P8 - Forces in Balance Flashcards
Vector
A physical quantity represented by an arrow with direction and magnitude.
(Remember: orange jumpsuit man)
Scalar
A physical quantity with only magnitude.
Examples of Vector quantities
force, velocity, displacement, acceleration, momentum.
difference between distance and displacement
Displacement is distance in a given direction.
- Displacement is a vector; displacement has a direction and magnitude.
- Distance is a scalar; distance only has a magnitude.
Force
A push or pull,
which acts on an object because of its interaction with another object.
Contact Force
A force that only acts on objects when the objects touch each other.
Non-Contact Force
A force that acts on objects, but the objects DO NOT touch each other.
Examples of Contact Forces
- Air resistance
- Tension
- Thrust
- Friction
Examples of Non-Contact Forces
- Electrostatic
- Magnetic
- Electromagnetic
- Gravitational
- Nuclear
Unit of Force
The Newton (N).
What can forces do on an object?
- Change the shape
- Change its motion
- Change its state of rest.
[True or False] Bikes travelling in different directions have different velocities.
True.
Why do bikes have different velocities if they are travelling at the same speed?
Because they are travelling in different directions; a vector involves direction and magnitude.
When two forces interact, they ___ exert ___ and ___ forces on ___ ___.
When two forces interact, they always exert equal and opposite forces on each other.
Newton’s Third Law
When two forces interact, the forces they exert on each other are equal and opposite.
Newton’s First Law of Motion
If the resultant force on an object is zero, the object stays at rest if it is stationary, or it keeps moving with the same velocity.
If resultant force on an object is equal to zero:
The object stays at rest / the same speed and direction.
If resultant force on an object > zero:
The speed or direction of the object will change.
If two forces act on an object along the same line in the same direction, the resultant force is …
… their sum.
If two forces act on an object along the same line in opposite directions, the resultant force is …
… their difference.
What does a free-body force diagram of an object show?
The forces acting on it.
What can resultant forces be?
Balanced or unbalanced.
[True or False] When the resultant force on an object is zero, it is stationary.
False.
It may also be travelling at the same velocity / the same constant speed in the same direction.
[True or False] Forces on a diagram should always be clearly labelled.
True.
Force diagrams usually show more than one force.
[Newton’s Third Law] If the forces are always equal, how does anything ever go anywhere?
The two forces are acting on different objects; the effects of these forces on each object depends on the masses of the objects.
Larger mass, smaller effect.
What is the driving force on a car?
The force that makes the car move.
This is sometimes called the engine force or the motive force.
How does the driving force push the car forward?
Because there is friction between the ground and the tyre.
[Friction in action] When the car moves forward:
- Force of the friction on the ROAD on the TYRE is in the forward direction.
- Force of friction of TYRE on the ROAD is in the reverse direction.
See kerboodle: pg101 Figure 3 - Driving Force diagram. (AQA Combined Science Physics)
When a force moves an object through a ___, ___ ___ ___ and ___ ___ ___ on the object.
When a force moves an object through a distance, ENERGY IS TRANSFERRED and WORK IS DONE on the object.
Centre of mass
The point where the mass of an object can be thought of as being concentrated.
Method to finding the centre of mass of an irregular polygon / irregularly-shaped object
- Hang your card from a clamp stand, along with a plumb bob.
- Draw a line along the plumb bob’s string.
- Repeat two or more times.
- The centre of mass will be located at the intersection of the lines.
When an object is freely suspended …
it comes to rest with its centre of mass directly underneath the point of suspension.
[True or False] The centre of mass of a symmetrical object is along the axis of symmetry.
True.
Stable Equilibrium
An object is said to be in stable equilibrium:
- If it returns to its original position when it is disturbed.
Unstable Equilibrium
An object is said to be in unstable equilibrium:
- If it falls or topples when it is disturbed.
Neutral Equilibrium
An object is said to be in neutral equilibrium:
- If it remains in its new position when disturbed.
Example of Stable Equilibrium
A book resting on the ground.
Example of Unstable Equilibrium
Apex of an ice cream cone balancing on top of a book.
The movement of the book will disturb the position of the ice cream cone.
Example of Neutral Equilibrium
A marble or rolling ball on a flat horizontal surface.
Factors affecting Stability
- Base width
- Height of centre of mass
Centre of Gravity, Centre of Mass
Essentially the same thing for the time being.
Gravitational Force
The force of attraction between masses
Formula for weight of an object IF you know its mass (m) and strength of the gravitational field (N/kg)
Weight (N) = Mass (kg) x Gravitational Field Strength (N/kg)
For Earth, it is 9.8 N/kg
For the Moon, it is around 1.6 N/kg
Mass is ___ ___ to Weight
Directly Proportional
Resultant force
The overall force on a point or object.
If a ___ Force ___ an object, ___ ___ ___
If a RESULTANT Force MOVES an object, WORK IS DONE.
The parallelogram of forces is a ___ ___ of two force ___
The parallelogram of forces is a SCALE DIAGRAM of two force VECTORS.
Use of the parallelogram of forces
Used to find the resultant of two forces that do NOT act along the same line.
What is the resultant force vector of a parallelogram of forces?
The resultant is the diagonal of the parallelogram that starts at the origin of the two forces.
Normal Contact Force
The equal and opposite force that surfaces exert to prevent solid objects from passing through each other.
The normal contact force is always perpendicular to the gradient / slope an object is on
Normal = 90 degrees; perpendicular ... Contact = Touching
What does it mean to resolve a force?
Finding components perpendicular to each other that have a resultant force that is equal to the force in question.
This is informally known as “Splitting a vector into its components”.
How to resolve a force in two perpendicular directions
Draw a rectangle with adjacent sides ALONG the perpendicular components; along the two directions.
This is so that the diagonal shows the force vector.
What is the resultant force for an object in equilibrium?
Zero.
Why would an object at rest be in equilibrium?
Because the resultant force acting on it is zero.
‘Adding’ Vectors
Finding the resultant force when given any amount of vectors.
Informally: “Adding any number of vectors acting in the same direction”.
An object is in ___ if the ___ on it are ___
An Object is in EQUILIBRIUM if the FORCES on it are BALANCED.
What is the Parallelogram of forces?
A geometrical method to find the resultant of two forces that don’t act along the same line.
Friction
The force opposing the relative motion of two solid surfaces in contact.
