TOPIC 5 FORCES PART 2 Flashcards
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.
State the equation used to calculate pressure. Give appropriate units.
Pressure = Force/Area
Pressure (Pascals), Force (Newtons), Area (metre2)
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 depths?
Pressure due to a column of liquid(Pa) = Column height(m) x density(kg/m3) x gravitational field strength(N/kg)
P = hρg
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
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 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 canonly 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.
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
Give an approximate value for the acceleration of an object in free fall under gravity near the Earth’s surface.
9.8 m/s2
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 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.
What is inertia?
The tendency of an object to continue in its state of rest or 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 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 2. Drugs 3. 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
To stop a car in a given distance, if its velocity is increased, what must happen to the braking force applied?
The braking force must also be increased.
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 = Mass x Velocity
What is the unit used for momentum?
kg m/s
kilogram metres per seconds
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.
State an equation linking change in momentum, force and time
Force x Time = Change in Momentum
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
Perujan
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