Flashcards in P5 Forces Deck (54):
Describe Vector and Scalar Quantities
- A scalar quantity is one that has magnitude only:
• length, area, speed, density, energy, work, power, time, distance
- A vector quantity has both magnitude and direction:
• displacement, velocity, acceleration, momentum, force (including weight)
Describe contact and non-contact forces
Contact forces act between objects that are touching one another:
• normal reaction, air resistance, tension, friction (opposes motion)
Non-contact forces act between objects even when they are not touching one another:
• magnetic, gravitational, electrostatic
- Forces allow objects to interact with one another
- Forces can change:
• speed of an object
• direction in which it's moving
• the shape of an object (stretch, compress and bend)
What is meant by weight and mass? How do you calculate weight?
- Weight is the force acting on an object due to gravity (measured in Newtons) -> vector
- Mass is telling how much mass is in an object (measured in g/kg) -> scalar
Weight = Mass x Gravitational Field Strength
• Weight is directly proportional to mass
• g = 9.8kg/N on Earth
All objects are attracted to each other -> force of this attraction increases when the objects have a larger ass or are closer together
What is meant by the centre of mass?
The centre of mass of an object is the point from which its weight can be considered to act from
What is a resultant force?
When 2 or more forces acting on an object can be replaced by a single force that has the same effect as the original forces combined
Describe the forces acting on the following isolated objects:
• A moving car
• A skydiver
• normal contact force
• driving force
• drag/air resistance
What is meant by work done and how is it calculated?
- When a force causes an object to move through a distance work is done on the object
- The amount of work done on an object is equal to the amount of energy transferred to it
- A force has done work -> when a force moves an object through a distance, energy is transferred and work is done
- Work is not done when the object doesn't move in the direction of the force
Work done (J) = Force (N) x distance (m)
Define the unit of work
One joule of work done is on an object when a force of one newton causes its displacement to change by one metre
if F=1N and s=1m,
1J = 1Nm
Explain why work is done against frictional forces causes a rise in temperature of an object
- Energy is being transferred to the kinetic energy store of the object because it starts moving
- Some energy is transferred into the thermal energy stores due to friction
Explain why to change the shape of an object more than one force has to be applied
More than one forces need to be acting on the object or the object would just move in the direction of applied force
What is elastic and inelastic deformation?
Elastic Deformation: when an object is deformed elastically it will return to it original shape if the forces acting on it are removed
Inelastic Deformation: when an object is deformed inelastically it will not return to its original shape if the forces acting on it are removed
Describe the extension of an elastic object
Force = spring constant x extension (or compression)
• Force is directly proportional to extension
Limit of Proportionality: the point beyond which the force applied to an object is no longer proportional to its extension
How is Elastic Potential Energy Calculated
Elastic potential energy = 1/2spring constant x extension/compression^2
- If an object is deformed elastically the amount of work done on it = elastic potential energy stored in it
What is the principle of moments?
Principle of moments: when an object is in (rotational) equilibrium, the sum of the anti-clockwise moments about a pivot is equal to the sum of the clockwise moments
What is the equation for moment?
M = Fd
-> d = the perpendicular distance from the pivot to the line of action force
• a moment is the turning effect of a force (the moment of a force is the turning effect it has about a pivot)
• as we apply the force further from the pivot the moment exerted by it increases
Explain how levers and gears transmit the rotational effects of forces
- Levers increase the distance from the pivot at which the force is applied, makes it easier to do work
- Gears used to transmit the rotational effect of a force from one place to another -> sizes of gears change the moment of the force
Describe a fluid
- A substance that can flow as particles are able to move around and can either be a liquid or a gas
How is pressure created in a fluid?
The pressure in a fluid always produces a force which acts at right angles (normal) to the surface of the walls of its container
How is pressure calculated in a fluid?
p = F/A
p -> pressure (Pa)
F -> force normal to surface (N)
A -> area of surface (m^2)
Explain why the pressure at a point in a fluid increases with the height of the column above
- The water at a given depth has to support the weight of all the water in the vertical column above it
- The deeper the greater the weight, therefore the greater the pressure
What is the equation for the pressure due to the column of liquid above?
p = hρg
p -> pressure due to liquid above (Pa)
h -> height of column (m)
ρ -> density of liquid (kg/m^3)
g -> gravitational field strength (N/kg)
Upthrust = weight of fluid displaced
• if the density of the completely submerged object is less than the fluid around it, then the upthrust force will be greater than the weight of the object and the object will float
• if the object is more dense than the fluid its weight will be greater than the upthrust forces acting on it, the object will sink
What is the box is ρb < ρw
Less dense than water:
• as it moves upwards the weight of water its displacing will decrease
• come to rest in a partially submerged position
• upthrust and weight forces become equal
• has to displace less water for the upthrust force to equal its weight (why it sits higher on the water)
If ρb > ρw
More dense than water = sinks!
