P8,9,10,11 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.
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 magnitude and an
associated direction
Give three examples of vector quantities.
- Velocity
- Displacement
- Force
Give some 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?
● 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 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?
Nm = 1 J
How much work is done by the force of 10N acting on and object over a distance of 5m?
10 x 5 = 50 Nm
= 50 J
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 (plastic) 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
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.
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 lever be used as a force multiplier?
- Greater effort-to-pivot distance than pivot-to-load means greater force on the load.
- Moment on effort = Moment on load
- Applies principle of moments
Give an example of when a lever may be used to multiply a force.
A wrench, which has a long handle so that the force applied by the user is multiplied.
What determines 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.
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?
- Depth increase → More liquid mass above.
- More mass → Increased force due to gravity.
- Force increases → Pressure increases (constant area).
Why does pressure in fluids increase with density?
- Higher fluid density → more particles in given volume → greater fluid weight.
- Increased fluid weight → stronger force from fluid above.
- Stronger force → higher pressure at that point.
Why are objects in a fluid subject to an upthrust?
- Submerged objects have higher pressure beneath than above due to fluid weight.
- Result: Upwards force called upthrust acts on submerged objects.
- Floating or sinking depends on upthrust compared to object’s 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 = weight of displaced fluid.
- High object density = less displaced volume.
- Insufficient upthrust = object sinks.
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 quantity 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
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
How can the distance travelled by an object be calculated from a 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^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?
The tendency of an object to continue in its state of rest or uniform motion.
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 forces 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 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
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 they 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
