Module 3.2 - Forces In Action ✓

Question 1

What is a free body force diagram?

Answer 1

A diagram showing all the forces acting on a single body

Question 2

Name some common forces acting on objects?

Answer 2

• Weight
• Normal contact force
• Friction
• Tension
• Driving Force

Question 3

Define weight?

Answer 3

Weight is the force acting on a mass due to a gravitational field

Question 4

What is the ‘centre of mass’ of an object?

Answer 4

The centre of mass of an object is the single point you can consider all of its weight to act through

Question 5

What affects how stable an object is?

Answer 5

The height above it’s base of the centre of mass. And the size of the base area.

Higher centre of mass, less stable.
Smaller base area, less stable.

Question 6

How, by knowing an objects centre of mass can you tell if it will topple?

Answer 6

An object will topple if a vertical line drawn downwards from it’s centre of mass falls outside it’s base area.

Question 7

What are the two different types of friction?

Answer 7

• Contact Friction (between solid surfaces)

- Fluid Friction/Drag (acts in a liquid/gas)

Question 8

What is terminal velocity?

Answer 8

Terminal Velocity - The highest velocity attainable by an object, occurring when the sum of frictional forces acting on it are equal to the sum of driving forces acting on it

Question 9

What two requirements are there for an object to reach a terminal; velocity?

Answer 9

1) A driving force that stays the same all the time

2) A frictional/drag force that increases with speed

Question 10

Describe and explain the motion of an object falling from rest through the air.

Answer 10

Stage 1:
The object has no resistive forces acting on (as it is stationary) it and a constant weight providing it with a constant acceleration downwards.
Stage 2:
As the object’s speed increases, the drag force acting in the opposite direction will increase proportionally. This will result in a decreasing resultant force and so a decreasing acceleration.
Stage 3:
When the drag force acting on the object is equal to its weight the object will have reached its terminal velocity, and will continue to fall at constant speed.

Question 11

Explain the motion of a parachutist after they have opened their parachute.

Answer 11

When the parachutist opens his parachute, the air resistance will increase to be greater than his weight. This will decelerate the parachutist until the air resistance has reduced to be equal to his weight again, giving him a new terminal velocity.

Question 12

What is density?

Answer 12

Density is the relationship between the mass of an object and its volume (a measure of how compact an object is)

Question 13

How can you tell if an object will float on a fluid?

Answer 13

An object will float on a fluid if it has a lower density than that fluid. (Same applies for fluids on fluids)

Question 14

Define pressure?

Answer 14

Pressure is the force applied per unit area. It is measured in pascals (Pa) > N/m^2

Question 15

What is the equation for pressure in a liquid?

Answer 15

pressure = depth * density * gravitational field strength

p = hroeg

Question 16

Why do objects experience an upthrust in fluids?

Answer 16

Upthrust is caused by the difference in pressure acting on the top and bottom of a submerged object
(p = hpg) different pressures top and bottom so different forces acting on top/bottom

Question 17

Define upthrust

Answer 17

Upthrust is the upward force that a fluid exerts on a completely or partially submerged object

Question 18

State Archimedes’s principle

Answer 18

Archimedes’ principle - When a body is completely or partially submerged in water it experiences an upthrust equivalent to the weight of the fluid it has displaced

Question 19

How do you calculate moment?

Answer 19

Moment = Force * perpendicular distance form pivot to line of action of the force

Question 20

State the principle of moments

Answer 20

Principle of moments - for a body to be in equilibrium, the sum of clockwise moments about any point equals the sum of anticlockwise moments about that point

Question 21

Lever terminology XD

Answer 21

In a lever, an effort force acts against a load force using a rigid object rotting about a pivot

Question 22

What is a couple?

Answer 22

A couple is a pair of forces of equal size acting parallel to each other but in opposite directions. A couple doesn’t cause any resultant linear force

Question 23

What is torque?

Answer 23

Torque is the resultant turning force produced by a couple (It is a moment but we call it torque)

Question 24

How do you calculate the torque of a couple?

Answer 24

Torque of couple = size of one of the forces * perpendicular distance BETWEEN FORCES (NOT from one force to pivot)

T = F*d

Question 25

What is the unit for moment (and torque)?

Answer 25

Nm

Question 26

What does it mean for an object to be in equilibrium?

Answer 26

An object in equilibrium has no net force or moment acting on it

Question 27

What is interia?

Answer 27

Inertia is the tendency of an object to remain at the same velocity (Its resistance to a change in motion)

Question 28

What three things does fluid friction depend on?

Answer 28

• The thickness of the fluid (viscosity)
• The speed of the object
• The shape/size of the object

Question 29

How do you calculate the upthrust acting on a submerged object?

Answer 29

Calculate the force acting on the top and bottom of the object due to the pressures, the upthrust is the resultant force