P5: FORCES Flashcards
What is a scalar quantity and give three examples
It is a physical quantity which has magnitude (size) only. For example: time, voltage , energy
What is a vector quantity? Give three examples
It has a magnitude and a direction. For example:
velocity, force, displacement
How can a vector quantity be drawn and what does it show?
- as an arrow
- length of arrow = the magnitude
- the arrow points in the associated direction
what is a force? Give the two types
- a push or pull acting on an object due to an interaction with another object
- contact forces
- non-contact forces
give three examples of contact forces
- friction
- Air resistance
- Tension
give three examples of non-contact forces
- gravitational forces
- electrostatic forces
- magnetic forces
Give three examples of vector quantities
- velocity
- Displacement
- Force
Give 5 examples of scalar quantities
- temperature
- time
- mass
- speed
- distance,
- energy
what is weight?
Weight is the force acting on an object due to
gravity.
what quantities does weight depend on? Include equation and units
Weight = mass x gravitational field strength
- The object’s mass
- the gravitational field strength at the given position in the field
weight: Newtons , N
g = Newtons per kilogram , N/kg
What is the 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-metre
define ‘ the resultant force’
- the name given to the single force that is equivalent to all the other forces acting on a given object
State the effect of balanced forces on
an object’s motion.
- a stationary object stays still
- a moving object continues to move at
the same speed and in the same
direction
Describe how stationary and moving objects move when a resultant force acts.
- a stationary object starts to move in the
direction of the resultant force - a moving object changes speed and/or
direction in the direction of the
resultant force
What does it mean to “resolve a
force”?
A single force can be resolved into two
components acting at right angles to each
other. The two component forces together have the same effect as the single force.
What does it mean if a force is said to do ‘ work’
A force does work on an object when the force causes a displacement of the object.
What is the equation used to calculate work done? Give appropriate units.
Work done = Force x distance
work done = joules
force = newtons
distance= metres
Wat 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 N/m are equal to 1 joule of energy
1 Nm = 1J
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 - Hooke’s Law
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= Spring Constant x Extension
Force: N
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
When work is done to stretch a spring,
how much elastic potential energy is
stored?
Provided that the spring is not inelastically deformed, the work done on the spring and the elastic potential energy stored are equal
Define moment.
The turning effect of a force around a
point/pivot
State the equation for
moment of a force and the units
Moment = force x distance
moment: Newton metre (Nm)
force: Newton
distance: metre
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 sum of the clockwise moment about the pivot is equal to the sum of the anticlockwise moment about the pivot
State two ways to increase the size of a moment
- Increase the size of the applied force
- Increase the perpendicular distance
from the line of action of the force to
the pivot
What three parts make up a lever system?
- Load
- Effort
- Pivot
Explain how a lever can act
as a force multiplier
- if the distance between the effort and the pivot is greater than the distance between the pivot and load, the force applied o the load is greater than the effort force.
- this is since the moment on both the effort and load must be the same
- ( less force is needed to get the same moment )
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 what a gear is and
what function it serves in
machinery
- A gear is a circular disc with teeth. In
machinery it is interlocked with other gears - in order to transmit the rotational effects of a force from one place to another
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 = force / area
pressure: Pascals
Force: Newtons
Area: Metre”2
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 of the Earth
How does pressure in fluids increase with depth?
- As depth increases, the mass of liquid above the object 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
Describe and explain how
the pressure exerted by a liquid
on an object changes with
increasing depth
● As depth increases there is more liquid
above the object
● This increases the weight of the liquid
particles acting on the object
● As a result, pressure increases
Why does pressure in fluids increase with density?
- As the density of a fluid increases, the number of particles in a given volume increases
- 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
Which 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=hpg
why are objects in a fluid subject to an upthrust?
- When the pressure exerted by the liquid at the bottom of an object is greater than the pressure exerted at the top of an object.
- This creates a resultant force upwards (upthrust)
What is upthrust always equal to?
- the weight of the fluid that the object displaces
Give thee factors which 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.
How is atmospheric
pressure created on a surface?
