A Level Physics Revision: All of Forces | Moments, Torque, Centre of Gravity, Archimedes Principle

Newton's Second Law

• Newton's Second Law states that the net force is equal to the mass times the acceleration for a constant mass.

• The equation can be written as F = ma, where F is the net force, m is the mass, and a is the acceleration.

• The net force can be positive, negative, or zero depending on the forces acting on the object.

• A free body diagram is a visual representation of the forces acting on an object.

"Newton's Second Law says that the net force is equal to the mass times the acceleration for a constant mass."

Resolving forces

• Resolving forces allows us to determine the net force acting on an object.

• In this case, we are considering a force moving upwards, which we take as positive, and a force moving downwards, which we take as negative.

• The net force is equal to the difference between the upward force and the downward force, which can also be expressed as the product of mass and acceleration.

"The net force is equal to the tension minus the weight."

Drag force and terminal velocity

• Drag force is the resistance force experienced by an object moving through a fluid.

• The drag force increases as the velocity of the object increases and is proportional to the velocity squared.

• Eventually, the drag force and the weight of the object become equal, resulting in the object reaching terminal velocity where the net force is zero.

"As the object is going down, the drag force is increasing and it increases quite rapidly because drag force is proportional to the velocity squared."

Investigation of motion through a liquid

• To investigate the motion of an object falling through a liquid, such as a ball bearing in a tube filled with viscous fluid, consecutive intervals are marked on the tube.

• The time it takes for the ball bearing to reach each interval is recorded.

• Multiple readings are taken and the mean value is calculated.

• A graph of speed against time can be plotted, showing the decrease in acceleration and the attainment of terminal velocity.

"For the investigation, we need to mark consecutive equal intervals on the tube and record the time it takes for the ball bearing to reach each interval."

Forces on an inclined plane

• When an object is on an inclined plane, the weight of the object can be split into two components: one parallel to the plane and one perpendicular to the plane.

• The component parallel to the plane can be found using the sine of the angle of inclination, while the component perpendicular to the plane can be found using the cosine of the angle.

"The component which is parallel to the plane is mg sine theta, and the component which is perpendicular to the plane is mg cosine theta."

Component which is just

• The weight of an object can be split into parallel and perpendicular components.

• The parallel component is equal to the weight times the sine of the angle.

• The perpendicular component is equal to the weight times the cosine of the angle.

"In this scenario, the component which is parallel to the plate will be the weight times the sign of the angle, and the component which is perpendicular to the plane will be the weight times the cosine of the angle."

Center of gravity

• The center of gravity is the point at which the entire weight of an object seems to act.

• The center of gravity can be the geometric center of an object if it has an even mass distribution.

• For irregularly shaped objects, like a donut, the center of gravity does not necessarily coincide with the physical center.

• Determining the center of gravity can be done practically by suspending an object and observing where plumb lines intersect.

"The center of gravity is a point where the entire weight of the object seems to act, and it can be the geometric center or somewhere else depending on the object's shape. The center of gravity can be determined practically by suspending an object and finding the intersection of plumb lines."

Moments

• The moment of force is defined as the product of the force and its perpendicular distance from the pivot point.

• A moment can be either clockwise or counterclockwise, depending on the direction of the force and the resulting rotation.

• The moment of force can be calculated using the equation m = F * d, where m is the moment, F is the force, and d is the perpendicular distance.

"The moment of force is defined as the product of the force and its perpendicular distance from the pivot point. A moment can be either clockwise or counterclockwise. The equation for moment is m = F * d."

Principle of moments

• An object is in equilibrium if the sum of the anti-clockwise moments is equal to the sum of the clockwise moments.

• This principle can be applied to determine if an object will remain balanced or not.

• The sum of the anti-clockwise moments is calculated by multiplying the force by the perpendicular distance for each anti-clockwise moment.

• The sum of the clockwise moments is calculated in the same way for each clockwise moment.

"An object is in equilibrium when the sum of the anti-clockwise moments equals the sum of the clockwise moments. This principle helps determine if an object will stay balanced or not."

Resolving forces

• If f2 times x2 is equal to fx, then we should be consistent and give them subscripts. Let's call them f1 and x1.

• The sum of anticlockwise moments is equal to the sum of clockwise moments, which is the principle of moments.

• In an exam, the principle of moments can also be referred to as the absence of net moment.

"The sum of anticlockwise moments is equal to the sum of clockwise moments."

Force couples and torque

• A force couple consists of two equal and opposite forces acting in the same plane.

• Torque is defined as the net moment, which is equal to the magnitude of one of the forces multiplied by the distance between them.

"A force couple consists of two equal and opposite forces."

Triangle of forces

• The triangle of forces is a technique used when an object is in equilibrium.

• We start with one force (typically the weight) and draw the normal reaction and the force acting upwards.

• These forces form a 90-degree triangle, where the angles between the weight and the normal reaction are the same.

"The triangle of forces is used when an object is in equilibrium."

Density and pressure

• Density is defined as the mass per unit volume, with units of kilograms per meter cubed.

• Pressure is defined as the amount of force per unit area, with units of newtons per meter squared.

"Density is defined as the mass per unit volume."

Pressure inside a fluid

• The pressure at a height h inside a fluid is equal to the density of the fluid multiplied by the height multiplied by the gravitational acceleration.

"The pressure at a height h inside a fluid is equal to the density of the fluid multiplied by the height multiplied by the gravitational acceleration."

Archimedes principle

• Archimedes principle states that the upthrust acting on a submerged body is equal to the weight of the liquid that has been displaced.

• The upthrust can be greater than, equal to, or smaller than the weight depending on the density of the object and the fluid it is submerged in.

• The upthrust is calculated by multiplying the density of the fluid by the surface area of the object and the height submerged, and then multiplying by the gravitational acceleration> "Archimedes principle states that the upthrust is equal to the weight of liquid that has been displaced."