Forces - paper 2 Flashcards
To revise forces (Paper 2)
Define a scalar
A quantity that only has magnitude.
Give some examples of scalar quantities
Speed, distance, time, mass, volume
Define a vector quantity
A quantity that has magnitude and direction.
Give some examples of vector quantities
Velocity, displacement, acceleration, momentum, any force
State the difference between distance and displacement
Distance is a scalar - doesn’t involve direction
Displacement is a vector and is measured in a straight line from the start point to the finish point and the direction of that straight line.
State the difference between speed and velocity
Speed is a scalar - only has magniture
Velocity is a vector - has magniture and direction
State some examples of non contact forces
Friction, air resistance, drag, upthrust, tension, normal contact force
State some examples of contact forces
Gravity, magnetism, electrostatic attraction or repulsion
State the equation with all units for Weight
weight (N) = mass (kg) × gravitational field strength(N/kg)
What is the mathematical relationship between weight and mass.
They are directly proportional, if mass doubles then does weight
What equipment can be used to measure weight?
Weight is measured using a calibrated spring-balance called a newtonmeter
Define a resultant force
When a number of forces acting on an object may be replaced by a single force that has the same effect as all the original forces acting together.
Units of force
Newtons (N)
When is work done?
When a force causes an object to move through a distance
What is the relationship between work done and energy transferred
Work done = energy transferred
State the equation and all units for work done
work done (J) = force (N) × distance moved along the line of action of the force (m)
Define 1 joule of work
One joule of work is done when a force of one newton causes a displacement of one metre.
1 joule = 1 newton-metre
State Hooke’s law
The extension of an elastic object, such as a spring, is directly proportional to the force applied, provided that the limit of proportionality is not exceeded.
State Hookes law equation with all units
force (N) = spring constant (N/m) × extension (m)
When an object is stretched or squared what form of energy does it store?
Elastic potential energy.
Define elastic deformation
When the forces are removed the object returns to its original length
Define inelastic deformation
When the forces are removed the object doesn’t returns to its original length, it stays permanently stretched.
Define a moment
The turning effect of a force
State the equation for a moment, with all units
moment of a force (Nm) = force (N) × perpendicular distance (m)
If an object is balanced what can you say about the clockwise and anticlockwise moments
They must be equal
Define a fluid
Either a liquid or gas, particles must be able to move past each other
State the equation for pressure with all units
pressure = force / area
Pressure - Pa
Force - N
Area - m2
What is the relationship between pressure from a liquid and depth
As depth increases pressure increase due to a greater number of particles and force pushing on the object.
How does liquid pressure cause upthrust?
A partially (or totally) submerged object experiences a greater pressure on the bottom surface than on the top surface, (pressure increases with depth).
Force = Pressure x area so there is a greater force on the bottom surface than the top.
This creates a resultant force upwards. This force is called the upthrust.
Define the atmosphere
The atmosphere is a thin layer (relative to the size of the Earth) of air round the Earth.
State the relationship between altitude and the density of the atmosphere
he atmosphere gets less dense with increasing altitude.
As altitude increase why does atmospheric pressure decrease?
Air molecules colliding with a surface create atmospheric pressure. The number of air molecules (and so the weight of air) above a surface decreases as the height above ground level increases. So as height increases there is always less air above a surface than there is at a lower height. So atmospheric pressure decreases with an increase in height.
State the equation with all units to calculate speed
distance (m) = speed (m/s) × time (s)
how can speed be calculated from a distance - time graph
Using the gradient
State the equation to calculate average acceleration, with all units
acceleration = change in velocity / time
a = (v-u) / t
acceleration - m/s2
velocity - m/s
time - s
Define acceleration
Rate of change of velocity. An acceleration can be when an object:
- speeds up
- slows down
- changes direction
How can acceleration be found from a velocity - time graph
From the gradient
How can displacement be found from a velocity - time graph
From the area under the line
What is the initial rate of acceleration for an object falling close to the earths surface?
9.8 m/s2
What is terminal velocity and when does it occur?
the maximum velocity an object can fall at, this happens when forces are balanced (Weight = air resistance)
What factors can increase the size of air resistance
- Speed
- Surface area
- Density of the fluid which the object is falling through
State Newtons first law
if the resultant force acting on an object is zero the object will stay at rest or move at a constant velocity
The tendency of objects to continue in their state of rest or of uniform motion is called?
Inertia
State Newtons 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.
a = F/m
State the equation with all units for Newtons Second law
resultant force (N) = mass (kg) × acceleration (m/s2)
Define inertial mass
Inertial mass is a measure of how difficult it is to change the velocity of an object.
Inertial mass is defined as the ratio of force over acceleration.
m = F/a
Define inertia
the tendency of an object to continue in its state of rest or motion
State Newton’s third law
Whenever two objects interact, the forces they exert on each other are equal and opposite.
Define stopping distance
The stopping distance of a vehicle 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).
Stopping distance = thinking distance + braking distance
Define thinking distance
the distance the car travels whilst a driver reacts to a situation
Define braking distance
the distance the car travels whilst the brakes are applied
State the typical reaction times for a human
0.2 s to 0.9 s.
State some factors that can affect a persons reaction times
- tiredness,
- ages,
- drugs and alcohol,
- Distractions
State some factors that can affect the braking distance of a car
- Rain,
- ice,
- snow,
- poor quality brakes,
- poor quality tyres,
- gradient of the road,
- mass of the car
State the one factor that can increase both the thinking distance and braking distance
Speed
State the equation for momentum
momentum (kgm/s) = mass (kg) × velocity (m/s)
State the units for momentum
kg m/s
Define the conservation of momentum
In a closed system, the total momentum before an event is equal to the total momentum after the event.
Provided no external force is applied
What is required to change the momentum of an object
A force
Explain safety features such as: air bags, seat belts, gymnasium crash mats, cycle helmets and cushioned surfaces for playgrounds with reference to the concept of rate of change of momentum.
An air bag increases the time** for a person to stop. Therefore for the **same change of momentum** a **smaller force can be used.
Force = change in momentum/time
What is the centre of mass of an object?
The point at which the mass may be thought to be concentrated
Where does the centre of mass lie in symmetrical objects?
Along the line of symmetry
How can the centre of mass of an irregular shape be found?
by hanging the shape from a point and a plumb line is used to draw a vertical line downwards from the pivot. this is repeated from different points Where the lines meet is the centre of mass
What is a gear?
A rotating machine with cogs that mesh with another toothed part to transmit a moment
What do gears act as?
A force multiplier
What are the dangers of a large braking force?
Brakes overheating, loss of control and skidding
State the typical walking speed of a person
1.5 m/s
State the typical running speed of a person
3.0 m/s
State the typical cycling speed of a person
State the speed of sound
330 m/s
