P5 Resultant Forces and Work Done (page 203) Flashcards
What do Free Body Diagrams show?
All the forces acting on an object.
(you need to be able to describe all the forces acting on an isolated object or a system (p.167) - i.e every force acting on the object or system but none of the forces the object or system exerts on the rest of the world).
Explain the forces acting on a skydiver?
the sky divers’s weight acts on him pulling him towards the ground and drag (air resistance) also acts on him, in the opposite direction to his motion.
How can you show a skydivers act of forces?
by using a free body diagram (look at example on page 203 diagram 1.
On the free body diagram, on page 203, diagram 1, what do the sizes of the arrows show?
the relative magnitudes of the forces and the directions show the directions of the forces acting on the object.
What is a Resultant Force?
it is the Overall Force on a Point or Object.
(in most real situations there are at least two forces acting on an object along any direction).
If you have a number of forces acting at a single point, you can replace them with what?
a single force (so long as the single force has the same effect as the original forces together).
This single force is called The Resultant Force. (e.g. there’s a downward resultant force acting on the skydiver above).
If the forces all act along the same line (they’re all parallel), how is the overall effect found?
by adding those going in the same direction and subtracting any going in the opposite direction.
For the free body force diagram given, calculate the resultant force acting on the van? (see diagram 2 on page 203)
1) Consider the horizontal and verticle directions separately
2) State the size and direction of the resultant force
Vertical: 1500 - 1500 = O N
Horizontal: 1200 - 1000 N = 200 N
The resultant force is 200 N to the left
If a Resultant Force moves an object, what is this called?
Work is Done
When a force moves an object through a distance, Energy is transferred and work is done on the object).
To make something move (or keep it moving if there are frictional forces), what must be applied?
a force must be applied.
The thing applying the force needs a source of energy, give examples?
energy like fuel or food.
When the force does ‘work’ to move the object what happens?
energy is transferred from one store to another (see p.167).
(whether energy is transferred ‘usefully’ e.g. lifting a load, or is ‘wasted’ (p.170) you can still say that ‘work is done’. Just like Batman and Bruce Wayne, ‘work done’ and energy transferred’ are the same).
When you push something along a rough surface (like a carpet) you are doing work against frictional forces, explain why?
Energy is being transferred to the kinetic energy store of the object because it starts moving, but some is also being transferred to the thermal energy stores due to the friction. This causes the overall temperature of the object to increase (like rubbing your hands together to warm them up).
You can find out how much work has been done using what symbols?
W = Fs
(W - Work done (J))
F - Force (N)
s - Distance (moved along the line of action of the force) (m).
One Joule of work is done when……
a force of one newton causes an object to move a distance of one meter.
