Topic 5-Forces Flashcards
What is a scalar quantity?
A quantity that only has a magnitude
A quantity that isn’t direction dependent
What is a vector quantity?
A quantity that has both a magnitude and an associated direction
What are 5 examples of scalar quantities?
Speed
Distance
Time
Mass
Energy
What are 5 examples of vector quantity?
Velocity
Displacement
Acceleration
Force
Momentum
How can a vector quantity be drawn and what does it show?
It is drawn as an arrow
The length of the arrow represents the magnitude
The arrow points in the associated direction
What is a force?
A push or pull acting on an object due to an interaction with another object
What are the two categories that all forces can be split into?
Contact forces (objects touching)
Non-contact forces (objects separated)
Give three examples of contact forces
Friction
Air resistance
Tension
Give three examples of non-contact forces
Gravitational forces
Electrostatic forces
Magnetic forces
Is force a vector or a scalar quantity?
Vector
It has both a magnitude and an associated direction
What is weight?
The force that acts on an object due to gravity and the object’s mass
What quantities does weight depend on?
The objects mass
The gravitational field strength at the given position in the field
What is the unit used for weight?
The Newton (N)
What is the unit used for gravitational field strength?
N/kg
What is meant by an object’s centre of mass?
The single point where an object’s weight can be considered to act through
What piece of equipment can be used to measure an object’s weight?
A calibrated spring-balance or newton-meter
What is the name given to the single force that is equivalent to all the other forces acting on a given object?
The resultant force
Describe the forces acting upon a skydiver from when they initially jump to when they reach terminal velocity
As they fall, they accelerate, increasing their speed
As air resistance increases, the resultant force from weight decreases
So acceleration decreases, so they are not speeding up as quickly
Eventually they are equal and balance, so there is no resultant force.
So there is no acceleration when the resultant force is 0. They travel at terminal velocity
What does it mean if a force is said to do ‘work’?
The force causes an object to be displaced through a distance
Units for work done, force and distance
Work done (Joules)
Force (Newtons)
Distance (metres)
What distance must be used when calculating work done?
It must be the distance that is moved along the line of action of the force
Under what circumstance is 1 joule of work done?
When a force of 1 Newton causes a displacement of 1 metre
How many Newton-metres are equal to 1 joule of energy?
1Nm = 1J
How many Joules in a kilojoule?
1Kj=1000J
What occurs when work is done against frictional forces?
A rise in temperature of the object occurs because kinetic energy is converted to heat
Why does air resistance slow down a projectile?
The object does work against the air resistance
Kinetic energy is converted in to heat, slowing down the object
Explain the relationship between the force applied and the extension of an elastic object
The extension is directly proportional to the force applied, provided that the limit of proportionality is not exceeded (Hooke’s Law)
What is meant by an inelastic deformation?
A deformation which results in the object being permanently stretched
The object doesn’t return to its original shape when the force is removed
Units for force, spring constant and extension
Force (Newtons)
Spring Constant (N/m)
Extension (m)
What type of energy is stored in a spring when it is stretched
Elastic potential energy
What can extension be replaced with in the equation for spring force?
Compression
What is meant by the term ‘fluid’?
A liquid of a gas
In any fluid, at what angle do the forces due to pressure act on a given surface?
At right angles (normal to) the surface
Units for pressure, force and area
Pressure (Pascals)
Force (Newtons)
Area (Metres squared)
What happens to the density of the atmosphere with increasing altitude?
The atmosphere becomes less dense as altitude increases
Explain why atmospheric pressure decreases with an increase in height
Pressure is created by collisions of air molecules
The quantity of molecules (and so weight) decreases as the height increases
This means atmospheric pressure decreases with an increase of height
What is the Earth’s atmosphere?
A thin (relative to the magnitude of the Earth) layer of gas surrounding the Earth
How does pressure in fluids increase with depth?
As the depth increases, the mass of liquid above that depth also increases
This means that force due to the mass increases
Since the force has increases whilst the area has remained constant, the pressure will increase
Why does pressure in fluids increase with density?
As the density of a fluid increases, the number of particles in a given volume increases
Consequently the weight of the fluid is greater
This means that the force from the fluid above a certain point is larger
Since the force has increased, the pressure also increases
Units for pressure due to a column of liquid, column height, density and gravitational field strength
Pressure (Pa)
Column height (m)
Density (kg/metres cubed)
GFS (N/kg)
Why are objects in a fluid subject to upthrust?