When air molecules collide with a surface
Explain why atmospheric
pressure changes with altitude
Air molecules colliding with a surface
creates atmospheric pressure
● At increasing altitude, there are fewer
molecules of air above a surface so the
number of collisions between the air
molecules and the surface decreases
● As altitude increases, the weight of air
above the surface will decrease so
pressure will decrease
State a typical value for :
- human walking speed
- human running speed
- human cycling speed
- train speed
- plane speed
a. 1.5 m/s
b. 3 m/s
c. 6 m/s
d. 100 km/h
e. 900 km/h
State the equation linking distance , speed and time. Give appropriate units
distance = speed x time
distance : m
speed: m/s
time: s
why can 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
What piece of equipment can be used
to measure distance
Ruler, trundle-wheel
What piece of equipment can be used
to measure time?
Stop watch, light gates
. How can speed be calculated from a
distance-time graph?
the gradient
How can the speed of an accelerating
object be found from a distance-time
graph at a particular time?
Draw a tangent to the curve at that time and
find the gradient of the tangent.
What is the word and symbol equation for
acceleration?
Acceleration = change in velocity/time taken
a= v/t
units: m/s”2
How can acceleration be determined
from a velocity-time graph?
The gradient of a velocity-time graph.
How can the distance travelled by an object by an object be calculated from a velocity- time graph?
- it is equal to the area under the graph
What is the acceleration of objects near the Earth’s surface falling freely under gravity?
9.8 m/s2
. What is terminal velocity?
- An object falling through a fluid initially accelerates due to the force of gravity.
- air resistance will slowly equal weight and
- The resultant force will be zero and the object will move at its terminal velocity.
State Newton’s First Law:
- An object at rest will remain at rest and a
moving object will continue moving at constant velocity as long as no resultant force acts on the object
What is needed for velocity of an
object to change?
An unbalanced/resultant force
. What is inertia?
The tendency of objects to continue in their
state of rest or of uniform motion is called
inertia
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
State Newton’s Second Law:
The acceleration of an object is proportional to the resultant force acting on the object, and inversely proportional to the mass of the
object.
What is the word equation for
Newton’s Second Law?
Resultant force = mass x acceleration
. What is inertial mass?
A measure of how difficult it is to change the
velocity of an object. It is defined as the ratio of force over acceleration.
. State Newton’s Third Law
Whenever two objects interact, the forces they exert on each other are equal and opposite.
What is the stopping distance of a
vehicle?
- It is the sum of the distance the vehicle travels during the driver’s reaction time (thinking distance) and the distance it travels under the braking force (braking distance)
For a given braking distance, if the vehicle’s so speed is increased what can be said about its stopping distance
For a given braking force the greater the speed of the vehicle, the greater the stopping
distance.
What is a typical human reaction time?
0.2 - 0.9 s
Give three factors which can affect a driver’s reaction time.
Fatigue
drugs
alcohol
Give two factors which affect braking distance
- adverse road and weather conditions (such as wet and icy)
- poor condition of the vehicle (such as the brakes and tyres)
Describe the energy transfers that take place when a car applies its brakes
- When a force is applied to the brakes of a
vehicle - work done by the friction force
between the brakes - kinetic energy of the wheel is converted to heat and is dissipated to the surroundings through the brake discs
- temperature of brakes increases
In order to stop a vehicle within a
certain distance, how does the speed
of the vehicle affect the necessary
braking force
The greater the speed of a vehicle the greater
the braking force needed to stop the vehicle in a certain distance
What are the dangers of large braking
forces needed to slow down fast
moving vehicles?
- The greater the braking force the greater the deceleration of the vehicle.
- Large decelerations may lead to brakes overheating and/or loss of control.
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.
momentum = mass x velocity
What is the unit used for momentum?
- kg m/s
- kilogram metres per seconds
. What is the principle of conservation
of momentum?
In a closed system, the total momentum before an event is equal to the total momentum after the event.
State an equation linking change in momentum, force and time.
Force= change in momentum / time
What quantity is equal to the force experienced in a collision?
the rate of change of momentum
If an object’s change in 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 crumple zone
protects a driver in a crash?
- Increases time for driver to stop
- Decreases the rate of change of
momentum - Reduces force on driver
Explain how a seatbelt improves a passenger’s safety during a collision
- passenger must decelerate from the vehicles velocity at impact to zero, meaning they undergo a fixed change of momentum
- the force they experience is equal to rate of change of momentum
- seatbelt increases time over which force is applied
- reducing the rate of change of momentum
