Forces
Contact and Non-Contact Forces
• Contact Force – e.g. friction, air resistance
• Non-Contact Force – e.g. gravity, magnetic forces
Scalars and Vectors
• A scalar quantity is one that has a size or magnitude. E.g. mass, length
• A vector quantity is one that has a size and a direction. E.g. pressure, momentum
• Vectors can be represented by arrows:
o The length of the arrow represents the magnitude of the vector.
o The direction of the arrow represents the direction of the vector.
Adding Vectors:
• Two vectors acting in the same direction can simply be added together:
Resultant= F1 + F2
• If the two vectors act in opposite directions, we need to take one direction as positive, and the other as
negative, before resolving them:
Resultant= F1 – F2
• To find the resultant of two vectors acting at 90° to each other, we draw the vectors. The resultant
is diagonal of the rectangle. There are two ways to find the resultant:
o By calculation using Pythagoras Theorem
o By scale drawing
Scale Drawing:
1. Decide on a scale … often 1cm=1N
2. Work out how long each vector should be.
3. Draw one of the vectors.
4. “Move” a vector so that they follow on from each other, but ensure the 4N
angle is the same.
5. Join the start and the end with the resultant vector.
6. Measure the length and then convert into a force.
3N
Pythagoras Theorem:
1. a² + b² = c² √4! + 3!
2. F1² + F2² = Resultant²
Work Done
• When any objects are moved around work will need to be done on it to get it to move.
• We can work out the amount of work done in moving an object using the formula:
Work Done (J) = Force (N) x Distance Moved (m)
Weight and Mass:
• Mass is a measure of how much “stuff” – matter there is in an object. Measured in kg.
• Weight is a force acting on the mass because of gravity. Measured in N.
, Weight (N) = Mass (kg) x Gravitational Field Strength (N/kg)
Spring Constants:
• Hook’s Law:
• F =ke
Force (N) = Spring Constant x Extension
The spring does not go back to its original
length. This is “inelastic deformation”.
The spring returns to its original The ‘limit of
shape- this is “elastic proportionality’
deformation”.
Force is proportional to extension as
long as you don’t go past the “limit
of proportionality”. There is a linear
relationship up to this point.
• If an object returns to its normal shape after being stretched or squashed it is considered to be elastic.
• When an object is stretched it stores elastic potential energy.
• Ee = ½ke²
Elastic Potential Energy (J) = ½ x Spring Constant (N/m²) x Extension (m)
Moments:
• A moment is the turning effect of a force.
• A moment will act around a pivot.
• A moment can act clockwise or anti-clockwise.
• Moment (Nm) = Force (N) x Perpendicular Distance of the Force (m)
Gears:
• If wheel A has the same force as wheel B applied to it the moment of wheel B is double the
size because it had twice the radius of wheel A.
Pressure:
• Every time the particles hit the side of the container the particles exert a force act at right angles on the
container – this is called “pressure”.
• The pressure at the base of a column of fluid is:
Pressure = phg
Pressure (N/m²) = density (kg/m²) x height of the container (m) x gravitational field strength (10)
• Pressure increases with depth. This is because the water at the bottom of the container is pushed on by the
weight of the water further up, which causes it to be under higher pressure.
• It is the same with atmospheric pressure – at the top of a mountain there are less air molecules up here. Less
air molecule = fewer collisions = less pressure.
Contact and Non-Contact Forces
• Contact Force – e.g. friction, air resistance
• Non-Contact Force – e.g. gravity, magnetic forces
Scalars and Vectors
• A scalar quantity is one that has a size or magnitude. E.g. mass, length
• A vector quantity is one that has a size and a direction. E.g. pressure, momentum
• Vectors can be represented by arrows:
o The length of the arrow represents the magnitude of the vector.
o The direction of the arrow represents the direction of the vector.
Adding Vectors:
• Two vectors acting in the same direction can simply be added together:
Resultant= F1 + F2
• If the two vectors act in opposite directions, we need to take one direction as positive, and the other as
negative, before resolving them:
Resultant= F1 – F2
• To find the resultant of two vectors acting at 90° to each other, we draw the vectors. The resultant
is diagonal of the rectangle. There are two ways to find the resultant:
o By calculation using Pythagoras Theorem
o By scale drawing
Scale Drawing:
1. Decide on a scale … often 1cm=1N
2. Work out how long each vector should be.
3. Draw one of the vectors.
4. “Move” a vector so that they follow on from each other, but ensure the 4N
angle is the same.
5. Join the start and the end with the resultant vector.
6. Measure the length and then convert into a force.
3N
Pythagoras Theorem:
1. a² + b² = c² √4! + 3!
2. F1² + F2² = Resultant²
Work Done
• When any objects are moved around work will need to be done on it to get it to move.
• We can work out the amount of work done in moving an object using the formula:
Work Done (J) = Force (N) x Distance Moved (m)
Weight and Mass:
• Mass is a measure of how much “stuff” – matter there is in an object. Measured in kg.
• Weight is a force acting on the mass because of gravity. Measured in N.
, Weight (N) = Mass (kg) x Gravitational Field Strength (N/kg)
Spring Constants:
• Hook’s Law:
• F =ke
Force (N) = Spring Constant x Extension
The spring does not go back to its original
length. This is “inelastic deformation”.
The spring returns to its original The ‘limit of
shape- this is “elastic proportionality’
deformation”.
Force is proportional to extension as
long as you don’t go past the “limit
of proportionality”. There is a linear
relationship up to this point.
• If an object returns to its normal shape after being stretched or squashed it is considered to be elastic.
• When an object is stretched it stores elastic potential energy.
• Ee = ½ke²
Elastic Potential Energy (J) = ½ x Spring Constant (N/m²) x Extension (m)
Moments:
• A moment is the turning effect of a force.
• A moment will act around a pivot.
• A moment can act clockwise or anti-clockwise.
• Moment (Nm) = Force (N) x Perpendicular Distance of the Force (m)
Gears:
• If wheel A has the same force as wheel B applied to it the moment of wheel B is double the
size because it had twice the radius of wheel A.
Pressure:
• Every time the particles hit the side of the container the particles exert a force act at right angles on the
container – this is called “pressure”.
• The pressure at the base of a column of fluid is:
Pressure = phg
Pressure (N/m²) = density (kg/m²) x height of the container (m) x gravitational field strength (10)
• Pressure increases with depth. This is because the water at the bottom of the container is pushed on by the
weight of the water further up, which causes it to be under higher pressure.
• It is the same with atmospheric pressure – at the top of a mountain there are less air molecules up here. Less
air molecule = fewer collisions = less pressure.
