3.2 Forces in Action Flashcards
If you apply a net force on an object, what will happen?
The object will accelerate in the direction of the net force applied.
What equation relates force, mass and acceleration?
Force (N) = Mass (kg) x Acceleration (ms-2)
What does 1 Newton give?
1 Newton gives 1kg of mass an acceleration of 1ms-2 in the direction of the force.
What is the difference between weight and mass?
Weight is a gravitational force acting on an object - varies with location - and mass is the amount of matter of an object - constant.
How is the weight of an object calculated?
Weight (N) = Mass (kg) x Gravitational force
What device can be used to determine the weight of an object?
A newtonmetre.
What is friction?
The force that arises when two surfaces rub against each other.
What is drag?
The resistive force on an object travelling through fluid (liquid/gas).
What is tension?
The force within a stretched cable or rope.
What is upthrust?
An upward buoyancy force acting on an object when it is in fluid (liquid/gas).
What is a normal contact force?
A force arising when one object rests against another object.
How are forces represented in a free body diagram?
Each force is represented by an arrow labelled with the force, they’re drawn to scale (longer arrow = greater force).
How do you find the vertical component of a force?
mgsinθ
How do you find the horizontal component of a force?
mgcosθ
What is a fluid?
A fluid is a liquid or gas substance.
What is a drag force?
The frictional force that opposes motion of an object.
What is the magnitude of drag dependent on?
- Speed of an object
- Object’s cross-sectional area
How is drag related to speed?
Drag is directly proportional to speed².
Describe how forces of weight and drag change when an object undergoes a vertical fall.
- As the object starts to fall, there is no drag force and acceleration is g.
- As it falls, the speed increases and the drag force increases, resultant force decreases and the acceleration becomes less than g.
- The object will reach terminal velocity, so drag and weight are equal and opposite, speed is constant and the acceleration is 0.
What is the magnitude of drag at the instant an object begins to fall?
Zero.
What is terminal velocity?
Terminal velocity is the constant speed reached when the drag force and weight are equal and opposite on an object.
What is an experiment conducted to investigate the motion of an object falling through a fluid?
- A falling object is attached to a pulley and is dropped through a cylinder of liquid, pulling a light polystyrene ball upwards on the other side of the pulley.
- The motion of this ball is identical to that of the object falling through the fluid.
- The exact values of data can be collected using data logger software. and a motion sensor.
What is the experiment to determine terminal velocity of a metal ball falling through fluid?
- Have two ball bearings made with the same material, but with two different radii.
- Draw marks on the cylinder that are a known distance apart.
- Start a stop watch and slow motion camera, and drop one of the bearings into a cylinder of glycerine.
- Do this multiple times and repeat with the second bearing, dropping it from the same height.
- When the time taken between markings becomes equal, it has reached terminal velocity.
What is centre of gravity?
An imaginary point at which the entire weight of an object appears to act.
What is centre of mass?
A point through which any externally applied forces produce straight line motion but no rotation.
What is the experiment used to determine the centre of gravity of a 2D object?
- Make holes along the edges of card (2D object) and insert pins firmly in them.
- Allow the object to swing freely and it will come to rest with the centre of gravity vertically below the pin.
- Use a plumb line to hang from the pin and draw a line across the string.
- Repeat for the other holes, and the point of intersection of the lines is the centre of gravity.
What is meant by equilibrium?
A body is in equilibrium when the net force and net movement acting on it are 0.
If an object is in equilibrium, what does this mean in terms of the horizontal and vertical forces acting on the object?
They must be equal so resultant force is 0.
What is meant by the moment of a force?
The moment of a force is the turning effect of a force about an axis or a point.
How is the moment of a force determined?
Moment (Nm) = Force (N) x Perpendicular Distance of the Line of Action of Force from the Point of Rotation (m)
What is the principle of moments?
For a body in rotational equilibrium, the sum of anticlockwise moments about any point is equal to the sum of the clockwise moments about that same point.
How can you make an object rotate without any translational motion?
Apply a pair of equal and opposite, parallel forces to the object, that are applied along different lines.
What is a couple?
A pair of equal and opposite forces acting on a body but not in the same straight line.
What is torque?
Torque is the moment of a couple.
How is torque calculated?
Torque (Nm) = Perpendicular Separation Between Forces (m) x One of the Forces (N)
How is density defined?
Density is the mass per unit volume of a substance.
How is density calculated?
Density (kgm-3) = Mass (kg)/Volume (m3)
How do you find density of a regular object?
- Mass = weigh using scales/balance.
- Volume = Measure with rule/micrometre/digital calliper.
- Density = m/v
How do you find the density of an irregular object?
- Mass = weigh using scales/balance.
- Volume = Displacement (putting object in water and measuring the volume before/after it is put in).
- Density = m/v
How do you find the density of a liquid?
Find the volume using a measuring cylinder, and weigh using scales or a balance for mass.
Density = m/v
How is pressure defined?
Pressure is the normal force exerted per unit cross-sectional area.
How is pressure calculated?
Pressure (Pa or Nm-2) = Force (N)/Area (m2)
Why do gases and liquids exert pressure on surfaces?
Fluids exert pressure on surfaces because of constant bombardment by their molecules.
What is the equation used to calculate the pressure exerted by a vertical column of liquid?
Pressure (Pa or Nm-2) = Height (m) x Density (kgm-3) x Acceleration by Free Fall (ms-2)
What does pressure of a fluid depend on
Pressure is directly proportional to height, so water pressure increases with depth and denser liquids exert greater pressure. Pressure of a fluid at a particular depth is the same in all directions.
Why do objects submerged in water experience an upwards force?
It happens due to pressure differences at its upper and lower surfaces that result in an upwards resultant force.
What is an expression for calculating upthrust?
Upthrust (N) = Volume of Liquid Displaced (m3) x Density of Liquid (kgm-3) x Acceleration due to Gravity (ms-2)
What is Archimedes’ principle?
The upthrust exerted on a body immersed in a fluid, whether fully or partially, is equal to the weight of the fluid the body displaces.
What condition is required for a body to sink?
The upthrust is less than the weight of the object.
What condition is required for a body to float.
The upthrust is equal to the weight of an object, and the weight must be equal to the weight of fluid displaced.
