Forces In Action

Question 1

Name the formulas for calculating pressure and the units

Answer 1

P= F/A , pascals
P = hpg , pascals

Question 2

Name Archimedes’ principle and the formula

Answer 2

The upthrust exerted on a body immersed in a fluid is equal to the weight of the fluid that body displaces

W= -mg
W = -pVg
W= weight of water displaced
p= density
V = volume of object
g= acceleration due to free fall

Question 3

What is the centre of mass and where is it located? ( for objects of both uniform and non-uniform density)

Answer 3

The centre of mass is where all the mass of an object appears to be concentrated/acting.
Objects of uniform density will have the centre of mass be located in the geometric centre (the middle of an object)
For objects of non-uniform density, the centre of mass will be where the centre of gravity is

Question 4

How do you cause an object to only have translational movement (movement in space) without rotational movement?

Answer 4

Apply the force to the centre of mass. The object will produce straight line motion without rotation

Question 5

How do you find the centre of gravity?

Answer 5

Objects come to rest in line with the centre of gravity. Suspend an object and use a plumb-bob to trace a line along the centre. Rotate the object and draw another line. These lines intersect at the centre of gravity.

Question 6

Give the definition of drag and its relation to speed

Answer 6

Drag is a frictional force that opposes motion in a fluid. The magnitude of drag depends on the speed, shape, and texture of the object, the objects cross-sectional area, and the density of the fluid.

Drag is proportional to speed squared.

Question 6

What is a moment and what formula do you use for moments?

Answer 6

A turning effect of a force. Can be clockwise or anti-clockwise.

M=Fx, Newton metres
F= force
x= perpendicular distance of the line of rotation of force from the pivot
For forces at angles, use:
M= FxCos(Theta)

Question 8

Explain the meaning of the equation
eV = hc/lambda

Answer 8

The energy of an electron is equal to the energy of a photon
(This is true due to the law of energy conservation)
OR
Minimum energy required to emit a photon

Question 9

Describe an experiment that uses voltage to find the threshold frequency of LEDs of differing colours

Answer 9

-Connect an LED to a power supply with variable voltage
-Increase voltage until LED turns on
-Repeat several times and average to find V
-Repeat for each LED
-Shield LED in opaque tube to judge strike more accurately