Chapter 4 - Force, Mass And Weight

Question 1

Define the centre of mass

Answer 1

The centre of mass of an object is the mean point of all the mass of an object

Question 2

Define the centre of gravity and where is it?

Answer 2

The centre of gravity is the point where the entire weight of an object can be thought to be concentrated. (Appears to act). It is in the same place as the centre of mass.

Question 3

How can you find the centre of gravity of an object?

Answer 3

By using 2 points of suspension on a 2D object (flat object) you can use a plumb line to draw a vertical line across the object. The point of intersection of these two lines is the centre of gravity.

Question 4

What are the 6 main forces to be considered in almost any free-body diagram?

Answer 4

Weight
Friction
Drag
Tension
Upthrust
Normal

Question 5

What are the two main factors in causing an objects terminal velocity?

Answer 5

The objects speed and cross-sectional area

Question 6

What is drag directly proportional to?

Answer 6

Speed^2

Question 7

What is the equation for the moment of an object?

Answer 7

Moment = force x distance perpendicular to the direction of the line of action of the force

Question 8

What is the unit for a moment?

Answer 8

Nm (Newton metre)

Question 9

Define a couple of forces

Answer 9

When two equal forces parallel to each other but on different lines that are of equal magnitude affect two different sides of an object causing it to spin without moving (rotating and not translating)

Question 10

What is the torque of a couple?

Answer 10

The torque of a couple is the moment between the pair of forces

Question 11

What is the equation for the torque of a couple?

Answer 11

Torque (Nm) = Force (N)(of one of the forces) x distance (m)(between the two forces not the centre)

Question 12

What needs to be included alongside the answer and units for a question that wants the torque of a couple?

Answer 12

The direction of motion of the torque (clockwise or anti-clockwise)

Question 13

What is an easy way to help visualise and thus solve a triangle of forces question?

Answer 13

Draw the completed triangle of forces

Question 14

What is the equation for pressure? (Basic)

Answer 14

P = F / A

Question 15

What is the unit for pressure and what is its unit in SI units?

Answer 15

Pascals (Pa) or (Nm^-2)

Question 16

What is the equation for the pressure of a vertical column of liquid?

Answer 16

P = height x density x g

Question 17

What unusual property is true about the pressure in a fluid?

Answer 17

The pressure at any particular depth is the same in all directions.

Question 18

What is the equation for the upthrust of an object underwater and how is this obtained?

Answer 18

Upthrust = x (depth of the object) * cross sectional area * density of the fluid * g

This is obtained by subtracting the pressure acting at the top of any object from the pressure acting at the bottom of the object. This is what gives x as distance and not depth since x is the distance between the two places that the pressure is being measured.

Question 19

What is the Upthrust of an object in a fluid equal to?

Answer 19

The weight of the the fluid displaced by the object

Question 20

What is archimedes principle?

Answer 20

The Upthrust exerted on a body immersed in a fluid wether fully or partially submerged is equal to the weight of the fluid that the body displaces.

Question 21

What could you change in the ball and tube of liquid experiment to affect the balls terminal velocity?

Answer 21

Change the liquid
Change the size of the ball
Change the shape of the item being dropped
Change the mass of what’s being dropped

Question 22

What is Archimedes principle?

Answer 22

When a body is partially or fully submerged in a fluid, it experiences an upthrust equal to the weight of the fluid displaced