Forces Flashcards
What is a scalar quantity
A quantity that only has a magnitude/ a quantity that isn’t direction dependant
What is a vector quantity?
A quantity that has both magnitude and an associated direction
How can a vector quantity be drawn and what does it show?
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 magniude and an associated direction
Give three examples of vector quantities
- velocity
- displacement
- force
Give three examples of scalar quantities
temperature, time, mass, speed, distance, energy
What is weight?
The force that acts on an object due to gravity and the object’s mass
What quantities does weight depend on?
Weight = mass x gravitational field strength / the object’s 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
What does it mean if a force is said to do ‘work’?
The force causes an object to be displaced through a distance
What is the equation used to calculate work done?
Work done (Joules) = Force (Newtons) x 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 much work is done by the force acting on the below object over a distance of 5m?
10 x 5 = 50 Nm
=50J
What occurs when work is done against frictional forces?
A rise in temperature of the object occurs, 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
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
State the equation relating force, spring constant and extension. Give appropriate units
Force (N) = Spring constant (N/m) x 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 an alternative name for the turning effect of a force?
A moment
State the equation used to calculate the moment of a force. Five appropriate units.
Moment of force (Nm) = Force (N) x Distance (m)
What distance measurement is used when calculating a moment?
The perpendicular distance from the pivot to the line of action of the force
If an object is in equilibrium, what can be said about the moments acting on the object?
The clockwise moments are equal to the anticlockwise moments
What three parts make up a lever system?
- Load
- Effort
- Pivot
How can a alever be used as a force multiplier?
If the distance between the effort and pivot is greater than the distance vetween the pivot and the load, the force applied on the load is greater than the effort force. This is since the moment on both the effort and load must be the same. (I hated typing that)
Give an example of when a lever may be used to multiply a force
A wrench has a long handle so that the force applied by the user is multiplied
What determies the moment of a gear wheel?
The size of the wheel
Describe the moment and speed of a larger gear being driven by a smaller gear
The larger wheel will rotate more slowly but will also produce a larger moment
What is meant by the term ‘fluid’?
A liquid or 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
State the equation used to calculate pressure. Give appropriate units
Pressure (pascals) = Force (Newtons) / Area (metre^2)
What happens to the desity of the atmosphere with increasing altitude?
The atmosphere becomes less dense as altitude increases
Explain why atmospheric pressure decreses 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 the force due to the mass increases. Since the force has increased 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.
What equation shows the magnitude of pressure in liquids at different depths?
Pressure due to a column of liquid(Pa) = Column height(m) x density(kg/m^3) x gravitational field strength(N/kg)
P = hρg
Why are objects in a fluid subject to an upthrust?
When an object is submerged in a fluid, it has a higher pressure below it that 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 weigh. This means that it will sink.
Does a distance quantity require a specific direction? i.e. is it a scalar or vector quantity?
No specific direction is required so it is a scalar quantity
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 quanitity so it also involves direction
The object starts and ends at the same point
State a typical value for the speed of sound
330 m/s
What is a typical value for human walking speed?
1.5 m/s
What is a typical value for human running speed?
3 m/s
What is a typical value for human cycling speed?
6 m/s
State the equation linking distance, speed and time. Give appropriate units.
Distance (m) = speed (m/s) x time (s)
Why can an object travelling at a constant speed in a circle not have a constant velocity? (Higher)
Speed is a scalar quantity
Velocity is 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? (higher)
Drawing a tangent to the curve at the required time
Calculating the gradient of the tangent
State the equation for the average acceleration of an object. Give appropriate units
Acceleration (m/s^2) = (Change in Velocity (m/s)) / (time taken (s))
How can the distance travelled by an object be calculated from a velocity-time graph? (higher)
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^2
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 is 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 a resultant force
What is inertia? (higher)
The tendancy 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
F = ma
State Newton’s Second Law in words
An object’s acceleration is directly proportional to the resultant force acting on it and inversely proportional toW its mass
What is interial mass? (higher)
A measure of how difficult it is to change a given object’s velocity
The ration of force over acceleration
What is the symbol used to represent an approxite value?
~
State Newton’s Third Law
Whenever two objects interact, the forces they exert on each other are always equal and opposite.
(Every action has an equal and opposite reaction - Thomas Jefferson)
What is the stopping distance of a vehicle equal to?
The sum of thinking distance and braking distance
Give a typical range of values for human reaction time.
0.2 seconds - 0.9 seconds
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 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 distance, 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
State the equation used to calculate an object’s momentum (higher)
momentum = mass x velocity
What is the unit used for momentum? (higher)
kg m/s
kilogram metres per seconds
In a closed system, what can be said about the momentum before and after a collision? (higher)
The total momentum begore is equal to the total momentum afterwards
State an equation linking change in momentum, force and time (higher)
force x time = change in momentum
F Δt = mΔv
What quantity is equal to the force experienced in a collision? (higher)
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? (higher)
Increase the length of time over which the change of momentum occurs
Explain how a seatblet improves a passenger’s safety during a collision. (higher)
Passenger must decelerate from the vehicle’s velocity at impact to zero, meaning they undergo a fixed change of momentum
The force they experience is equal to the rate of change of momentum
Seatblets increase the time over which the force is applied, reducing the rate of change of momentum and therefore reducing the force experienced.
