mechanics

Question 1

moment of the force

Answer 1

The turning effect of a force is call the moment of the force

The moment of a force is defined by the equation:

𝑚𝑜𝑚𝑒𝑛𝑡 𝑜𝑓 𝑎 𝑓𝑜𝑟𝑐𝑒 (Nm) = 𝑓𝑜𝑟𝑐𝑒 (N) × Perpendicular distance from the pivot to the line of action of the force (m)

𝑀=𝐹𝑑

Question 2

The Principle of Moments

Answer 2

If an object is balanced: the total clockwise moment about a pivot equals the total anticlockwise moment about that pivot.

Question 3

Centre of Mass

Answer 3

The weight of an object may be considered to act at a single point referred to as the objects Centre of mass

Question 4

Use the principle of moments to calculate the unknown forces and distance shown in the diagrams below:

Answer 4

Question 5

lever

Answer 5

A lever is a rigid beam with a fulcrum. The effort (input force) is applied on one end of the beam and a load (output force) is applied to the other end of the beam. The fulcrum is the point on which the beam pivots.

The point at which the lever will just begin to move will be the point where the moment created by effort is equal to the moment created by the load.

Question 6

principal of the lever

Answer 6

The moment of the load and effort are equal which means that if the effort is further from the pivot than the load (its perpendicular distance is greater) the effort must be smallerthan the load.

Answer 7

A pair of gears exert forces on each other that are equal in magnitude but opposite in direction, which means that adjacent gears rotate in opposite directions. Because the magnitude of the force on pair of gears is the same, the turning effect on the gear is directly proportional to the radius of the gear (which is the distance between the force and the pivot). However the larger the gear the lower the rate of radiation.

(𝑅𝑎𝑑𝑖𝑢𝑠 𝑜𝑓 𝐺𝑒𝑎𝑟 𝐴)/(𝑅𝑎𝑑𝑖𝑢𝑠 𝑜𝑓 𝐺𝑒𝑎𝑟 𝐵)=(𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑇𝑒𝑒𝑡ℎ 𝐴)/(𝑁𝑢𝑚𝑚𝑒𝑟 𝑜𝑓 𝑇𝑒𝑒𝑡ℎ 𝐵)=(𝑀𝑜𝑚𝑒𝑛𝑡 𝑜𝑓 𝐺𝑒𝑎𝑟 𝐴)/(𝑀𝑜𝑚𝑒𝑛𝑡 𝑜𝑓 𝐺𝑒𝑎𝑟 𝐵)=(𝑅𝑎𝑡𝑒 𝑜𝑓 𝑟𝑜𝑡𝑎𝑡𝑖𝑜𝑛 𝐵)/(𝑅𝑎𝑡𝑒 𝑜𝑓 𝑟𝑜𝑡𝑎𝑡𝑖𝑜𝑛 𝐴)

Question 8

gear example

Answer 8

When two gears share the same axel they must have the same moment. Because they have the same moment the larger gear must exert a larger force,

Question 9

Momentum

Answer 9

𝑝=𝑚𝑣
momentum (Kgm/s) = mass (Kg) x Velocity (m/s)

Question 10

“Conservation of momentum” means…

Answer 10

The momentum in a closed system remains constant

Question 11

A toy train of mass 1kg is travelling at 3m/s when it collides with a second train of equal mass travelling in the opposite direction at 1m/s. They stick together due to magnetic catches. How fast do they travel immediately after impact?

Answer 11

Momentum before collision = Momentum after collision
(1x3)+(1x-1)= (2xv)
2=2v
v=1 m/s

Question 12

The trains then slow down. Why is momentum no longer being conserved?

Answer 12

External forces such as air resistance and friction act on the trolleys (the momentum is transferred to the air /ground which are not part of the system)

Question 13

Force and rate of change of momentum

Answer 13

𝐹=𝑚∆𝑣/∆𝑡

Resultant Force (N) = (Mass(kg) x Change in velocity(m/s)) / Change in time (s)

Question 14

Safety features

Answer 14

Helmets, crash mat, seat belts, etc. all protect humans in a similar way:
They increase the time taken for a person to come to a stop
The change in momentum is the same
So the rate of change of momentum is lower
Which means the resultant force is less
Which means the risk of injury is reduced

Question 15

inertia

Answer 15

the tendency of an object to continue in its state of rest or motion

Question 16

Formula for pressure

Answer 16

Pressure (Pa) = Force (N) / Area (M²)
P=F/A
The unit of pressure is the pascal (Pa)
One Pa is equal to one N/M²

Question 17

Pressure in a column of Liquid

Answer 17

As you move deeper in a liquid the pressure increases due to a greater weight of liquid above you.
The increase in pressure caused due to a column of liquid can be calculated with the equation:
pressure (Pa) =density (Kg/M³) ×gravitational field strength (N/Kg) ×height of column (M)
p= ρgh

Question 18

Upthrust

Answer 18

The weight of fluid displaced by an object is equal to the upthrust acting on the object.
If an object is fully immersed the volume of fluid, it displaces is equal to its own volume.
A fully immersed object will float if the upthrust is greater than its weight, which is only true if the weight of the fluid displaced is greater than the weight of the object itself. The only way this can be true is if the density of the fluid is greater than the density of the object.
If an object is denser than the surrounding fluid it will sink
If an object is less dense than the surrounding fluid it will float

Question 19

Atmospheric Pressure

Answer 19

Air molecules collide with a surface create atmospheric pressure.
At increasing altitude, the distance between air molecules increases.
So, the frequency of collisions of air molecules on the surface must decrease.
Which means that air pressure decreases with altitude.

Question 20

effect of radiation on the earths temperature

Answer 20

1) During the day, lots of radiation (like light) is transferred to the Earth from the Sun and absorbed. This causes an increase in local temperature.

2) At night, less radiation is being absorbed than is being emitted, causing a decrease in the local temperature.

3) Changes to the atmosphere can cause a change to the Earth’s overall temperature. If the atmosphere starts to absorb more radiation without emitting the same amount, the overall temperature will rise until absorption and emission are equal again.