3.2 Forces in Action Flashcards
What is a force? (2)
An interaction between two objects involving a push or pull
A vector with direction in space
What are the four fundamental forces? (4)
Gravitational
Electromagnetic
Strong nuclear
Weak nuclear
What force acts on an object in water, allowing it to float?
Upthrust
What force acts on an object when it is pulled by a string?
Tension
What force acts between a surface and an object when they are in contact with each other?
Normal contact force
What is friction?
Particles colliding with a moving object, causing kinetic energy to be lost as thermal energy
What is a free body diagram?
A diagram using a dot to represent an object and arrows to scale to show the direction and magnitude of the forces acting on it
What is a plumb line?
A mass suspended by a length of string, allowing straight, vertical lines to be drawn
How is the centre of mass of a 2D object found? (4)
Choose a point on the object and allow it to swing freely from a pin before coming to rest
Using a plumb line, draw a vertical line through the pin
Repeat this twice more, creating a point where all three lines cross
The point where they cross is the centre of mass
What is drag?
The frictional force experienced by an object travelling through a fluid
What factors affect drag? (5)
Speed of the object Cross-sectional area Shape of the object Roughness/texture of the object Density of the fluid that is travelled through
How is drag force and speed linked?
Drag force is proportional to speed^2
What are the forces on an object in the instant that it falls? (2)
No drag
Weight (acceleration = g)
What happens as a falling object’s speed increases? (2)
Drag force increases
Instantaneous acceleration decreases (less than g)
What happens when an object reaches terminal velocity? (2)
Drag is equal and opposite to weight
Acceleration is zero so speed is constant
What is the moment of a force?
The turning effect of a force about a pivot
Moment = force x perpendicular distance from pivot
What is a couple?
A pair of equal but opposite forces acting parallel and on different lines
They cause a turning effect but no translational movement
What is the torque of a couple? (2)
The moment of a couple
Torque = one of the forces x perpendicular distance between forces
What is the principle of moments? (2)
When a body is in equilibrium, the net force acting on it is zero and its net moment is zero
The sum of clockwise moments = the sum of anti-clockwise moments
What is the centre of gravity?
The point at which the entire weight of the object can be said to act
What is the difference between a torque and a moment?
A moment can have a non-zero resultant force, thereby causing movement whereas a torque always has a resultant force of zero
What is density? (2)
Mass per unit volume
Density = mass / volume
What is pressure? (2)
Force per unit area (acting perpendicular to the surface)
Pressure = force / area
What is the difference between solid and liquid pressure? (2)
Solid pressure - the smaller the area, the higher the pressure
Liquid pressure - in a fluid at rest, pressure at any point acts equally in all directions and increases with depth
What is Archimedes’ Principle?
The upthrust that a floating or submerged object feels is equal to the weight of displaced fluid
How is the upthrust on an object in a fluid calculated?
Pressure = height of liquid above x density of liquid x acceleration due to gravity
How is the density of an object calculated? (2)
Use a Eureka can - volume of water displaced = volume of object
Use a balance to find the mass of the object
