Module 3.2 - Forces in Action

Question 1

What is the resultant force?

Answer 1

A single force that which has the same effect as the sum of all the forces acting on a body.

Question 2

What does one newton mean?

Answer 2

The force that causes a mass of 1kg to have an acceleration of 1ms^-2.

Question 3

How do you work out net force?

Answer 3

F = ma

Question 4

How do you work out the weight of an object?

Answer 4

W = mg

Question 5

Define tension

Answer 5

Force experienced by any rope, string, cable or wire that is being pulled, hung, rotated or supported

Question 6

Define normal contact

Answer 6

Force that acts perpendicular to the point of contact of a body

Question 7

Define upthrust

Answer 7

Upwards force that a liquid/gas exerts on a body floating on it.

Question 8

Define friction

Answer 8

Force between two surfaces in contact, it opposes the motion at the point of contact.

Question 9

Define drag

Answer 9

Frictional force experienced by an object travelling through a fluid. Opposite to motion.

Question 10

What factors affect drag?

Answer 10

Density and Cross-sectional area

Question 11

Describe the motion of objects falling in a uniform gravitational field in the presence of drag

Answer 11

> Initially, the object accelerates because the downwards gravitational force is greater than the resistive force
However, the drag increases as the object accelerates.
Eventually the drag becomes equal to the weight so the resultant force becomes zero - constant velocity

Question 12

Define terminal velocity

Answer 12

The velocity at which an object’s drag equals to its accelerating force. Therefore there is no resultant force and no acceleration.

Question 13

What is equilibrium?

Answer 13

When the resultant force is zero in the same plane

Question 14

What is a triangle of forces?

Answer 14

Represents the direction and magnitude of three coplanar forces that are on an object in equilibrium.

Question 15

What is a moment (Nm) of a force?

Answer 15

Force (N) x Perpendicular distance (m)

Question 16

State the principle of moments

Answer 16

The sum of CLOCKWISE moments = the sum of ANTI-CLOCKWISE moments for the object to be in rotational equilibrium.

Question 17

What is a couple?

Answer 17

When two equal and anti parallel forces act to produce a rotation - no linear motion occurs.

Question 18

What is the moment/turning effect of a couple called?

Answer 18

A torque

Question 19

Define the centre of mass

Answer 19

Single point at which ALL of the mass of an object can be assumed to be situated - usually the centre of objects with a constant density.

Question 20

What is the centre of gravity?

Answer 20

Single point through which the entire weight of the object can be thought to act.

Question 21

How do you work out density?

Answer 21

Density (kg m^-3) = mass (kg) ÷ volume (m^3)

Question 22

How do you work out pressure for solids, liquids and gases?

Answer 22

p = F ÷ A

Question 23

Why does pressure increase with depth in a fluid?

Answer 23

Due to the force exerted by the increased weight of the fluid above.

Question 24

How do you work out the pressure of a fluid at depth?

Answer 24

p = depth x density of fluid x g

Question 25

What is Archimedes’ principle?

Answer 25

UPTHRUST of an object in a fluid equals to the weight of the water displaced.

Question 26

How do you work out the upthrust of a fluid?

Answer 26

Due to Archimedes principles where Upthrust = weight displaced:

Upthrust = pressure x area

             = (depth x density x g) x area of object