Unit 5: Forces
Scalar and Vector Quantities
Scalar quantity has magnitude only, e.g. energy, mass, temperature
Vector quantity has magnitude and direction, e.g. velocity, displacement, forces
Drawing Vectors
Arrows can be used to represent vector quantities:
- length of the arrow shows the magnitude
- arrow points in the direction that the vector quantity is acting
Displacement vs Distance
When we give a direction along with a distance we call it displacement
If we run 800m that is a distance, if we run that 800m on a circular track, although we have covered this
distance, we end up at the same place we started our total displacement is 0.
Contact and Non-Contact Forces
A force occurs when two or more objects interact and are either:
Contact forces: can be exerted between two objects when they touch, e.g. the tension as two people pull
against one another
Non-contact forces: can be exerted between two objects that are physically separated, e.g. the force of gravity
acts even when the objects are not touching
CONTACT FORCES NON-CONTACT FORCES
Friction force that opposes objects moving relative to Gravitational force the force by which all things with
each other, it acts between two sliding surfaces and can mass or energy are brought towards one another. Earth
act to stop something beginning to move. pulls us downwards with/without contact. The Sun’s
Air-resistance/drag force that acts in the opposite gravitational attraction keeps Earth and planets in orbit.
direction to an object moving through the air. Electrostatic force the force that acts between charged
Tension is exerted through a rope when we pull objects. By rubbing a balloon, you can charge it and stick it
something. to the wall.
Normal contact force supports an object that is resting Magnetic force is a magnet that can attract objects
on a surface such as a table or the floor. made from iron or steel towards it.
Upthrust is the upward force that a fluid exerts on a body
floating in it.
Gravity
Force of attraction between all masses.
Force of gravity close to Earth is due to the gravitational field around the planet (10N/kg)
The mass of an object is related to the amount of matter it contains and is constant
Weight
Force acting on an object due to gravity
Weight of an object is considered to act through a single point = the centre of mass of
the object
Measured using a newton meter (a calibrated spring-balance)
Weight of an object depends on the gravitational field strength where the object is and
is directly proportional to its mass
- weight ∝ mass
weight ( W , N )=mass ( m , kg ) × gravitational field strength ( g , N / kg )
, Resultant Forces
When more than one force acts on an object they are seen as a single force with the same effect as
all the forces acting together. (overall force acting on an object)
Equilibrium = when there is no resultant force acting on an object.
A force is a vector so we can represent the size and direction with an arrow:
If we have two forces acting in the same direction we add them together to find the resultant force:
1N
3N
2N
If we have two forces acting in opposite directions we take one away from the other to find the
resultant force:
1N 2N 1N
If there is a resultant force acting on an object, it will accelerate in that direction.
Vector Diagrams
A free body diagram show all the different forces acting on an object:
Water pushes
boat up
(buoyancy)
Resistive force Driving force
Gravity pulls boat
down (weight)
Scale Vector Diagrams
Used to illustrate the overall effect when more than one force acts on an object:
- Forces are added together to find a resultant force, with both magnitude and direction
- Vectors are added tip to tail and a resultant force arrow is drawn
Used when a force is acting diagonally:
- To work out what effect the force will have express the diagonal force as two forces at right-
angles to each other
- We can split the force at an angle into horizontal and vertical components:
- The force FR can be broken down into F1 and F2
F1 is the same length as the length of FR in the horizontal direction
F2 is the same length as the length of FR in the vertical direction
- FR is also the vector found by adding F1 and F2 tip to tail
F1
F2 F2
FR FR
FR
F1
Scalar and Vector Quantities
Scalar quantity has magnitude only, e.g. energy, mass, temperature
Vector quantity has magnitude and direction, e.g. velocity, displacement, forces
Drawing Vectors
Arrows can be used to represent vector quantities:
- length of the arrow shows the magnitude
- arrow points in the direction that the vector quantity is acting
Displacement vs Distance
When we give a direction along with a distance we call it displacement
If we run 800m that is a distance, if we run that 800m on a circular track, although we have covered this
distance, we end up at the same place we started our total displacement is 0.
Contact and Non-Contact Forces
A force occurs when two or more objects interact and are either:
Contact forces: can be exerted between two objects when they touch, e.g. the tension as two people pull
against one another
Non-contact forces: can be exerted between two objects that are physically separated, e.g. the force of gravity
acts even when the objects are not touching
CONTACT FORCES NON-CONTACT FORCES
Friction force that opposes objects moving relative to Gravitational force the force by which all things with
each other, it acts between two sliding surfaces and can mass or energy are brought towards one another. Earth
act to stop something beginning to move. pulls us downwards with/without contact. The Sun’s
Air-resistance/drag force that acts in the opposite gravitational attraction keeps Earth and planets in orbit.
direction to an object moving through the air. Electrostatic force the force that acts between charged
Tension is exerted through a rope when we pull objects. By rubbing a balloon, you can charge it and stick it
something. to the wall.
Normal contact force supports an object that is resting Magnetic force is a magnet that can attract objects
on a surface such as a table or the floor. made from iron or steel towards it.
Upthrust is the upward force that a fluid exerts on a body
floating in it.
Gravity
Force of attraction between all masses.
Force of gravity close to Earth is due to the gravitational field around the planet (10N/kg)
The mass of an object is related to the amount of matter it contains and is constant
Weight
Force acting on an object due to gravity
Weight of an object is considered to act through a single point = the centre of mass of
the object
Measured using a newton meter (a calibrated spring-balance)
Weight of an object depends on the gravitational field strength where the object is and
is directly proportional to its mass
- weight ∝ mass
weight ( W , N )=mass ( m , kg ) × gravitational field strength ( g , N / kg )
, Resultant Forces
When more than one force acts on an object they are seen as a single force with the same effect as
all the forces acting together. (overall force acting on an object)
Equilibrium = when there is no resultant force acting on an object.
A force is a vector so we can represent the size and direction with an arrow:
If we have two forces acting in the same direction we add them together to find the resultant force:
1N
3N
2N
If we have two forces acting in opposite directions we take one away from the other to find the
resultant force:
1N 2N 1N
If there is a resultant force acting on an object, it will accelerate in that direction.
Vector Diagrams
A free body diagram show all the different forces acting on an object:
Water pushes
boat up
(buoyancy)
Resistive force Driving force
Gravity pulls boat
down (weight)
Scale Vector Diagrams
Used to illustrate the overall effect when more than one force acts on an object:
- Forces are added together to find a resultant force, with both magnitude and direction
- Vectors are added tip to tail and a resultant force arrow is drawn
Used when a force is acting diagonally:
- To work out what effect the force will have express the diagonal force as two forces at right-
angles to each other
- We can split the force at an angle into horizontal and vertical components:
- The force FR can be broken down into F1 and F2
F1 is the same length as the length of FR in the horizontal direction
F2 is the same length as the length of FR in the vertical direction
- FR is also the vector found by adding F1 and F2 tip to tail
F1
F2 F2
FR FR
FR
F1