When an object is submerged in fluid, it has a higher pressure below it than it does above, as there is more weight above it at the bottom than there is at the top.
This leads to an upwards force called upthrust. You float or sink depending on whether the upthrust is more or less than your weight
What is upthrust always equal to?
The weight of the fluid that the object displaces
What factors influence whether an object will sink or float?
Upthrust
Weight
Density of fluid
Explain why an object with a density greater than that of water can never float
Upthrust is equal to the weight of the displaced fluid
If the density of the object is high, there would not be enough volume displaced to produce an upthrust larger than the object’s weight. This means that it will sink
If an object moves 3 metres to the left and then 3 metres back to its initial position, what is the object’s total displacement?
The object has zero displacement
Displacement is a vector quantity so it also involves direction
The object starts and ends at the same point
Typical value for human walking speed?
1.5m/s
Typical value for the speed of sound
330 m/s
Typical value for human running speed?
3 m/s
Typical human cycling speed?
6 m/s
Why can an object travelling at a constant speed in a circle not have a constant velocity?
Speed is a scalar quantity
Velocity is a vector quantity which means it can only be constant if the direction is constant
In circular motion, the direction is continuously changing
How can speed be calculated from a distance-time graph?
The speed is equal to the gradient of the graph
What must be done to calculate speed at a given time from a distance-time graph for an accelerating object?
Drawing a tangent to the curve at the required time
Calculating the gradient of the tangent
State the units for acceleration, velocity and time
Acceleration (m/s squared)
Velocity (m/s)
Time (s)
How can the distance travelled by an object be calculated from velocity-time graph?
It is equal to the area under the graph
Give an approximate value for the acceleration of an object in free fall under gravity near the Earth’s surface
9.8 m/s squared
What can be said about the resultant force acting on an object when it is falling at terminal velocity?
The resultant force is zero
When at terminal velocity, the object is moving at a constant speed and so isn’t accelerating
State Newton’s first law for a stationary object
If the resultant force on a stationary object is zero, the object will remain at rest
State Newton’s first law for a moving object
If the resultant force on a moving object zero, the object will remain at constant velocity (same speed in same direction)
What can be said about the braking forces and driving forces when a car is travelling at constant velocity?
The braking forces are equal to the driving forces
If an object changes direction but remains at a constant speed, is there a resultant force?
Since there is a change in direction, there is a change in velocity and so there must be resultant force
What is inertia?
The tendency of an object to continue in its state of rest or uniform motion
State the defining equation for Newton’s Second Law
Resultant force = Mass x Acceleration
State Newton’s Second Law in words
An object’s acceleration is directly proportional to the resultant force acting on it and inversely proportional to its mass
What is inertial mass?
A measure of how difficult it is to change a given object’s velocity
The ratio of force over acceleration
What is the symbol used to represent an approximate value?
~
State Newton’s Third Law
Whenever two objects interact, the force that they exert on each other are always equal and opposite
What is the stopping distance of a vehicle equal to?
The sum of thinking distance and braking distance
For a given braking distance, if the vehicle’s speed is increased, what can be said about its stopping distance?
The stopping distance is increased with an increase in speed
Give a typical range of values for human reaction time
0.2 seconds - 0.9 seconds
Give three factors which can affect a driver’s reaction time
Tiredness
Drugs
Alcohol
Give two factors which may affect braking distance
Adverse (wet/icy) road conditions
Poor tyre/brake conditions
Describe the energy transfers that take place when a car applies its brakes
Work is done by the friction force between the brakes and wheel
Kinetic energy of the wheel is converted to heat and is dissipated to the surroundings through the brake discs
To stop a car in a given, if its velocity is increased, what must happen to the braking force applied?
The braking force must also be increased
State two consequences of a vehicle undergoing very large decelerations
Kinetic energy converted to heat is very high causing brakes to overheat
Loss of control of the vehicle
What is the unit used for momentum?
kg m/s
Kilogram metres per seconds
In a closed system, what can be said about the momentum before and after a collision?
The total momentum before is equal to the total momentum afterwards
What quantity is equal to the force experienced in a collision?
The rate of change of momentum
If an object’s change of momentum is fixed, what is the only way to reduce the force that the object experiences?
