Force Flashcards
What is a scaler quantity?
A quantity that only has magnitude.
What is a vector quantity?
a quantity that has both magnitude and direction
Examples of scalar quantities
speed, distance, time, mass, temperature, energy
Examples of vector quantities
force, velocity, displacement, acceleration, momentum
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 on an object due to the 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 scalar quantity?
● Vector
● It has both a magnitude and an associated direction
What is weight?
the force of gravity acting on an object due to gravity and the objects mass
What quantities does weight depend on?
Weight = mass x gravitational field strength
● The object’s mass
● The gravitational field strength at the
given position in the field
What is the unit used for weight?
Newton (N)
What is the the unit used for gravitational field strength?
N/kg
What is meant by an objects 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?
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 is the equation used to calculate work done?
work done (joules) = force (newtons) × distance (metres)
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 joules of work done?
When a force of 1 Newtons causes a displacement of 1 metre
How many newtons-metres are equal to 1 joule of energy?
1 Nm = 1 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 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 constants and extension
Force (N) = Spring constants (N/m) x 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
What is the alternative name for the turning effect of a force?
moment
State the equation used to calculate the moment of a force
moment of a force (Nm) = force (N) x distance (m)
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?
If the distance between the effort and pivot is greater than the distance between the pivot and load, the force applied on the load is greater than the effort force. This is since the moment on both the effort and load must be the same.
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’?
liquid or 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
● Pressure (Pa) = Force (N) / Area (m2)
● P = F / A
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?
As the depth increases, the mass of liquid above that depth 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.
Why does pressure in fluids increase with density?
As the density of a fluid increases, the number of particles in a given volume increases. Consequently 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.
What equation shows the magnitude of pressure in liquids at different depth?
Pressure due to a column of liquid (Pa) = Column height (m) x density (kg/m^3) x gravitational field strength (N/kg)
Why are objects in a fluid subject to an upthrust?
When an object is submerged in a fluid, it has a higher pressure below it than it does above, as there is more weight above it at the bottom than there is at the top. This leads to an upwards force called upthrust. You float or sink depending on whether the upthrust is more or less than your 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 liquid
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.
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.
State a typical value for the speed of sound.
330 m/s
What is the 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 of human cycling speed?
6 m/s
State the equation linking distance, speed and time
distance (m) = speed (m/s) x time (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
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
State the equation for the average acceleration of an object
Acceleration = change in velocity / time taken
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 the 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 their state of rest or of uniform motion
State the defining equation for Newton’s Second Law.
Resultant force = mass x acceleration
F = ma
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 the velocity of an object
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 equal to?
The sum of the thinking distance and the braking distance
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
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 (kgm/s) = Mass (kg) x Velocity (m/s)
P = mv
What can be said about the momentum before and after a collision?
The total momentum before and after are equal.
State an equation linking change in momentum, force and time
Force x Time = Change in Momentum
F Δt = PF Δt = mΔv
What quantity is equal to the force experienced in a collision?
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
