4 - Forces in action Flashcards
What is mass?
A physical property of an object which describes how much matter it contains.
Define net force.
The sum of all of the forces acting on an object.
What does a net force cause?
Acceleration in the direction of the force.
What is weight?
The gravitational force acting on an object.
What is an object’s centre of mass?
A point in an object through which its entire weight can be thought to act.
How would you carry out an experiment to determine an object’s centre of mass?
- Hang object - free to spin
- Hang plumb line and mark line
- Repeat in two other positions
- Point where lines cross is centre of mass.
What is each force represented by in a free body diagram?
A labelled arrow, proportional to the size of the force.
What is friction?
The force that arises when two surfaces rub against each other.
What is drag?
The resistive force acting on an object that is travelling through a fluid.
What is tension?
The force within a stretched cable or rope.
What is upthrust?
An upwards buoyancy force acting on a liquid.
What is normal contact force?
A force arising when one object rests against another object.
What is drag proportional to?
velocity squared x surface area
What is the drag force experienced by objects travelling through air commonly called?
Air resistance
Define terminal velocity.
When an object stops accelerating and has a constant velocity because the drag is equal and opposite to its weight.
Describe how a falling object reaches terminal velocity.
- At release, acceleration is equal to g, downward force = mg, net force = mg
- As object accelerates, velocity increases, downward force = mg, upwards force = D, net force = mg - D
- Eventually, drag becomes equal to weight and so net force = 0, drag cannot increase because speed is no longer increasing
What is the only factor that can affect an objects terminal velocity when it is already travelling at terminal velocity?
Surface area
e.g. parachute
What is a moment?
The turning effect of a force around a point.
How do you calculate a moment?
Force x distance from pivot
What must you always ensure (and sometimes have to calculate) before calculating a moment?
That the force and the distance are perpendicular.
What is the principle of moments?
When a body is at rotational equilibrium, the sum of the clockwise moments must be equal to the sum of the anticlockwise moments.
What is a couple?
A pair of equal, parallel forces acting in opposite directions.
What is torque?
The moment of a couple.
How do you calculate torque?
One of the forces x distance between forces.
