Forces – Revision Notes
What is a force?
A force is a push or pull that causes a change in speed, direction, or shape.
Scalar and Vector





Scalar quantities are a physical measurement. They only have a magnitude (or size)
Examples of scalar quantities: temperature, mass, energy, distance, speed, density
Vector quantities have both magnitude and an associated direction.
Examples of vector quantities: force (N), displacement, velocity, acceleration, momentum
The direction of a vector can be given in a written description or denoted by an arrow. The
length of an arrow represents magnitude of quantity
Resultant Force




Resultant force is a single force that has the same effect as 2 or more forces acting
together
Resultant Forces in the same direction produce a resultant force that is greater than either
individual force. To find the resultant force, just add the 2 forces together
Two forces in the opposite directions produce a resultant force that is smaller than either
individual force. To find the resultant force, subtract the smaller magnitude from the
larger magnitude
DON’T FORGET TO WRITE THE DIRECTION; FORCE IS A VECTOR QUANTIY!
Contact Forces



Contact forces are forces that act between 2 objects that are physically touching
Examples: Reaction force, Tension, Friction, Air resistance
When a contact force acts between 2 objects, both experience the same sized force, but in
opposite direction.
Non – Contact


Non-Contact forces are forces that act between 2 objects that are not physically touching
each other.
Examples: Magnetic Force, Electrostatic force, Gravitational force
Gravity


All objects with mass produce a gravitational field
The more mass an object has the greater its gravitational field will be


The Gravitational field strength at a point is the gravitational force
exerted per unit mass at a point.
The gravitational field strength of the earth is 9.8 N/m (approx. 10N/m)
Weight




Weight is the force acting on an object due to gravity
It has the unit newtons (N)
It acts towards the centre of the gravitational field
The weight of an object can be measured using a Newton Metre
W = mg


The weight of an object and its mass are directly proportional, because gravitational field
strength is a constant
Weight = Mass x Gravitational field strength
 Weight is measured in newtons (N)
 Mass is measured in kilograms (kg)
 Gravitational field strength is measured in (N/kg)
Free Body Diagram




Free body diagrams are drawings that represent the
forces acting upon an object.
The object or the ‘body’ is usually shown as a box or a
dot or a circle
The forces acting on it are shown by thin arrows that
point in the direction of the force applied (either away
from or towards the centre)
There should be different arrows for each force applied
upon the object
The Consequence of force

When a force acts on an object, it may:
 Change in shape or size
 Start/stop moving
 Change in speed of a moving object
 Change in the direction of motion of the moving object
Change in shape


When force acts on an object, the object may change shape by bending, stretching, or
compressing
There must be more than one force acting on the shape of a stationary object
 The forces can bend an object’s ends past each other
 The object’s ends can be pulled apart
 Push an object’s ends together

The change in shape is called deformation:
 Elastic deformation can be reversed when force is removed
 Inelastic deformation cannot be fully reversed when the force is removed
Hooke’s Law – Force and Extension are directly proportional


Extension happens when an object increases in length and
compression happens when it decreases in length.
The Extension of an elastic object can be described as:
Force = Spring Constant x Extension
F = ke
 Force is measured in Newtons (N)
 Spring Constant is measured in Newtons per metre (N/m)
 Extension is measure in metres (m)
Energy Stored in a Spring
 Elastic Potential Energy is the energy stored within a spring
 Work done is equal to the elastic potential energy stored.
 Elastic Potential energy can be calculated by:
E = ½ ke2
 Force is measured in Newtons (N)
 Extension is measure in metres (m)
 Elastic Potential energy is measured in Joules (J)
Motion in a straight line

The distance travelled by an object and moving at a
constant speed can be calculated using the equation:
Distance = Speed x Time
D = 𝑠𝑡
 Distance travelled is measured in metres (m)
 Speed is measured in metres per second (m/s)
 Time is measured in seconds (s)
Newton’s Laws of Motion
1) If all the forces on an object are balanced (the resultant force = 0), the object will remain at
a constant velocity or at rest
2) Resultant Force = mass x acceleration
F = m𝛼
If the forces acting on an object are unbalanced, the object’s acceleration will be:
 In the direction of the resultant force
 Directly proportional to the resultant force
3) When 2 objects interact, they exert an equal and opposite force on each other; Every
action has an equal and opposite reaction.
Velocity



Velocity of an object is the measure of its speed in a particular direction
Velocity is a vector because it has both magnitude and an associated direction
Formula for velocity
𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦 =
Displacement
Time
Acceleration
 Acceleration is the rate of change in velocity
 Change in velocity can be calculated using the equation:
𝐶ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 = 𝑓𝑖𝑛𝑎𝑙 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 − 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦
∆𝑣 = 𝑣 − 𝑢
𝑎𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛 =
α=



