3.2 Forces in Action

Question 1

Define what mass is

Answer 1

Mass, which is a physical property of an object, is the amount of matter an object contains

Question 2

Describe what happens when a resultant force acts on an object

Answer 2

When a resultant (net) force is acting on an object, it will make the object accelerate in the direction of the net force

Question 3

What is the equation linking the mass, acceleration, and net (resultant force)?

Answer 3

F = ma
- Where Net Force (F) is measured in Newtons (N)
- Mass (m) is measured in kg
- Acceleration is measured in ms⁻²

Question 4

Using the formula F = ma, what happens if we give an object of 1kg mass an acceleration of 1ms⁻²?

Answer 4

Using F = ma, if we give an object of 1kg mass, an acceleration of 1ms⁻², then we say that a force of 1 Newton has been applied to it

Question 5

Define what weight is

Answer 5

Weight is defined as the gravitational force acting upon an object

Question 6

What is g?

Answer 6

g is the acceleration an object in free fall will have, which is 9.81ms⁻² on Earth, but can be different on other planets

Question 7

What formula can we use to determine the weight of an object?

Answer 7

W = mg, where:
- Weight (W) is measured in (N) Newtons
- (M) Mass is measured in kg
- (g) is the acceleration of an object in free fall on Earth - 9.81ms⁻²

Question 8

How do we derive the weight equation W = mg ?

Answer 8

We can derive the formula W = mg from F = ma, where:
- The Force is the Weight
- The acceleration is

Question 9

Describe what Tension is

Answer 9

Tension is the force within a stretched cable or rope

Tip: Tension acts away from the mass

Question 10

Describe what Normal Contact Force is

Answer 10

Normal contact force is the force arising when one object rests against another object. It acts at a 90° angle to the plane of contact.

Question 11

What is friction

Answer 11

Friction is the force that arises when two surfaces rub against each other

Question 12

What are free body diagrams used for?

Answer 12

Free body diagreams are used to model all of the forces that are acting on an object

Question 13

What are the rules for drawing free body diagrams

Answer 13

→ Each force vector is represented by an arrow labelled with the force it represents
→ Each arrow is drawn to scale (the longer the arrow, the greater the force)

Question 14

Do questions on free body diagrams on a slope (Page 52)

Answer 14

Tip: It is often easier to resolve the forces in to the component acting parallel to the slope, and the component acting perpendicular to the slope, rather than horizontally and vertically

Question 15

What are fluids?

Answer 15

Fluids are substances that can flow - such as gases and liquids

Question 16

What is drag?

Answer 16

Drag is the frictional force that opposes the motion of an object travelling through a fluid

Remember, air is also a fluid

In air, it is called air resistance

Question 17

What are the factors that affect the magnitude of the drag force on an object?

Answer 17

The 2 most important factors that affect the magnitude of the drag force:
- The speed of the object
- The cross-sectional area of the object

The other factors that affect the magnitude of the drag force on an object:
- Speed of the object
- Shape of the object
- Roughness or texture of the object
- The density of the fluid through which the object travels

Question 18

What happens to the drag force for an object with a large cross-sectional area

Answer 18

Objects with a large cross-sectional area experience a greater drag force

Question 19

Describe the relationship between the speed of an object travelling through a fluid, and its drag force

Answer 19

For an object travelling through a fluid, including air, the drag force is directly proportional to the square of speed

Question 20

Describe the motion of an object falling through air or any fluid

Answer 20

• ## During a vertical fall through air or another fluid, the weight of the object remains constant but the speed increases due to acceleration (F = ma). This in turn results in an increase of the drag force
• At the instant an object starts to fall, there is no drag force on the object. The total force is equal to the weight ( W = mg )
• The acceleration of the object is g, the acceleration of free fall
• As the object falls, its speed increases and this in turn increases the magnitude of the opposing drag force
• The resultant (net) force on the object decreases and the instantaneuous acceleration of the object becomes less than g
• Eventually, the object reaches terminal velocity - when the drag force on the object is opposite and equal to its weight.
• At terminal velocity, the object has zero acceleration and zero net force, and its speed is constant. It has reached terminal velocity
• The object will no longer accelerate and will remain at this acceleration for the duration of the fall before reaching the ground

Tip: you can also think of g as the gravitational field strength being the force acting on the object

Question 21

Terminal Velocity Diagram

Answer 21

Question 22

Define the moment of a force

Answer 22

The moment of a force can be described as the __turning effect__ of a force about some axis or point

Question 23

What is the formula to find a moment?