Increase the length of time over which the change of momentum occurs
Explain how a seatbelt improves a passenger’s safety during a collision
Passenger must decelerate from the vehicle’s velocity at impact to zero, meaning the undergo a fixed change of momentum
The force they experience is equal to the rate of change of momentum
Seatbelts increase the time over which the force is applied, reducing the rate of change of momentum and therefore reducing the force experienced
The weightlifter has lifted a weight of 2250 newtons above his head. The weight is held still
What force is used by the weightlifter to hold the weight still
Give a reason for your answer
2250N
Forces must be balanced
At the end of the lift, the powerlifter hold the bar stationary, above his head, for two seconds
How much work does the powerlifter do on the bar during these two seconds
Give a reason for your answer
0
Work is only done when a force makes an object move
A slide is made of plastic
The child becomes electrically charged when he goes down the slide
Explain why
There is friction between the child and slide
This causes electrons to transfer between the child and the slide
Going down the plastic slide causes the child’s hair to stand on end
What conclusion about the electrical charge on the child’s hair can be made from this observation? Give a reason for your answer
All the charges on the hair are the same
Charges/hairs are repelling
Why would the child not become electrically charged if the slide was made from metal?
The charge would pass through the metal to earth
Ice hockey players wear protective pads filled with foam
Explain how the protective pads help to reduce injury when the players collide
Protective pads increases the time taken to stop during the collision
So the rate of change of momentum decreases
Reducing the force (on the ice hockey player)
As the aircraft moves along the runway to take off, its acceleration decreases even though the force from the engines is constant. Explain why.
As speed increases, air resistance increases
This reduces the resultant force
What happens to the kinetic energy of the bus as it is braking?
Kinetic energy decreases and is transferred by heating to the breaks
A fielder, as he catches a cricket ball, pulls his hands backwards.
Explain why this action reduces the force on his hands.
It increases the time to change/reduce momentum
Applying the brakes of the car causes the temperature of the brakes to increase.
Explain why.
Work done by friction between brakes and wheel
Causes a decrease in KE and increase in thermal energy
Hybrid cars have an electric engine and a petrol engine. This type of car is often fitted with a regenerative braking system.
A regenerative braking system not only slows a car down but at the same time causes a generator to charge the car’s battery.
State and explain the benefit of a hybrid car being fitted with a regenerative braking system.
The battery needs recharging less often and increases the range of the car as the efficiency of the car is increased
The decrease in kinetic energy charges the battery
The data needed to draw the bar chart was obtained using a robotic athlete fitted with electronic sensors
Why is this data likely to be more reliable than data obtained using human athletes
Conditions can be repeated
With humans, the conditions needed to repeat tests may not be constant
Once a skateboard starts to recoil, it slows down and its kinetic energy decreases
Explain why
Work is done
Against friction
Transforming kinetic energy into heat
In a collision, momentum is not always conserved
Why?
An external force acts on the colliding objects
The skateboard moves backwards as the skateboarder jumps forwards.
Explain, using the idea of momentum, why the skateboard moves backwards.
Momentum before jumping=momentum after jumping
Before jumping, momentum and skateboarder is zero
After jumping skateboarder has momentum forwards so skateboard must have equal momentum backwards
The student used the spring, a set of weights and a ruler to investigate how the extension of the spring depended on the weight hanging from the spring.
Before taking any measurements, the student adjusted the ruler to make it vertical.
Explain why adjusting the ruler was important.
To reduce the error in measuring the extension of the spring
As the ruler at an angle would make the measured extensions shorter
The student used the spring, a set of weights and a ruler to investigate how the extension of the spring depended on the weight hanging from the spring.
Describe one technique that you could have used to improve the accuracy of the measurements taken by the student.
Attach a horizontal pointer to the bottom of the spring
So that the pointer goes across the ruler scale
A student investigated the behaviour of springs. She had a box of identical springs.
The student suspended a spring from a rod by one of its loops. A force was applied to the spring by suspending a mass from it.
No other masses were provided.
Explain how the student could test if the spring was behaving elastically.
Remove the mass
Observe if the spring returns to its original length
Explain why atmospheric pressure decreases with increasing altitude
Air molecules colliding with a surface create pressure
At increasing altitude distance between molecules increases
So number of collisions with a surface decreases
The diagram shows the cross-section of one type of aircraft window.
Explain why the window has been designed to have this shape.
Force from air pressure acting from inside to outside bigger than force acting inwards
So keeps the window in position
What property of a liquid enables a hydraulic brake system to work?
Liquids are incompressible
Musicians often use loudspeakers
The loudspeaker cone vibrates when an alternating current flows through the coil.
Explain why.
The current flowing through the coil creates a magnetic field around the coil
This magnetic field interacts with the permanent magnetic field
This produces a resultant force and the cone moves
When the direction of the current changes, the direction of the force changes to the opposite direction
