Forces Flashcards
What is the unit used for momentum?
kg m/s
In a closed system, what can be said about the momentum before and after a collision?
The total momentum before is equal to the total momentum afterwards
What quantity is equal to the force experienced in a collision?
The rate of change of momentum
If an objects 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
Does a distance quantity require a specific direction? (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 a 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 for human cycling speed?
6 m/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 is continuously changing
How can speed be calculated from a distance-time graph?
The speed is equal to the gradient of the graph
How can the distance travelled by an object be calculated from a velocity-time graph?
It is equal to the area under the graph
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
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 its state of rest or uniform motion
State Newton’s second law
An object’s acceleration is directly proportional to the resultant force acting on it and is inversely proportional to its mass
What is inertia mass?
- a measure of how difficult it is to change a given object’s velocity
- the ratio of force over acceleration
State Newton’s third law
Whenever two objects interact, the forces that they exert on each other are always equal and opposite
What is the stopping distance of a vehicle equal to?
The sum of thinking distance and 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 condtions
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
What is meant by the term fluid?
A liquid or a gas
In any fluids, at what angle do the forces due to pressure act on a given surface?
At right angles to the surface
What happens to the density of the atmosphere with increasing altitude?
The atmosphere becomes less dense and altitude increases
Explain why atmospheric pressure decreased 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
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 is upthrust always equal to?
The weight of the fluid that the object displaces
What factors influence whether an object will sink of 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
What is an alternative name for the turning effect of a force?
A moment
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
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
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 does it mean if a force is said to do work?
The force causes an object to be displaced through a distance
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 joule of work done?
When a force of 1 Newton causes a displacement of 1 metre
How mant Newton-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
What is a scalar quantity?
A quantity that only has a magnitude
What is a vector quantity?
A quantity that has both a megnitude and an associated direction
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 acting on an object due to an interation with another object
What are the two categories that all forces can be split into?
- 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
Is force a vector or a scalar quantity?
- vector
- it has both a magnitude and an associated direction
Give three examples of vector quantities
- velocity
- displacement
- force
Give three examples of scalar quantities
- temperature
- time
- mass
What is weight?
The force that acts on an object due to gravity and the object’s mass
What quantities does weight depend on?
- the objects mass
- the gravitational field strength at the given position in the field
What is the unit used for weight?
The Newton (N)
What is the unit used for gravitational field strength?
N/kg
What is 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
What is the name given to the single force that is equivalent to all the other forces acting on a given object?
The resultant force
