backwards, what is the distance travelled? What is the object’s displacement?
The total distance travelled is 4 + 2 = 6 metres. Distance does not depend on direction.
The displacement is 4 - 2 = 2 metres. The second part of the journey is taken in the opposite direction so we take away this part of the journey. The object is only 2 metres from where it was to begin with.
If a car travelling at a velocity of +10m/s suddenly turns around and continues at the same speed, what is its new velocity?
-10m/s
How is an objects speed shown on a distance-time graph
An object’s speed is shown by the gradient (slope) of the distance-time graph
If an object’s speed increases, the object will travel a longer distance in the same amount of time.
What will this do to the gradient ?
The slope (gradient) of the distance-time graph will become steeper
If an object’s speed decreases, then the object will travel a shorter distance in the same amount of time.
What does this do to the gradient ?
The slope (gradient) of the distance-time graph will become less steep.
On a speed-time graph if there is a horizontal line what does this mean ?
The object is moving at a constant speed
On a velocity-time graph if there is a horizontal line what does this mean ?
The object has stopped
The area under a velocity-time graph is equal to what ?
The area under a velocity-time graph is equal to the distance travelled by an object.
To find the area under the graph, what do we do ?
break it down into small shapes and add the areas of the shapes
How do you work out area of a triangle ?
The area of a triangle = 0.5 x base x height.
How do you work out area of a rectangle ?
The area of a rectangle = base x height.
If an object travels at 10 m/s constantly for 1 minute, how far will it have travelled?
600 m
How do you work out average speed ?
The average speed is the total distance travelled divided by the total time taken
What can we use to mesure speed ?
We can measure speed using light gates
Average speed = ? ÷ ?
Average speed = total distance ÷ total time.
It is usually measured in m/s.
How do light gates work ?
Light gates record how long it takes for an object to pass through them, so can be used to calculate the speed:
The length of a piece of card is measured with a ruler, and attached to a trolley.
The trolley is rolled down a ramp so that the card passes through the light gate.
The speed of the trolley when it passes the gate is calculated using v = x ÷ t.
x is the length of the card and t is the amount of time it takes to pass through the gate.
Usain Bolt broke a world record when he ran 100m in 9.58 seconds. What was his average speed?
10.4 m/s
How do you work out acceleration ?
An object’s acceleration (measured in metres per second squared, m/s2) is equal to its change in velocity (metres per second, m/s) divided by the time taken (seconds, s) to change its velocity. Deceleration is negative acceleration (speed decreasing).
A ball rolling down a hill accelerates uniformly from rest to a velocity of 5 m/s in 10 seconds. What is the acceleration of the ball?
0.5m/s^2
How do you work out final speed ?
Final speed = initial speed + (acceleration x time).
v = u + at
Final speed^2 - initial speed^2
(Final speed)2 – (initial speed)2 = 2 × acceleration × distance.
v2 - u2 = 2as
What is resultant force ?
The resultant force is the sum of all of the forces acting on an object.
What is the change in an object’s motion caused by ?
The change in an object’s motion is caused by the resultant force.
If the forces acting on an object are unbalanced (not equal) ,what does this mean ?
it means that a resultant force is acting on the object
What is newtons second law ?
A resultant force causes an acceleration.
The acceleration can be calculated with this equation.
Resultant force (F) = mass (m) x acceleration (a).
How can you work out resultant force ?
Resultant force (F) = mass (m) x acceleration (a).
Resultant Force = forces acting in one direction−forces acting in the opposite direction
What does newtons first law state ?
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.
If an object is moving and there is no resultant force acting on it, what does this mean for the velocity ?
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.
If an object is stationary (not moving) and there is no resultant force acting on what will happen ?
If an object is stationary (not moving) and there is no resultant force acting on it, it will stay stationary.
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.
When 2 objects interact, the forces they exert on (apply to) each other are equal and opposite. This is a statement of what law?
Newrons third law
How can you slove the momentum of an object ?
The momentum of an object is its mass multiplied by its velocity (p = mv).
How can you slove for change in momentum ?
Change in momentum = force x time.
Δp = F x t.
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.
What does the law of conservation of momentum say ?
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 collision.
What are the units of momentum?
