Module 3.2 - Forces in Action Flashcards
(28 cards)
What is meant by the term ‘resultant force’?
Single force which has the same effect as the sum of all the forces acting on a body
What is meant by the term a ‘newton’?
The force that causes a mass of one kilogram to have an acceleration of one metre per second squared
What does it mean when an object is in equilibrium?
All forces acting on them in the same place (coplanar forces) are balanced - there’s zero net or resultant force. The object is either stationary or travelling at a constant velocity
What is meant by the term ‘moment of a force’/’turning moment’?
The product of a force and the perpendicular distance of its line of action from the point or axis
What is meant by the term ‘torque’?
The turning moment due to a couple is the product of one of the forces and the perpendicular distance between them. Units: Nm
What is meant by the term ‘centre of mass’?
The single point at which all the mass of an object can be assumed to be situated. For a symmetrical body of constant density, this will be the centre of the object
What is meant by the term ‘density’?
Mass per unit volume. SI unit: kgm^-3
What is meant by the term ‘Archimedes’ principle’?
States that the upward buoyant force (upthrust) exerted on an object immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the object displaces
What is Newton’s first law?
When the resultant force of an object is zero, the object remains stationary/at a constant velocity
What is Newton’s second law?
F=ma
What is weight and how is it calculated?
-force causing falling objects to accelerate due to gravity
-weight(N) = mass(kg) x acceleration of free fall(ms^-2)
W=mg
What are the four fundamental forces of nature?
- gravitational force
- electromagnetic force (holds atoms and molecules together)
- weak interaction (responsible for radioactive decay)
- strong interaction (holds subatomic particles together in the nucleus of atoms - is the strongest)
What causes tension and what affect can this have on an object?
- experienced by any rope, string, cable or wire being pulled, hung, rotated or supported
- can cause objects to accelerate or change shape
What is the normal contact force?
- reaction force
- perpendicular to the point of contact of the body
What is upthrust?
- upward force a liquid or gas exerts on a body floating in it due to the water displaced
- a body falling through a liquid will experience both drag and upthrust
What is friction?
-acts between 2 surfaces in contact with one another
-caused by interatomic and intermolecular forces when surfaces are in contact
friction opposes motion at the point of contact
What should a free body diagram contain?
- body on which the forces act (often shown as a point)
- direction of application of each force
- type of each force
- size of each force
Is friction dependent on velocity?
No
Is drag dependent on velocity?
Yes -frictional force of drag when passing through a fluid does depend on velocity
What is the equation for the force of drag and what equation derives from it?
Fd = 0.5ρCdAv^2
force of drag(N) = 0.5 x density of fluid(kgm^-3) x coefficient of drag x cross sectional area of the moving object(m^2) x velocity of moving object^2(ms^-1)
-simplified to Fd =kv^2 showing F is directly proportional to v^2
How does a free falling object reach terminal velocity?
- initially accelerates as gravitational force is greater than resistive forces
- drag increases as it accelerates
- eventually drag = weight so resultant force = 0N so it falls at constant velocity (terminal velocity)
(EX) Describe an experiment to measure terminal velocity of a falling body through a liquid.
- Use a sphere of known mass & let it fall through fluid e.g. wallpaper paste, heavy oil or liquid detergent
- Carefully drop a ball bearing into liquid & start timer. At given points use elastic bonds to mark vertical position of the ball
- When ball reaches the bottom of the tube, measure the distance between each consecutive pair of elastic bands
- Magnet has to be used to take out the ball bearing to repeat
- For each time period, measure distance between consecutive elastic bands, record the time period for each & use this to calculate average velocity, v, of the ball
- When you have several measurements, calculate the mean distance travelled for each time period & use this to calculate velocity, v, for each time period
- Plot graph of v (y axis) against cumulative time from release of ball, t (x axis). Draw a smooth curve through the points. Identify time ball reaches terminal velocity & use graph to determine ‘best’ value for terminal velocity
What is the equation for the turning moment?
turning moment (Nm) = magnitude of force (N) x perpendicular distance from a fixed point (m)
What is the principle of moments?
For an object to be in rotational equilibrium, sum of clockwise moments = sum of anticlockwise moments
