Physics Topic 5 - Forces Flashcards
What is a scalar quantity?
-a quantity that only has a magnitude
-a quantity that isn’t direction dependent
What is a vector quantity?
-a quantity that has both magnitude 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 interaction with another object
What are the two categories that all forces can be split into?
1-contact forces
2-non-contact forces
Give 3 examples of contact forces
-friction
-air resistance
-tension
Give 3 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
1- velocity
2-displacement
3-force
Give some examples of scalar quantities
- temperature
-time
-mass
-speed
-distance
-energy
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 object’s 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 is the name given to the single force that is equivalent to all the other forces acting on a given object?
The 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 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 circumstances is 1 joule of work done?
When a force of 1 newton causes a displacement of 1 metre
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 objects 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 ineleastic deformation?
- a deformation which results in the objects being permanently stretched
-the object doesn’t return to its original shape when force is removed
What type of energy is stored in string when it is stretched?
Elastic potential energy
What can extension be replaced with in the equation for spring force?
Compression
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 form 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?
1-load
2-effort
3-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 wrenching, 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’?
A liquid or a 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
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 decreases as the height increases
-this means atmospheric pressure decreases with an increase of height
What is the earth’s atmosphere?
A thin 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 volumes 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 or float?
-upthrust
-weight
-density of fluid
Does a distance quantity require a specific direction?
No specific direction is required so it is a scalar quantity
State a typical value for the speed of sound
330m/s
What is a typical value for human walking sped?
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 a circular motion, the direction is continuously changing
How can speed be calculated from a distance - time graph?
Th 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 tot he curve at the required time
-calculating the gradient of the tangent
What must be done to calculate speed at a given time from a distance - time graph for an accelerating object?
-drawing a tangent tot he curve at the required time
-calculating the gradient of the tangent
How can distance travelled by an object be calculated from a velocity - time graph?
It is equal to the area underneath the graph
State newton’s 1st law from a stationary object
If the resultant force on a stationary object is zero, the object will remain at rest
State newton’s 1st law for a moving object
If the resultant force on a moving object is zero, the object will remain at constant velocity
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 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 2nd law
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 a given object’s velocity
-the ratio of force over acceleration
State newton’s 3rd 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
For a given braking distance, if the vehicle’s speed is increased, what can be said about its stopping distance?
The stopping distance is increased with an increase in speed
Give three factors which can affect a driver’s reaction time
1-tiredness
2-drugs
3-alcohol
Give two factors which may affect braking distance
1-adverse road conditions
2-poor tyre/brake conditions
To stop a car in a given distance, if its velocity is increased, what must happen to the breaking force applied?
The braking force must also be increased
State two consequences of a vehicle undergoing very large declarations
1-kinetic energy converted to heat is very high causing brakes to overheat
2-loss of control of the vechile
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 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 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 experienced
