Paper 2 - P5 - Forces Flashcards
Vector quantity
> Vector quantities have magnitude and direction.
E.g. force, velocity, displacement, acceleration, momentum.
They can be represented by an arrow:
-The length of the arrow shows the magnitude and the direction of the arrow shows the direction of the quantity.
Scalar quantity
> Scalar quantities only have magnitude and NO direction.
>E.g. speed, distance, mass, temperature, time.
How does velocity (vector) follow on from speed (scalar)?
> Velocity is a vector, but speed is a scalar quantity.
Both bikes are travelling at the same speed but they have different velocities because they are travelling in different directions.
What two groups can forces be put into?
> All forces are either contact and non-contact.
Force - defintion
> A force is a push or a pull on an object that is caused by it interacting with something.
Contact forces
> When two objects have to be touching for a force to act, that force is called a contact force.
E.g. friction, air resistance, tension in ropes, normal contact force.
Non-contact forces
> If the objects do not need to be touching for the force to act, the force is non-contact.
E.g. magnetic force, gravitational force, electrostatic force.
What happens when forces interact?
> When two objects interact, there is a force produced on both objects.
An interaction pair.
This is basically Newton’s 3rd law.
E.g. the Sun and the Earth are attracted to each other by the gravitational force. This is a non-contact force. An equal but opposite force of attraction is felt by both the sun and the Earth.
E.g. a chair exerts a force on the ground, whilst the ground pushes back at the chair with the same force (the normal contact force). Equal but opposite forces are felt by both the chair and the ground.
Interaction pair - definition
> A pair of forces that are equal and opposite and act on two interacting objects.
Gravitational force
> Gravitational force is the force of attraction between masses.
Gravity attracts all masses, but you only notice it when one of the masses is really really big, e.g. a planet.
Anything near a planet or star is attracted to it very strongly.
It has 2 important effects.
The two important effects of gravity.
1) On the surface of a planet, it makes all things fall towards the ground.
2) It gives everything weight.
Mass - definition
> Mass is just the amount of ‘stuff’ in an object. For any given object this will have the same value anywhere in the universe.
Weight - definition
> Weight is the force acting on an object due to gravity (the pull of the gravitational force on the object).
Close to the Earth, this force is caused by the gravitational field strength around the Earth.
Gravitational field strength - info
> GFS varies with location.
>It’s stronger the closer you are to the mass causing the field, and stronger for larger masses.
Weight - info
> The weight of an object depends on the strength of the gravitational field at the location of the object. This means the weight of an object changes with its location.
E.g. an object has the same mass whether it’s on the Earth or Moon, but it’s weight will be less on the Moon as the GFS is weaker (1.6N/kg).
Weight is a force measured in newtons.
You can think of the force as acting from a single point on the object, called its centre of mass. For a uniform object this will be at the centre of the object.
Weight is measured using a calibrated spring balance (or newtonmeter).
Mass - info
> Mass is not a force.
>It’s measured in kilograms with a mass balance.
Relationship between mass and weight
> Mass and weight are directly proportional.
>Increasing the mass of a subject increases its weight, if you double the mass, you double the weight etc.
EQUATION - WEIGHT
Weight (N) = Mass (kg) x Gravitational Field Strength (N/kg).
What type of diagram show all the forces acting on an object?
> A free body diagram
Free Body Diagrams
> The size 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 the overall force on a point or object?
> Resultant force
Resultant Force
> The overall force on a point or an object.
If you have a number of forces acting at a single point, you can replace the single force (the resultant force).
If the forces all act along the same line, the overall effect is found by adding those going in the same direction and subtracting any going in the opposite direction.
Consider the horizontal and vertical directions separately. State the size and direction of the resultant force, e.g. 200N to the left.
Resultant Force Theory
> If a resultant force moves an object, work is done.
When a force moves an object through a distance, energy is transferred and work is done on the object.
1)To make something move, a force must be applied.
2) The thing applying the force must have a source of energy (like fuel or food).
3)The force does ‘work’ to move the object and energy is transferred from one store to another.
4) Work done and energy transferred are the same thing.
EQUATION - WORK DONE
> Work Done (J) = Force (N) x Distance (moved along the line of action of the force) (m).
> 1 Joule = 1Nm