3.2 Forces in action

Question 1

What is the equation for net force?

Answer 1

Net force = mass x acceleration (F = ma)

Question 2

What is the definition of 1 Newton?

Answer 2

The force needed to make a mass of 1 kg accelerate by 1 ms^-1.

Question 3

What is the equation for weight?

Answer 3

Weight = mass x acceleration of free fall (W = mg)

Question 4

What are the four fundamental forces?

Answer 4

Gravitational forces, electromagnetic forces, strong force, weak force.

Question 5

What is Newton’s First Law?

Answer 5

An object will travel at the same velocity unless a net force acts on it.

Question 6

What is Newton’s Second Law?

Answer 6

Force is proportional to the rate of change of momentum.

Question 7

What is Newton’s Third Law?

Answer 7

If object A exerts a force, F, onto object B, object B exerts a force, -F, onto object A (equal in magnitude and opposite in direction).

Question 8

What are features of dynamics?

Answer 8

Net F = ma, in motion.

Question 9

What are features of statics?

Answer 9

Net F = 0 (also at constant velocity), stationary.

Question 10

What is tension?

Answer 10

Force in string, cable, rope, etc which acts along it.

Question 11

What is normal reaction force?

Answer 11

Force that acts perpendicular to the point of contact of a body and the surface with which it is in contact.

Question 12

What is friction?

Answer 12

Force that occurs between two surfaces in contact with each other, caused by inter-atomic and inter-molecular forces, acts in the opposite direction to the direction the object is moving in or trying to move in.

Question 13

What is upthrust?

Answer 13

Force that a fluid exerts on a body floating in it due to the water displaced, equal to the weight of the fluid displaced, always acts up.

Question 14

What is drag?

Answer 14

A combination of friction and air resistance (or water resistance).

Question 15

What does a plateau in a velocity time graph mean?

Answer 15

The object has reached its terminal velocity.

Question 16

What is centre of mass?

Answer 16

The point at which the mass appears to act.

Question 17

What is the centre of gravity?

Answer 17

The point at which the weight appears to act.

Question 18

What is the moment of a force?

Answer 18

The product of a force and the perpendicular distance of its line of action from the point or axis.

Question 19

What is the equation for the moment of a force?

Answer 19

Moment = force x perpendicular distance

Question 20

What are moments measured in?

Answer 20

Newton-metres (Nm)

Question 21

What is the principle of moments?

Answer 21

When a body is in equilibrium, there is no net moment.

Question 22

What are the two conditions of equilibrium?

Answer 22

No net force and no net moment.

Question 23

What is the equation for pressure?

Answer 23

Pressure = force / area (P = F/A)

Question 24

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

Answer 24

Pressure = density x gravitational field strength x height of column (P = ρxgxh)

Question 25

What is the equation for density?

Answer 25

Density = mass / volume (ρ = m/V)

Question 26

How do you calculate density?

Answer 26

Find mass using a balance, find volume by LxWxH or measuring diameter and using 4/3πr^3 for a sphere, or using a water displacement can for irregular objects.

Question 27

What is the definition of density?

Answer 27

Mass per unit volume.

Question 28

What is a resultant force?

Answer 28

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

Question 29

What is the weak interaction force responsible for?

Answer 29

The process of radioactive decay.

Question 30

What is the strong interaction force responsible for?

Answer 30

Holding subatomic particles together in the nucleus of atoms.

Question 31

What are three examples of non-contact forces?

Answer 31

Gravitational, magnetic, and electrostatic forces.

Question 32

What are five examples of contact forces?

Answer 32

Tension, normal contact force, upthrust, friction and drag.

Question 33

What two upwards forces will a body falling through any liquid experience?

Answer 33

Drag and upthrust.

Question 34

What should a free body diagram contain?

Answer 34

The body on which the forces act, the direction of application of each force, the type of each force, the size of each force.

Question 35

What do most frictional forces not depend on?

Answer 35

The velocity of the object.

Question 36

What frictional force does depend on velocity?

Answer 36

The frictional force of drag that an object experiences when moving through a fluid.

Question 37

How can the force of drag be calculated?

Answer 37

Force of drag = 0.5 x cross-sectional area of the moving object x the density of the fluid x the coefficient of drag x (the velocity of the moving object)^2 (Fd = 1/2 x ρ x Cd x A x(v)^2

Question 38

What is the force of drag directly proportional to?

Answer 38

Velocity squared.

Question 39

How does a falling object reach terminal velocity?

Answer 39

Initially, the downwards gravitational force is greater than the resistive forces, so the object accelerated, causing the drag on it to increase, eventually drag becomes equal to weight and the resultant force on the object is zero, so falls at a constant velocity (terminal velocity).

Question 40

What are the two ways the resultant of two forces acting on a body can be worked out?

Answer 40

By scale drawing, by calculation from a right angled vector triangle.

Question 41

What is torque due to a couple?

Answer 41

the product of one of the forces and the perpendicular distance between them.

Question 42

When does a couple occur?

Answer 42

When two equal anti-parallel (parallel but acting in opposite directions) forces act to produce a rotation, no linear motion occurs.