If ρb = ρw
density is equal = neutral buoyancy (stays at rest)
Describe the Earth's atmosphere
- Layer of air that surrounds the Earth, thin compared to the size of the Earth
- Atmospheric pressure is created by air molecules colliding with the Earth's surface
Why does atmospheric pressure decrease with height?
- Air pressure is caused by the force exerted when moving air particles collide with a surface
- As height increases, the density of the air decreases, which means that air particles collide into objects and other air particles less frequently
- This causes the force exerted by these particles on a given surface to decrease
- Air pressure decreases with height
Define distance and displacement and why they are scalar and vector quantities
Distance is a scalar quantity as it describes how far something has travelled
Displacement is a vector quantity as it describes how far and in which direction something has travelled from a particular point
How is speed calculated?
v = s/t
v -> speed (m/s)
s -> distance (m)
t -> time (s)
Describe some typical speeds and explain the factors that are taken into account
Walking = 1.5m/s
Running = 3m/s
Cycling = 6m/s
• values depend on age, fitness, terrain, distance
car (v ≈ 60mph) = 30m/s
airplane (v ≈ 500mph) = 200m/s
speed of sound in air = 330m/s
How is acceleration calculated?
acceleration (m/s^2) = change in velocity (m/s) ÷ time
• deceleration is just negative acceleration
• What is meant by uniform acceleration?
• How is it calculated?
• (Constant acceleration) velocity increasing at a constant rate
• v^2 - u^2 =2as OR acceleration (m/s^2) = change in velocity (m/s) ÷ time
Describe the speed and velocity of an object travelling in a circle and give examples
- When an object is travelling at a steady speed in a circular path its velocity is constantly changing because it's direction of motion is always changing
- For example: a car travelling round a bend at a steady speed or the moon orbiting the Earth
Describe the parts of a distance-time graph
• Gradient shows the speed of the object
• Straight line = steady speed
• Flat sections = stationary
• Steepening curve = acceleration
• Levelling off curve = deceleration
* acceleration is calculated by finding the tangent of a line
Describe the parts of a velocity-time graph
• Gradient shows the acceleration
• Straight line = acceleration (if line is \ object is constantly decelerating
• Flat sections = steady speed
• Curve = changing acceleration
* Distance travelled = area underneath, if lines are curved count the squares underneath (distance = area of squares x no. squares)
What is drag and air resistance?
Drag is the resistance you get in a fluid (gas or liquid)
Air resistance is a type of drag, the frictional force produced by the air acting on a moving object
• Objects are shaped to be streamlined so fluid flows easily
• Parachutes want to create as much drag as possible
• As speed increases drag increases
What is meant by terminal velocity?
• The maximum an object can travel at as the weight and air resistance become equal
Describe how an object falling from rest in a fluid reaches terminal velocity
- Object is released, the only force acting on it will be its weight, a = 9.8 m/s, v = 0
- As object continues to fall air resistance and velocity increase, acceleration will decrease
- Object will eventually reach terminal velocity when the weight of the object is equal to the air resistance, resultant force goes to 0 and acceleration falls to 0
What is Newton's First Law of Motion?
• If the resultant force acting on an object is zero, then its acceleration will be zero
- If an object is at rest it will remain at rest
- If and object is moving it will continue to move with constant velocity
What is Newton's Second Law of Motion?
• An object will accelerate if there is a resultant force acting on it
• The acceleration of the object is:
- Proportional to the resultant force acting on it
- Inversely proportional to its mass
F -> resultant force (N)
m -> mass (kg)
a -> acceleration (m/s)
Inertia is the tendency of an object to remain at rest or at a constant velocity is called inertia
- The greater the inertial mass of an object, the more difficult it is to change its velocity
What is Newton's Third Law of Motion?
When an object exerts a force on another object, the second object will exert an equal and opposite force on the first. Both forces must be the of the same type
- applies to both contact and non-contact forces
What is the relationship between mass and acceleration
acceleration is inversely proportional to mass
What are the components of stopping distance
Stopping distance: total distance over which a vehicle (car) comes to rest
• stopping distance = thinking distance + braking distance
- Thinking distance: distance travelled while reacting
- Braking distance: distance travelled between point at which brakes are applied and point at which the car comes to rest
How can reaction time be measured?
- Ruler test
- Average person has a reaction time of 0.7 seconds
What is the braking distance affected by?
- condition of the brakes and tyres
- type of road surface
- weather conditions
- size of the braking force
What can affect reaction time?
Tiredness, distractions, alcohol consumption and drugs
What is momentum?
The momentum of an object is proportional to its velocity and its mass
- Vector quantity
What is the principle of conservation of momentum?
In a closed system, the total momentum before any event is equal to the total momentum after the event
How is momentum calculated?
Describe the momentum in a vehicle collision and an explosion
- If a moving vehicle collides with a stationary vehicle, the momentum of the moving vehicle will be shared, move slower
- In an explosion all particles move at the same speed by in different directions, so p=0 like before as everything was still
How can force be calculated (change in velocity)?
F = m x change in v ÷ t