Module 3 – Forces and motion Flashcards
What is the formula and SI unit for speed?
change in distance / time ; ms⁻¹
What is the formula and SI unit for velocity?
change in displacement / time ; ms⁻¹
Describe a method to determine the terminal velocity of a falling object.
The terminal velocity of a falling object can be determined using light gates connected to a timer or data logger. The time when each of two light beams are broken by the object is recorded, and these times, combined with the known distance between the light-gates, can be used to calculate the velocity of the object.
What is the formula for calculating the vertical velocity of a projectile?
v * sin(θ)
What is the formula for calculating the horizontal velocity of a projectile?
v * cos(θ)
What is meant by ‘stopping distance’?
The stopping distance of a car is defined as the total distance travelled from when the driver of the car sees a hazard requiring them to stop, to the point they actually stop. It consists of two components, the thinking distance and the braking distance.
What affects the thinking distance?
The thinking distance is affected by the initial speed of the car, and factors that affect the driver’s reaction time, such as tiredness, being under the influence of alcohol or drugs, and other distractions.
What affects the braking distance?
As well as the initial speed of the car, the braking distance is also increased by poor road conditions (e.g icy, wet), and the car conditions (e.g worn tyres, poor brakes, heavy load).
What is the SI unit for force?
kgms⁻²
Name and describe 6 commonly occurring forces.
- Weight – the gravitational force acting on an object, through its centre of mass.
- Friction – the force that arises when two surfaces rub against each other.
- Drag – the resistive force on an object travelling through a fluid (e.g. water or air).
- Tension – the force within a stretched cable or rope.
- Up-thrust – the upward buoyancy force acting on an object when it is in a fluid.
- Normal Contact Force – the force arising when an object rests against another object acting at a 90° angle to the plane of contact.
What is the formula for weight?
W = mg, where m is the mass of the object, and g is the acceleration due
to gravity.
How is drag linked to speed?
Drag is proportional to speed².
What is the formula and SI unit for moments?
Moment = fx - SI unit = kgm⁻s⁻²
What is a couple?
A couple is a pair of forces, that have equal magnitude and opposite direction, which are applied to a body in parallel with each other and along different lines.
What are the conditions for a body to be in equilibrium?
The net force acting on it is 0, and its net moment is also 0.
What is the principle of moments?
The principle of moments is the requirement for the net moment to be 0 for a body to be in equilibrium).
What is meant by the ‘centre of mass’ of an object?
The centre of mass is the point through which the application of an external force produces only motion in a straight line, with no rotation.
What is the SI unit for density?
kgm⁻³
Explain how the formula for kinetic energy can be derived from the equations of motion.
Explain how the formula for GPE can be derived.
What does Hooke’s law state?
Hooke’s law states that for a material within its elastic limit, the force applied is directly proportional to the extension of the material.
In other words, once the elastic limit of the material is reached, Hooke’s law is no longer obeyed and the material will not return to its original shape.
Describe this Force-extension graph for a spring.
Up until point A, the force acting on the spring is proportional to the extension. The spring shows elastic deformation, meaning that it will return to its original shape when the force is removed. The gradient of the line here is equal to the force constant.
Following point A, Hooke’s law is no longer obeyed. The spring shows plastic deformation, meaning that when the force is removed, the spring will experience permanent deformation and will not return to its original length.
Can you describe this stress-strain graph for a mild steel wire.
On a stress-strain graph for a wire, P is the limit of proportionality. Up until this point, Hooke’s law is obeyed. E is the elastic limit, and beyond this point the wire will experience plastic deformation. Y1 and Y2 are yield points where there is rapid extension. UTS is the ultimate tensile strength, the maximum breaking stress that can be applied to the wire. Strong materials have a high UTS.
Describe how the equation F = ma can be derived from Newton’s second law.
Only true where the mass of the object remains constant during the motion of the object.
Explain the purpose of using impulse.
The forces acting on a body may vary over time, so we can use impulse to analyse this motion. It is a measure of change in momentum, so we can use Newton’s second law to derive it. The impulse of a force is defined as the product of the force and the time for which it acts. The area under a force-time graph is the equal to the impulse over that time
duration, and is also equal to the change in momentum.
What does the the Principle of conservation of momentum state?
The principle of conservation of momentum states that for a system of interacting objects, the total momentum in a specified direction remains constant, as long as no external forces act on the system.
