P5 Flashcards
What is the difference between a scalar and vector quantity?
Scalar quantities - magnitude only
Vector quantities - have magnitude and direction
How can a vector quantity be represented?
using an arrow
- the length=magnitude
- the direction of the arrow= direction of the vector quantity
What is a force?
A push or pull that acts on an object due to their interaction with another object
What are contact forces?
The objects are physically touching
What are non-contact forces?
The objects are physically separated
Give examples of contact forces
- friction
- tension
- air resistance
Give examples of non-contact forces?
- gravity
- electrostatic force
- magnetic force
What is an interaction pair?
A pair of forces that are equal (in magnitude) and opposite in direction
Is force a scalar or vector quantity?
Vector
What is weight?
The force acting on an object due to gravity
How can you calculate the weight of an object
W= m x g
W= weight (N)
m= mass (kg)
g= gravitational field strength (N/kg)
Where does the weight of an object act?
at the object’s ‘centre of mass’
What is weight measured with?
a newton meter
Describe the relationship between the weight and mass of an object
the weight of an object is directly proportional to the mass of an object
What is the resultant force?
a number of forces acting on an object that may be replaced by a single force that has the same effect
What happens when the forces acting on an object combine to give a resultant force of 0?
The forces are balanced and the object is in equilibrium
When is work done on an object?
When a force causes an object to move through a distance (when the force causes a displacement of the object)
How can you calculate the work done by a force on an object?
W= F x s
W= work done (J)
F= force (N)
s= distance (m)
What happens when work is done against the frictional forces acting on an object?
It causes a rise in the temperature of the object
Why does more than one force have to be applied in order to change the shape of an object (by bending, stretching or compressing) ?
Otherwise the object (e.g. the string) would just move in the direction of the single applied force.
What is the difference between elastic deformation and inelastic deformation?
elastically deformed - if it can go back to its original shape and length after the force has been removed
- e.g. a spring, rubber band
inelastically deformed - if it does not return to its original shape and length after the force has been removed, its shape will have been changed permanently
Describe the extension of an elastic object
The extension of an elastic object is directly proportional to the force applied, provided that the limit of proportionality is not exceeded
What is the equation for force?
F= k x e
F= force (N)
k= spring constant (N/m)
e= extension (m)
What does a force that stretches or compress a spring do?
It does work and elastic potential energy is stored in the spring
provided the spring is not inelastically deformed, the work done and the elastic potential energy stored are equal
What is the equation used to calculate elastic potential energy (up to the limit of proportionality)?
Ee = 1/2 x k x e^2
What is a moment?
The turning effect of a force
How do you calculate the size of the moment?
M = F x d
M= moment (Nm)
F= force (N)
d= perpendicular distance from the pivot to the line of action of the force (m)
What is the role of a lever?
Levers increases the distance from the pivot at which the force is applied
this means less force is needed to get the same movement
Explain the function of gears
Gears are used to transmit the rotational effect of a force from one place to another
- their teeth interlock that when one turns another turns in the opposite direction
- the larger gear moves slower
Explain what happens when an object is balanced
The total clockwise moment about a pivot equals the total anticlockwise moment about that pivot
Describe the pressure in fluids (liquids/gases)
The pressure in fluids causes a force normal (at right angles) to any surface
What equation can be used to calculate the pressure at the surface of a fluid?
p= F/A
p= pressure (Pa)
F= force normal to a surface(N)
A= area of that surface (m^2)
How can you calculate the pressure due to a column of liquid?
pressure = height x density x gravitational field strength
p= h x p x g
p= pressure (Pa)
h= height (m)
p= density (kg/m^3)
g= gravitational field (N/kg)
Why (in a liquid) does pressure at a point increase with the height of liquid above that point and with the density of the liquid?
The pressure increases because there are more water particles on top of the point and the increased pressure is due to the weight of the water above
What does a partially or submerged object experience?
upthrust- a greater pressure on the bottom surface than on the top surface which creates a resultant force upwards
Why do objects weigh less when in fluids?
The weight of an object decreases as the object experiences upthrust when submerged in a fluid. The resultant force of the weight and upthrust gives a reduced weight
What does Archimedes principle state?
When a body is partly or totally immersed in a fluid, there is an upthrust that is equal to the weight of fluid displaced
What is the atmosphere?
a thin layer of air around the Earth
(it gets less dense with increasing altitude)
What creates atmospheric pressure?
Air molecules colliding with a surface
What happens to the number of air molecules above a surface as the height of the surface above ground level increases?
The number of air molecules decreases
What happens as height increases?
atmospheric pressure decreases
True or False: Distance is a vector quantity?
False- distance is scalar
What is displacement?
This includes both the distance an object moves (from start point to finish point) and the direction of the straight line
What type of quantity is speed?
Speed is scalar
Give typical values for the speed of walking running and cycling
- walking - 1.5 m/s
- running- 3 m/s
- cycling- 6 m/s
What is a typical value for the speed of sound in air?
330 m/s
How can you calculate the distance travelled in a specific time ( at a constant speed)?
s= v x t
s= distance (m)
v= speed (m/s)
t= time (s)
What is velocity?
The speed of an object in a given direction
- so it is a vector quantity
What is meant by terminal velocity?
after a while, a falling object would have a constant velocity
What is the equation for acceleration?
acceleration = change in velocity/ time taken
Define inertia
The tendency of an object to continue in its current state of motion
What is Newton’s first law?
If the resultant force acting on an object is 0:
- if it is stationary, it will remain stationary
- if it is moving, it will move at a constant speed
What is Newton’s second law?
The resultant force acting on an object is directly proportional to the acceleration and indirectly proportional to the mass
F= ma
What is Newton’s third law?
When two objects interact, the forces they exert on each other are equal and opposite
(Every action has an equal opposite reaction - Thomas Jefferson)
What is momentum?
mass x velocity
Describe the conservation of momentum principle
In a closed system, the momentum before an event is equal to the total momentum after
What happens when a car crashes (into a brick wall)?
- The momentum decreases to 0 almost immediately
- the massive change in momentum means that anyone inside the car would experience a huge force
What is meant by stopping distance?
The distance it takes a car to stop in an emergency
(thinking distance + braking distance)
What is ‘thinking distance’?
The distance travelled between when the driver realises they need to brake and when they apply the brake
What is ‘braking distance’?
The distance the car travels between the driver applying the brakes and the car coming to a stop
Factors affecting thinking distance
- drugs/alcohol
- tiredness
- distractions (e.g. mobile phone)
factors affecting braking distance
- road conditions
- tire conditions
- brakes conditions
- initial car speed
What are dangers of large decelerations?
the greater the braking force the greater the deceleration.
- A larger deceleration can cause overheating/skidding
- It will also transfer more stopping force to the passengers which would harm the passengers