Answer 23

moment = force x perpendicular distance of the line of action of a force from the axis or point of rotation

moment = Fx

• Moment is in N m
• Force is in N
• Distance is in m

Question 24

Introduction to couples

Answer 24

Question 25

Describe what a couple is

Answer 25

A couple is a pair of equal and opposite forces that act on the same plane in parallel with each other but along __different lines__ of the plane/body, in order to produce a rotational force only (no translational motion)

Question 26

What is a torque?

Answer 26

A torque is known as the total moment of a couple

Question 27

How do we work out the torque of a couple

Answer 27

torque of a couple = one of the forces x perpendicular SEPARATION between the forces

Question 28

Describe what the centre of mass is

Answer 28

The centre of mass of is the imaginary point where the entire weight of an object appears to act

Question 29

What is the centre of gravity?

Answer 29

The centre of gravity is the point where the resultant gravitational force of an object (W - its total weight) will act.

Question 30

Describe a similiarty between the centre of mass and centre of gravity

Answer 30

The centre of gravity and centre of mass coincide at the same point

In other words they are in the same location

Question 31

Describe what density is

Answer 31

The density of a substance is defined as it mass per unit volume

Question 32

What is the formula for density

Answer 32

ρ = m/v

where m is mass in kg and V is volume in m⁻³

Question 33

How do we determine the density of an object

Answer 33

In order to determine the density of an object, we need to work out its mass and volume:

• To measure the mass, a digital balance can be used
• To measure the volume:
  1. For a liquid, a measuring cyclinder can be used to determine the volume
  2. For a regular shaped solid, measurements can be taken with a ruler to work out the volume
  3. For an irregular shaped solid, the volume can be determined by the displacement of a liquid and its change in volume when an object is fully submerged into it

Question 34

What is pressure?

Answer 34

Pressure is the normal force exerted per unit cross-sectional area

Question 35

Give the equation for pressure

Answer 35

P = F / A
pressure = force / cross-sectional area

• Pressure is in Nm⁻² or Pa (Pascals)
• Force is in N
• Area is in m⁻²

Question 36

How do fluids (gases and liquids) exert pressure?

Answer 36

Gases, such as air, exert pressure on surfaces because of the constant bombardment by their molecules. Liquids also exert pressure for the same reason.

Question 37

Describe what happens to an object immersed in a column of fluid

Answer 37

An object immersed in a column of fluid experiences pressure due to the weight of the fluid

Question 38

Why does pressure increase with depth in a fluid?

Answer 38

Pressure increases with depth in a fluid because of the force exerted by the increased weight of the fluid above

Question 39

Describe the pressure around the atmosphere of the Earth

Answer 39

The pressure exerted by the atmosphere of the Earth varies with altitude.
The more depth there is, the more weight there is, so the pressure increases as the altitude goes down

Question 40

How do we calculate the pressure p, exerted by a vertical column of any liquid

Answer 40

We can calculate the pressure p, exerted by a vertical column of any liquid from its weight and the cross-sectional area of the base, using the formula:

p = h ρ g
- Where h is the height of the vertical column
- ρ is the density of the liquid
- g is the acceleration of free fall (9.81ms⁻²)
——————————–
1. This is derived from the pressure at the base using the formula pressure = force / area
2. The only force is the weight, so you can change it to pressure = weight / area
3. The weight is equal to the mass of the column x g (W = mg), so p = mg / a
4. The mass of the column can be substituted with the density x the volume as ρ = m/v.
5. So p = ρ v g / a
6. The volume v can be substituted with volume = area x h. So p = ρ a h g / a
7. So p = h ρ g

Question 41

Give an unusual feature about the pressure in a fluid

Answer 41

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

Question 42

Define what Upthrust is

Answer 42

Upthrust is an upward buoyancy force acting on an object when it is submerged in a fluid.

This happens because the pressure at the bottom surface of the object is greater than at its top surface.

Definition: buoyancy - the ability or tendency of something to float in water or other fluid.

Question 43

Describe how upthrust works on this submerged wooden block

Answer 43

Upthrust happens because the pressure at the bottom surface of the object is greater than at its top surface

Question 44

What does Archimedes’ principle state?

Answer 44

The upthrust exerted on a body immersed in a fluid, whether partially or fully submerged, is equal to the weight of the fluid that the body displaces

Question 45

When does an object sink, and when does it float?

Answer 45

An object will sink if the upthrust is less than the weight of the object

For a floating object, the upthrust must equal to the weight of the object. This in turn means that the weight of the object must be equal to the weight of the fluid it displaces - due to Archimedes’s principle.