Chapter 4: Forces in action

Question 1

What is the equation that links Force, Mass, and Acceleration?

Answer 1

The equation that links Force, Mass and Acceleration is:

Force = Mass x Acceleration

Question 2

What is the equation that links Weight, Mass and Gravity?

Answer 2

The equation that links Weight, Mass and Gravity is: Weight = Mass x Gravity

Question 3

What is the net (resultant) force?

Answer 3

The Net Resultant Force is equal to the sum of all forces acting on an object.

Question 4

What is the Centre of Gravity?

Answer 4

The Centre of Gravity is an imaginary point in the centre of an object where weight is assumed to act through.

Question 5

What is a Free-Body Diagram?

Answer 5

A Free-Body Diagram is a diagram which isolates and presents all the forces acting on an object.

Question 6

What is Drag Force?

Answer 6

Drag Force is a resistive force that arises when an object moves through a fluid such as Air or Water.

Question 7

What direction does Friction act towards?

Answer 7

Friction acts in the opposite direction of motion.

Question 8

What factors affect Drag Force?

Answer 8

Factors that affect Drag Force include:

• The Surface Area of the Object.
• The Relative Speed that the object is travelling in.
• The Density of the fluid the Object is moving through.
• The Texture and Shape of the Object.

Question 9

What is the definition of Terminal Velocity?

Answer 9

Terminal Velocity is defined as the constant velocity reached by an object when the drag force and upthrust is equal and opposite to the weight of the object.

Question 10

Describe the First Stage Motion when an object starts to fall.

Answer 10

When an Object starts falling:

• The total force is equal to the weight of the object.
• The acceleration of the object is equal to the acceleration of Freefall. (a = g)
• There is no Drag Force.

Question 11

Describe the Second Stage of Motion when an object is falling.

Answer 11

When an Object is falling:
- The Speed of the Object increases.
- The Resultant Force starts to decrease.
- Drag Force starts to increase.
- The (instantaneous) acceleration of the Object starts to decrease (less than g).
a = (mg - D) / m

*Talk about Speed, Resultant Force, and Acceleration.

Question 12

Describe the Third Stage of Motion when an object is falling.

Answer 12

At the last stage when an object is falling:

• The Resultant Force is 0.
• Acceleration is 0. (a = 0)
• The object has reached Terminal Velocity. Meaning, Speed / Velocity is constant.

Question 13

What is the definition of a Moment of a Force?

Answer 13

The Moment of a Force is defined as the turning effect of a force about some Axis. The Equation for a Moment is:
Moment = Force x Perpendicular Distance from the Pivot.

Question 14

How do you find the perpendicular distance from a force applied at an Angle?

Answer 14

To find the Perpendicular Distance from a force applied at an Angle, use Trigonometry:
x = Distance from Pivot x CosΘ

Question 15

What is the Principle of Moments?

Answer 15

The Principle of Moments state that for a body in rotational equilibrium, the sum of the anticlockwise moments is equal to the sum of the clockwise moments about the same point.

Question 16

How do you find the total moment?

Answer 16

To find the total moment within one side, add all the moments from each force from the pivot individually together.

Question 17

What is the definition of a Couple?

Answer 17

A Couple is a pair of equal and opposite parallel forces acting on a body but not in a straight line.

*This can allow an object to rotate it without translating it from its original position.

Question 18

What is the definition of Torque, and its equation?

Answer 18

Torque is defined as the moment of a couple, and can be calculated using the equation:

Torque = (Force 1) x Perpendicular Separation of the forces.

Question 19

What are the rules for making a triangle of forces?

Answer 19

When drawing a triangle of forces:

• The resultant force must be zero, and the object is in equilibrium.
• The triangle is closed and must have an angle.
• The end of one side must be the start of another side.

Question 20

What is the definition of Density, and its Equation?

Answer 20

Density is defined as the mass per unit volume of an object or substance., and the equation for Density is:

Density = Mass / Volume (p = m / v)

Question 21

What is the equation for Pressure in a cylinder column?

Answer 21

For a liquid within a cylinder column, the equation for Pressure is:
p = hϼg
p = Pressure, h = Height/Depth of Object in cylinder, ϼ = Density of Liquid, g = 9.81ms-2

Question 22

What is the definition of Upthrust, and its Equation?

Answer 22

Upthrust is defined as the upward buoyancy force exerted on a body immered in a liquid, and its Equation is:

Upthrust = Axϼg, 
A = Cross-sectional area of cylinder, x = Height/Depth of Object within Cylinder, ϼ = Density of Liquid, g = Acceleration of Freefall

Question 23

What is Archimedes’ Principle?

Answer 23

Archimedes’ Principle states that the upthrust exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces.

Question 24

When will an object in a fluid sink?

Answer 24

An object in a fluid will sink if the upthrust (upwards buoyancy force) acting on the object is less than the weight of the object.