3.2 - Forces in Action Flashcards
define resultant force
is a single force which has the same effect as the sum of all the forces acting on a body
what is the formula for resultant (or net) force? and what law is this?
f = ma
where f = resultant force
m = mass of object
a = acceleration
form of Newton’s second law when mass is constant
when can you use f = ma?
when mass is constant (e.g not a rocket)
what is the unit of force and define it?
newtons, N
1 newton is the force required to accelerate a mass of 1kg by 1ms^-2 in the direction of the force
if the forces on an object are balanced the object is said to be in…?
equilibrium
no resultant force, no acceleration
what is the formula for weight force?
w = mg
where w = weight force
m = mass of object
g = acceleration due to gravity
define tension
is the force experienced by any rope, string, cable or wire that is being pulled, hung, rotated or supported
define normal contact force (or reaction force)
is the force that acts perpendicular (at right angles) to the point of contact of a body and the surface with which it is in contact
define upthrust
the upwards force that a liquid or gas exerts on a body floating in it due to the fluid displaced
define friction
the force of friction occurs between two surfaces in contact with one another and resists the motion at the point of contact
what are free body force diagrams and what should they contain?
simplified labelled drawing where the object is modelled as a point, it should contain…
the body on which the forces act
the direction of application of each force
the type of each force
the size of each force
If you drop two objects of different masses, which will hit the ground first (ignoring air resistance)?
Both hit the ground at the same time because acceleration is independent of mass; objects fall at the same rate.
What is drag?
The frictional force experienced by an object travelling through a fluid (can be liquid or gas).
What factors affect drag for an object travelling through air?
Cross sectional area of the object and the density of the fluid.
What is the relationship between drag caused by air resistance and the cross sectional surface area of the object?
Drag ∝ cross sectional area (directly proportional).
What is the relationship between drag caused by air resistance and velocity of the object?
Drag ∝ velocity^2 (directly proportional to the square of velocity).
What is terminal velocity?
The velocity at the point during freefall when the object stops accelerating as the weight force is balanced by the air resistance (becomes equal); object falls at constant velocity.
Outline an investigation to determine the terminal velocity in fluids.
Set up a clamped cylinder containing a viscous liquid (e.g., glycerol) and place elastic bands at fixed distances down the tube. Drop a steel ball bearing into the tube at the top and use a timer to record the time at which the ball reaches each band. Repeat to reduce error and record results in a table. Measure the distances between each consecutive band and use average times to calculate average velocities between each pair of elastic bands. Plot a graph with v on the y-axis against cumulative time t on the x-axis, draw a smooth curve, and identify the time at which the ball reaches its terminal velocity (lines become constant/flat).
What does it mean for an object to be in equilibrium?
Objects are in equilibrium when all the forces acting on them in the same plane (coplanar forces) are balanced; there is zero net or resultant force. In terms of motion, the object is either stationary or is travelling at constant velocity.
What is a moment?
A turning effect of a force.
What is the definition/equation of a moment?
The moment of force (or turning moment) is the product of a force and the perpendicular distance of its line of action or axis. Moment = force x perpendicular distance from pivot. Moment = F x d.
What is the principle of moments?
The principle of moments states that for an object to be in rotational equilibrium, the sum of the clockwise moments must be equal to the sum of the anticlockwise moments (moments must be balanced or the object will turn).
What is a couple?
A couple occurs when two equal, anti-parallel forces act to produce a rotation (no linear motion occurs).
What is the definition/equation of a torque of a couple?
The turning moment (torque) due to a couple is the product of one of the forces and the perpendicular distance between them. Torque of couple = size of one of the forces x perpendicular distance between them. T = F x d.
What are examples of objects that experience moments?
Muscles, bones, and joints act as levers; an effort force acts against a load force by means of a rigid object rotating around a pivot.
Define the centre of mass.
The centre of mass of an object is the single point at which all of the mass of the object can be assumed to be situated.
Define the centre of gravity.
The centre of gravity of an object is the single point through which the entire weight of the object can be thought to act.
For a symmetrical body of constant density, where is the centre of mass?
At the centre of the body.
Outline an investigation to determine the centre of mass or centre of gravity of an object.
Make three holes in random places on the edge of the shape. Hang the shape from the first hole by placing through a pin, let it swing until it comes to rest. Use a plumb line to draw a vertical line down from the point of suspension to the bottom of the shape. Repeat these steps but with the different holes. Where the lines intersect is the centre of mass.
What does a wide base and a low centre of mass mean for an object?
It has more stability.
How does an object become unstable?
If an object is tilted so that the centre of mass lies outside the width of the base of the object, it will become unstable and will topple and fall.
What is neutral stability and give an example?
Neutral stability is when the centre of mass can never fall outside the width of the base of the object (that is touching the ground). For example, objects that can roll like a football.
What does resolving a force mean?
Splitting it into its horizontal and vertical components using trigonometry.
What does coplanar forces mean?
Forces that act in the same plane.
What does a closed loop mean in vector triangles?
There is no resultant force; acceleration is zero.
What does a velocity-time graph look like for terminal velocity?
Line starts increasing with high gradient and slowly flattens out (curves to gradient of zero).
What does a velocity-time graph look like for terminal velocity of a skydiver with a parachute being opened?
Line starts increasing with high gradient and slowly flattens out (curves to gradient of zero); then once parachute opened, steep drop to lower terminal velocity, continues straight with zero gradient.
What is the formula for density?
ρ = m / v (the mass per unit volume).
What is the symbol of density?
ρ, rho.
How do you calculate the density of an irregular shape?
Measure the object’s mass using scales, then calculate the volume by putting the object in a cylinder of water of known volume (cm^3) and work out the volume of water displaced (this will be equal to the volume of the object).
What is Archimedes’ principle?
Archimedes’ principle states that the upward buoyant force (upthrust) exerted on an object immersed in a fluid is equal to the weight of the fluid the object displaces.
What is the formula for pressure exerted by a fluid at any depth?
p = ρhg, where p = pressure, ρ = density of fluid, h = depth of fluid, g = acceleration due to gravity.
How do you derive p = ρhg?
Density = m / v; therefore m = ρ × v. For a cylinder/column shape, volume is equal to h × A (where h is the height and A is the cross-sectional area); therefore m = ρhA where m is the mass of the fluid. The weight of this mass is equal to mg, so weight force = ρhAg. Pressure = force / area, so ρhAg / A gives you pressure = ρhg.
What happens when a force is applied through the centre of mass of a body?
The body will accelerate in the direction of the applied force in a straight line with no rotation.
What is the formula for pressure for solids, liquids, and gases?
p = F / A.
