Measurements and Units
Instruments for measurement
- Ruler
- Micrometer screw gauge
- Measures to the nearest 1/100 mm
● Centre of mass
- Point on the object where mass is concentrated
- Point where weight of the object acts
To determine centre of mass
● Centre of mass affects stability
- Object is stable if weight is inside the base of the object
- Unstable If weights outside base of object
- Low centre of mass and large base increases stability of object
Describe how the student can find the period of oscillation more accurately
- Check the zero error of the stopwatch
- Count the number of oscillations in 20 seconds
- Divide number of oscillations by 20
- Repeat 3 times and find the mean (average)
,Describe a method used to determine the centre of mass of the card
- Pin the card to a stand and allow it to rotate freely
- Then, poke 2 holes in the card and insert a plumb line into the first hole
- Mark position where card stops rotating and draw line
- Repeat the same for the other hole
- Intersection of lines is the centre of mass
Explain why the card falls anticlockwise when the angle is large
- Centre of mass lies outside the base
- So anticlockwise moment is created
, Forces and Motion
● Velocity: rate of change of displacement
- Vector quantity with magnitude and vector
● Acceleration: rate of change of velocity
● Mass:
- Amount of matter contained within an object
- All masses have inertia
, - Inertia is the resistance to change in motion
- Larger mass = larger inertia
● Weight
- Force on a mass due to the earth’s gravity
- w=mg
● Density
- How closely packed the particles are
A force can:
- Change the shape
- Accelerate
- Larger force = larger acceleration
- Mass must be constant
- Larger mass = smaller acceleration
- Force constant
- Decelerate
- Change direction
Hooke’s Law: extension of a spring is directly proportional to the force applied if the limit
of proportionality is not reached
- F=kx
● Centripetal force
- Force that causes an object to move in a circle
- Force acts 90* to direction of motion
- Force does not do any work to object
- Because object does not move in the direction of the force
- Force changes direction without changing speed
Instruments for measurement
- Ruler
- Micrometer screw gauge
- Measures to the nearest 1/100 mm
● Centre of mass
- Point on the object where mass is concentrated
- Point where weight of the object acts
To determine centre of mass
● Centre of mass affects stability
- Object is stable if weight is inside the base of the object
- Unstable If weights outside base of object
- Low centre of mass and large base increases stability of object
Describe how the student can find the period of oscillation more accurately
- Check the zero error of the stopwatch
- Count the number of oscillations in 20 seconds
- Divide number of oscillations by 20
- Repeat 3 times and find the mean (average)
,Describe a method used to determine the centre of mass of the card
- Pin the card to a stand and allow it to rotate freely
- Then, poke 2 holes in the card and insert a plumb line into the first hole
- Mark position where card stops rotating and draw line
- Repeat the same for the other hole
- Intersection of lines is the centre of mass
Explain why the card falls anticlockwise when the angle is large
- Centre of mass lies outside the base
- So anticlockwise moment is created
, Forces and Motion
● Velocity: rate of change of displacement
- Vector quantity with magnitude and vector
● Acceleration: rate of change of velocity
● Mass:
- Amount of matter contained within an object
- All masses have inertia
, - Inertia is the resistance to change in motion
- Larger mass = larger inertia
● Weight
- Force on a mass due to the earth’s gravity
- w=mg
● Density
- How closely packed the particles are
A force can:
- Change the shape
- Accelerate
- Larger force = larger acceleration
- Mass must be constant
- Larger mass = smaller acceleration
- Force constant
- Decelerate
- Change direction
Hooke’s Law: extension of a spring is directly proportional to the force applied if the limit
of proportionality is not reached
- F=kx
● Centripetal force
- Force that causes an object to move in a circle
- Force acts 90* to direction of motion
- Force does not do any work to object
- Because object does not move in the direction of the force
- Force changes direction without changing speed
