Section 4: Mechanics Flashcards
What is a scalar quantity?
Only has magnitude. No direction.
What is a vector quantity?
Has a magnitude and direction.
What are some examples of scalar quantities?
length/distance, speed, mass, temperature, time, energy
What are some examples of vector quantities?
displacement, velocity, force, acceleration, momentum
When you have the angle to the horizontal, how do you find the horizontal component of a vector?
horizontal component=
resultant vector x cos(angle)
When you have the angle to the horizontal, how do you find the vertical component of a vector?
Vertical component=
resultant vector x sin(angle)
What does it mean if an object is in equilibrium?
All the forces acting on the object are balanced and cancel each other out There is no resultant force. When only two forces act on the object, the object is in equilibrium if they are equal and opposite.
What are the two ways that you can solve equilibrium problems?
- Force vectors in a closed loop.
Forces acting on a object in equilibrium form a closed loop when you draw them tip-to-tail. This is called a vector triangle. - Resolving forces in two perpendicular directions.
If an object is in equilibrium, the sum of the components in each direction must be equal to zero.
What is a moment of a force defined as?
Moment of a force= Force (size of force in Newtons) x distance (perpendicular distance between the line of action of the force and the turning point in metres)
What does the principle of moments state?
For a body to be in equilibrium, the sum of the clockwise moments about any point equals the sum of the anti-clockwise moments about the same point. If the moments aren’t balanced, the object will turn.
What is a couple?
A couple is a pair of forces of equal size, which act parallel to each other, but in opposite directions. A couple doesn’t cause any resultant linear force, but does produce a turning effect.
How do you find the moment of a couple?
moment of couple in Nm = size of the force in N x perpendicular distance between the lines of the action of the forces in m
What does it mean by centre of mass?
The centre of mass of an object is the single point that you can consider its whole weight to act through (whatever the orientation). The object will always balance around this point.
How can you find the centre of mass of an object?
- Hang the object freely from a point (e.g. one corner)
- Draw a vertical line downwards from the point of suspension - use a plumb bob to get your line exactly vertical.
- Hang the object from a different point, and draw another vertical line.
- The centre of mass is where the two lines cross.
In terms of centre of mass and line of action of weight, when will an object experience a moment?
An object will topple over of the line of action of its weight (drawn down from the centre of mass) falls outside its base area. This is because a resultant moment occurs, which provides a turning point.