Kgm/s
What is the force experienced by a person whose momentum changes by 10kgm/s in 2s?
What is the force experienced by a person whose momentum changes by 10kgm/s in 2s?
F = Δp ÷ t
F = 10 ÷ 2
F = 5 N
If an object which has a lot of momentum is made to stop suddenly, what is needed to stop the car ?
a large force is required because of the change in momentum.
What are some of the saftey features cars have that absorb the kinetic energy transferred by collisions ?
Cars have safety features such as seat belts, air bags and crumple zones
How do the saftey features in a car reduce injuries to people ?
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.
Using ideas about momentum, describe how a rocket is launched.
The rocket starts at rest and has no momentum.
The rocket pushes the fuel out of the bottom of the rocket. The fuel has gained momentum in the direction of the ground (earth).
The conservation of momentum means that the rest of the rocket must gain momentum in the opposite direction. This propels (pushes) the rocket into the sky.
How do safety features in a car reduce the force felt by a person in a crash?
When a crash takes place, there is a certain change in momentum.
The force felt by the person is the change in momentum divided by the time taken.
Safety features of a car increase the time taken for the momentum to change.
Therefore reducing the force felt by the person in the crash.
An object’s mass is a measure of what ?
the amount of matter that it contains
What is 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 what ?
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.
What is weight equal to ?
Mass x gravitational field strength
How can you mesure weight ?
A digital balance (like weighing scales) can measure the weight of an object.
Digital balances on Earth use the equation w=mg to convert weight into mass because gravity is constant on the Earth’s surface.
Weight can also be measured using a calibrated spring-balance (newton meter).
What is mass equal to ?
Weight / gravitational field strength
If an object is in free fall, what is the only force acting on it ?
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.
An object in free fall will accelerate at a constant rate, what is this called ?
An object in free fall will accelerate at a constant rate. This constant rate is called the acceleration due to gravity (g).
What is the average value of the earths acceleration due to gravity ?
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.
Objects falling through the Earth’s atmosphere do not continue to accelerate indefinitely why is this ?
because air resistance slows them down.
What resitance slows down a falling object ?
Air resistance slows down a falling object.
The force due to air resistance increases as the speed of a falling object increases.
What type of force is air resistance ?
Air resistance is a frictional force that opposes the motion of objects moving quickly through air.
An object moving at a constant speed in a circle (circular motion) is constantly changing what ?
An object moving at a constant speed in a circle (circular motion) is constantly changing direction.
Velocity and acceleration are vector quantities, so an object moving in a circle is constantly changing velocity and acceleration.
Velocity and acceleration are vector quantities, so an object moving in a circle is constantly changing velocity and acceleration.
What is the resultant force acting towards the centre of a circle called ?
The resultant force always acts towards the centre of the circle.
This resultant force is called the centripetal force.
When on a roundabout, you have to pull yourself towards the middle to stop yourself falling off.
Therefore, the object is accelerating towards the centre of the circle.
What is 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 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.
This resultant force towards the centre of the circle is called the centripetal force.
An object falling from a plane towards the ground…
Is in the earths gravitational field
What are the features of air resistance ?
Is a frictional force
Slows a falling object
Opposes the motion (movement) of objects moving through air
What is stopping distance ?
Stopping distance is the distance it takes a car to stop in an emergency (i.e. when the car is braking suddenly).
What is stopping distance equal to ?
Stopping distance = thinking distance + braking distance.
What is 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 realises they need to brake and when they apply the brakes.
What is breaking distance ?
The distance the car travels between the driver applying the brakes and the car stopping.
What is the typical stopping distance for a car trvaelling 20mph ?
For a car travelling at 20mph, a typical stopping distance is 12m.
What is the typical stopping distance for a car trvaelling 50mph ?
For a car travelling at 50mph, a typical stopping distance is 53m.
What is the typical stopping distance for a car trvaelling 70mph ?
For a car travelling at 70mph, a typical stopping distance is 96m
What are factors that affect thinking distance ?
Distractions
Tiredness
Drugs and alchol
How do distractions increase thinking distance ?
Distractions will cause a driver to react more slowly in an emergency.
Examples include mobile phones or small children.
How does tiredness increase thinking time ?
Tired drivers will react more slowly in an emergency.