Change in velocity
time
∆𝑣
𝑡
Acceleration (𝛼) is measured in metres per second squared (m s-1)
Change in velocity (∆𝑣) is measured in metres per second (m/s)
Time taken (𝑡) is measured in seconds (s)
Velocity, acceleration, and distance

(final velocity)2 – (initial velocity)2 = 2 x acceleration x distance
𝑣 − 𝑢2 = 2𝑎𝑠
 Final velocity (v) is measured in metres per second (m/s)
 Initial velocity is measured in metres per second (m/s)
 Acceleration (a) is measured in metres per second squared (m/s2)
 Displacement (s) is measured in measured in metres (m)
2
Momentum:
 Momentum is the product of mass and velocity
 Momentum is a vector quantity
 Momentum can be calculated using the equation
Momentum = mass x velocity
P = mv
 Momentum (𝑝) is measured in kilograms meters per second (kg m/s)
 Mass (𝑚) is measured in kilograms (kg)
Work Done



When force causes an object to move, work is being done
Work is the measure of energy transfer when a force moves an object through a distance
When work is done, energy is transferred from on object to another, and is converted form
one form to another.
Energy transferred = Work Done

The equation used to calculate the work done is:
work done = force × distance
W=Fd

work done (W) is measured in joules (J)
SI Units


















Weight is measured in newtons (N)
Mass is measured in kilograms (kg)
Gravitational field strength is measured in (N/kg)
Force is measured in Newtons (N)
Spring Constant is measured in Newtons per metre (N/m)
Extension is measure in metres (m)
Distance travelled is measured in metres (m)
Speed is measured in metres per second (m/s)
Time is measured in seconds (s)
Acceleration (𝛼) is measured in metres per second squared (m s-1)
Change in velocity (∆𝑣) is measured in metres per second (m/s)
Final velocity (v) is measured in metres per second (m/s)
Initial velocity is measured in metres per second (m/s)
Acceleration (a) is measured in metres per second squared (m/s2)
Displacement (s) is measured in measured in metres (m)
Momentum (𝑝) is measured in kilograms meters per second (kg m/s)
Mass (𝑚) is measured in kilograms (kg)
Work done is measured in Joules (j)
Key Terms
Acceleration – The rate of change in velocity
Compression – A shortening of length
Contact force - Forces that act between 2 objects that are in physical contact
Deformation – A change in shape
Elastic deformation – An action that can be reversed when force is removed
Elastic Potential Energy – Energy stored in squashed, stretched or twisted materials.
Electrostatic force – A force of attraction between particles with opposite charges
Extension – Increase in length
Force - A force is a push or pull that causes a change in speed, direction, or shape
Friction - A force that opposes or prevents movement upon surfaces
Inelastic deformation – An action that cannot be reversed when the force is removed
Magnetic – The ability to be magnetised or attracted by a magnet
Magnitude – The size of a physical quantity
Mass – The amount of matter contained in the object
Momentum – The product of mass and velocity
Non-contact force – A force that can be activated without any physical contact between objects
Repel – To push away. Magnetically similar poles repel one another
Resultant Force - A single force that has the same magnitude as many other forces
acting together upon the same object.
Scalar – A physical measurement that includes only magnitude (Or size)
Tension – A pulling force exerted by each end of the object
Upthrust – The upwards force exerted by a fluid
Vector – A physical measurement that has both magnitude and direction
Weight – Mass with gravitational field strength
Work – Energy Transferred by a force
Required Practical – Investigating Hooke’s Law
Aim: To investigate the relationship between force and extension for a spring
Equipment:
-
Spring
Ruler
Clamp Stand
Weight
Variables:
-
Independent variable: Force applied to spring
Dependent variable: Extension of spring
Control variable: Spring
Method:
1.
2.
3.
4.
5.
Set up equipment as shown
Measure the original length of the spring with ruler
Attach a known weight (approximately 1N) to the spring
Measure the new length of the spring
Calculate extension of the spring by subtracting original length from the new length of
spring
6. Repeat Steps: 3-5 for up to 6N
Recording Results
1. Note the data found onto a table
2. Plot a scatter graph with force of weight on
y-axis and extension on x-axis
3. Draw the line of best fit; the gradient of this
line is the spring constant of the coil spring
Hazard and Control Measures
Hazard
Equipment Falling off the
table
Consequence
Heavy objects falling on feet
Control measures
Hold the base of the clamp stand
firmly on table
Sharp end of spring recoiling
if spring breaks/snaps
Damage to eyes or cuts on
skin
Masses falling to floor if
spring fails
Heavy objects falling to feet
Wear eye protection. Lower the
masses gently whilst landing the
spring
Gently lower load onto spring