Physics - Recall for Chapter 4; Forces in Action
Recall Questions
Define 1N?
The force required to give a mass of $1kg$ an acceleration of $1ms^{-2}$.
What is a resultant (net) force?
The vector sum of all the forces acting on an object.
What is the relationship between resultant force and acceleration, assuming mass remains constant?
Describe the motion of an object with a resultant force of $0N$ acting on it?
Zero acceleration (therefore stationary or constant velocity).
What direction does friction/drag act in?
The opposite direction to the direction of the motion causing it.
Define centre of mass?
A point at which any externally applied force produces a straight-line (translational) motion without rotation.
Define centre of gravity?
A point at which the entire weight of an object appears to act.
How can we find the centre of gravity of an object (long)?
Suspend the object freely from a point. Use a plumbline to draw a straight vertical line on the object. Repeat using different suspension points. The centre of gravity is located where the lines intersect.
Name seven forces?
Weight, thrust, friction, tension, drag, normal contact force, upthrust.
In which direction does the normal contact force act?
Perpendicular to the surface the object is in contact with.
What is a free-body diagram?
A diagram showing all forces (or the resultant force) acting on an object.
State three factors that affect the size of the drag force acting on an object?
The speed of the object, the cross-sectional area of the object perpindicular to the dirction of travel and the miscosity of the fluid the object is moving through.
Describe the motion of an object falling through a fluid at terminal velocity?
Constant velocity, as acceleration is $0 ms^{-2}$.
Explain why an object falling through a fluid reaches a terminal velocity (long)?
Drag forces increase with speed until they are equal to the weight of the object but acting in the oppoiste dirction. The resultant force is therefore $0N$, so the object no longer accelerates.
Define the moment of a force?
The product of the force applied and hte perpendicular distance of the line of action of the force to the pivot.
State two conditions needed for an object to be in equilibrium?
The net (resultant) force acting on the object must be zero and the net (resultant) moment on the object must be zero.
Define a couple?
A pair of forces that together cause a net (resultant) moment, but no net (resultant) force.
State the conditions needed for a pair of forces acting on the same object to be a couple?
The two forces must be the same size but acting in opposite directions.
Define the torque of a couple?
The product of the magnitude of one of the forces multiplied by the perpindicular distance between the forces ($T = Fd$).
What are coplanar forces?
Forces acting in the same plane.
What does it mean if the forces acting on an object form a closed triangle when a vector diagram is drawn?
The object is in equilibrium (net force is $0N$).
What is meant by “resolving” forces?
Finding two components of a force that are at right angles to each other (usually the vertical and horizontal components).
Define density?
The mass per unit volume of a substance.,
Describe how the density of an irregularly shaped object can be determined (long)?
Measure the mass of the object using scales. Fully submerge the object in water. The volume of water displaced is equal to the volume of the object. Calculate density using $\rho = \frac{m}{V}$.
Define pressure?
Pressure is the normal force exerted on a surface per unit cross-sectional area.
State the SI unit for pressure?
$Pa$ or $Nm^{-2}$.
State what is meant by a fluid?
A liquid or gas.
Explain why an object submerged in a fluid experiences upthrust?
The pressure on the bottom surface of the object is greater than the pressure at the top, causing a net force upwards.
State Archimede’s principle?
The upthrust exerted on a body immersed in a fluid, whether partially or fully submerged, is equal to the weight of the fluid that the body displaces.
State the relationship between the pressure exerted by a colymn of fluid and the height of the column of fluid?
Pressure is directly proportional to height and independent of cross-sectional area ($p = mhg$), assuming the density of the fluid remains constant.