How do drugs and alchol increase thinking distance ?
Drivers under the influence of drugs or alcohol will react more slowly in an emergency
What are factors that increase breaking distance ?
Condition of car
Inital speed
Road conditions
How does the conditions of the car increase breaking distance ?
If a car’s brakes or tyres are in a poor condition, then the braking distance will increase.
The higher the mass of the vehicle, the larger the stopping distance will be.
How does initial car speed increase breaking distance ?
The faster a car is travelling, the further it will travel before it comes to a stop.
How do road surfaces increase breaking distance ?
Wet or icy conditions will increase the braking distance.
The more friction there is between the tyre and the road the shorter the stopping distance will be.
How do we measure reaction times in humans ?
Ruler drop test
How do meaure the ruler drop test ?
One person holds a ruler vertically, with the zero mark at the bottom.
The second person, whose reaction time is being tested, places their thumb and first finger around the zero mark of the ruler, while the partner holds the ruler vertically.
The first person releases the ruler, and the second person closes their finger and thumb to catch it.
How do measure reaction time ?
The distance the ruler falls before being stopped can be used to measure the reaction time.
Acceleration is ‘g’ - the acceleration due to gravity - which can be taken to be 10m/s2.
We can use the equations v2 - u2 = 2as and a = (v-u) ÷ t to calculate reaction time.
A typical reaction time for humans is 0.25s.
What is a typical reaction time for a human?
0.25s
What is the typical walking speed ?
Typical walking speed is 1.4m/s.
What is the typical running speed ?
Typical running speed is 3.3m/s.
What is the typical speed of a bus ?
Typical speed of a bus in town is 10m/s.
What is the typicial speed of a car on a motorway ?
Typical speed of a car on the motorway is 31m/s.
What is the typical speed of wind ?
Typical speed of wind is 6m/s.
What is the typical speed of sound ?
Typical speed of sound is 340m/s.
Express 7.2km/h in metres per second.
7.2km = 7.2 × 1000m = 7200m
1h = 60 × 60s = 3600s
7.2km/h = 7200⁄3600 m/s = 2m/s
How is work dome when braking ?
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.
The greater the speed of a vehicle, the greater the braking force needed to stop the vehicle in a certain distance.
What does this mean for work done ?
The greater the speed of a vehicle, the greater the braking force needed to stop the vehicle in a certain distance.
This means that more work needs to be done on the brakes to stop the car.
The greater the mass of the vehicle, the greater the braking force needed to stop the vehicle.
What does this mean for work done ?
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.
For the same work done, how can the stopping distance be decreased ?
the stopping distance will decrease if the force (grip) between the road and the vehicle increases.
How do you estimate the work dome of a car ?
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.
What force is required to stop a car of mass 500kg over a distance 10m, if the car’s initial speed is 2m/s?
F d = ½mv2
F = ½mv2 ÷ d
F = ½ 500 × 22 ÷ 10
F = 100N
The greater the braking force, the greater the deceleration of the what ?
the greater the deceleration of the vehicle
What are the effects of a large deceleration ?
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
When a typical family car decelerates at a set of traffic lights, how large is the force exerted on (applied to) the passengers ?
only a small force is exerted on (applied to) the passengers.
This is because the deceleration happens over a long period of time.
The force doesn’t harm the passengers.
When a typical family car suddenly stops in the road to avoid a collision, how large is the force exerted on (applied to) the passengers ?
a greater force is exerted on (applied to) the passengers.
This is because the deceleration happens over a shorter period of time.
The force should not be enough to harm the passengers.
When a typical family suddenly is stopped by a crash, how large is the force applied to the passengers than the last example ?
an even greater force is applied to the passengers than the last example.
This is because the deceleration happens in even less time.
This could harm passengers.
How can the force in an emergency stop be calculated ?
Force = change in momentum ÷ time taken
The force in an emergency stop can also be calculated using the change in momentum divided by the time taken.
A car of mass 500kg comes to a stop from an initial speed of 2m/s in a time of 4s.
What is the force ?
Change in momentum = m(v − u) = 500 × (0 − 2) = - 1000kgm/s.
Force = change in momentum ÷ time taken.
Force = - 1000 ÷ 4 = - 250N.
