Module 3.2 - Forces In Action ✓ Flashcards
What is a free body force diagram?
A diagram showing all the forces acting on a single body
Name some common forces acting on objects?
- Weight
- Normal contact force
- Friction
- Tension
- Driving Force
Define weight?
Weight is the force acting on a mass due to a gravitational field
What is the ‘centre of mass’ of an object?
The centre of mass of an object is the single point you can consider all of its weight to act through
What affects how stable an object is?
The height above it’s base of the centre of mass. And the size of the base area.
Higher centre of mass, less stable.
Smaller base area, less stable.
How, by knowing an objects centre of mass can you tell if it will topple?
An object will topple if a vertical line drawn downwards from it’s centre of mass falls outside it’s base area.
What are the two different types of friction?
- Contact Friction (between solid surfaces)
- Fluid Friction/Drag (acts in a liquid/gas)
What is terminal velocity?
Terminal Velocity - The highest velocity attainable by an object, occurring when the sum of frictional forces acting on it are equal to the sum of driving forces acting on it
What two requirements are there for an object to reach a terminal; velocity?
1) A driving force that stays the same all the time
2) A frictional/drag force that increases with speed
Describe and explain the motion of an object falling from rest through the air.
Stage 1:
The object has no resistive forces acting on (as it is stationary) it and a constant weight providing it with a constant acceleration downwards.
Stage 2:
As the object’s speed increases, the drag force acting in the opposite direction will increase proportionally. This will result in a decreasing resultant force and so a decreasing acceleration.
Stage 3:
When the drag force acting on the object is equal to its weight the object will have reached its terminal velocity, and will continue to fall at constant speed.
Explain the motion of a parachutist after they have opened their parachute.
When the parachutist opens his parachute, the air resistance will increase to be greater than his weight. This will decelerate the parachutist until the air resistance has reduced to be equal to his weight again, giving him a new terminal velocity.
What is density?
Density is the relationship between the mass of an object and its volume (a measure of how compact an object is)
How can you tell if an object will float on a fluid?
An object will float on a fluid if it has a lower density than that fluid. (Same applies for fluids on fluids)
Define pressure?
Pressure is the force applied per unit area. It is measured in pascals (Pa) > N/m^2
What is the equation for pressure in a liquid?
pressure = depth * density * gravitational field strength
p = hroeg
Why do objects experience an upthrust in fluids?
Upthrust is caused by the difference in pressure acting on the top and bottom of a submerged object
(p = hpg) different pressures top and bottom so different forces acting on top/bottom
Define upthrust
Upthrust is the upward force that a fluid exerts on a completely or partially submerged object
State Archimedes’s principle
Archimedes’ principle - When a body is completely or partially submerged in water it experiences an upthrust equivalent to the weight of the fluid it has displaced
How do you calculate moment?
Moment = Force * perpendicular distance form pivot to line of action of the force
State the principle of moments
Principle of moments - for a body to be in equilibrium, the sum of clockwise moments about any point equals the sum of anticlockwise moments about that point
Lever terminology XD
In a lever, an effort force acts against a load force using a rigid object rotting about a pivot
What is a couple?
A couple is a pair of forces of equal size acting parallel to each other but in opposite directions. A couple doesn’t cause any resultant linear force
What is torque?
Torque is the resultant turning force produced by a couple (It is a moment but we call it torque)
How do you calculate the torque of a couple?
Torque of couple = size of one of the forces * perpendicular distance BETWEEN FORCES (NOT from one force to pivot)
T = F*d
What is the unit for moment (and torque)?
Nm
What does it mean for an object to be in equilibrium?
An object in equilibrium has no net force or moment acting on it
What is interia?
Inertia is the tendency of an object to remain at the same velocity (Its resistance to a change in motion)
What three things does fluid friction depend on?
- The thickness of the fluid (viscosity)
- The speed of the object
- The shape/size of the object
How do you calculate the upthrust acting on a submerged object?
Calculate the force acting on the top and bottom of the object due to the pressures, the upthrust is the resultant force
