Chapter 3: Forces

Question 1

What is the definition of mass?

Answer 1

Mass is a measure of matter in a body

Question 2

What are the non-contact forces?

Answer 2

Question 3

What are the contact forces

Answer 3

Question 4

Define and expain ‘mass’

Answer 4

Mass is a measure of the amount of matter in a body:
* It does not change with its location or shape
* Depends on the number and composition of atoms and molecules that make up the body

Mass is a scalar quantity & SI unit is kilogram (kg)

Question 5

Define and explain what is weight

Answer 5

• Weight is the gravitational force acting on an object that has mass
• The direction of weight is downward, i.e. towards the centre of Earth

Weight is a vector quantity, and the SI unit is newton (N)

W = mg

W = weight in newtons
m = mass of object in kilograms (kg)
g = gravitational acceleration (m/s^2)

Question 6

What is a gravitational field?

Answer 6

• A gravitational field is a region in which a mass experiences a force due to gravitational attraction
• For example, Earth has a gravitational field surrounding it. Any object within Earth’s gravitational field will experience a force exerted by Earth
• The gravitational force experienced is the strongest at the surface of Earth. It get weaker further away due to a decreasing gravitational field strength

Question 7

Define ‘contact force’

Answer 7

Contact forces are forces that act when objects are touchoing or in contact with each other

Question 8

Define ‘non-contact force’

Answer 8

Non-contact force are forces that act even when the objects are not touching or in contact with each other.

Question 9

Define and explain what is meant by ‘gravitational field strength’

Answer 9

g can also be expressed as g = m/h

When expressed this way, g is known as the gravitational field strength which tells us how much gravitational force acts on one kilogram of mass placed at that point.

For example, the gravitational field strength of Earth is 10N/kg, meaning every 1kg of mass has a weight of 10N

Question 10

What does Newton’s First Law state

Answer 10

Every object will continue in its state of rest or uniform motion in a straight line unless a resultant force acts on it.

Question 10

Define ‘inertia’

Answer 10

The inertia of an object refers to the reluctance of the object to change its state of rest or motion, due to it’s mass

Question 10

Define what is stated in Newton’s Second Law of Motion

Answer 10

When a resultant/net force acts on an object of a constant mass, the object will accelerate in the direction of the resultant force.

F = ma

Question 11

Define what is stated in Newton’s Third Law of Motion

Answer 11

If body A exerts a F(AB) on body B, then body B will exert an equal and opposite force(BA) on body A.

for every action (force) in nature there is an equal and opposite reaction.

Question 12

What is terminal velocity?

Answer 12

When air resistance acting against an object equals it weight, the object starts to travel at a constant speed known as terminal velocity

Question 13

What is the resultant force, when the forces acting on a body are balanced?

Answer 13

The resultant force is zero

Question 14

Based on Newton’s third law, what are the characteristics of action and reaction forces?

Answer 14

1. Forces always occur in pairs. Each pair is made up of an action
   and a reaction forces.
2. Action and reaction forces are equal in magnitude.
3. Action and reaction forces act in opposite directions.
4. Action and reaction forces act on different bodies.

Each action and reaction pair of forces must be of the same nature

Question 15

What can a force do?

Answer 15

A force can cause a

• body at rest to move;
• moving body to increase in speed;
• moving body to decrease in speed; or
• moving body to change direction.

Question 16

What is friction?

Answer 16

Friction is a contact force which arises when surfaces or bodies are in contact with each other

Question 17

Give examples of when friction is and isn’t useful

Answer 17

Friction is useful when:
* It allows is to walk without slipping
* It allows us to apply brakes to slow down our moving vehicles

Friction is not useful when:
* Moving parts in motors and energ lose energy when they move against friction
* Moving parts experience wear and tear, and have to be replaced regularly

Question 18

Define ‘air resistance’

Answer 18

Air resistance or drag is a form of friction due to an object moving in a fluid (a liquid or a gas)
* When an object moves through a fluid, it collides with the fluid particles and this collision produces a force that opposes the direction of the object’s motion.

This force is called air resistance when the object moves through air; and drag when the object moves through other fluids

Question 19

What are the factors that affect air resistance or drag?

Answer 19

• Speed of the object
  The faster the object moves through the fluid, the larger the air resistance or drag
• Cross-sectional area of the moving object
  In general, the larger the cross-sectional area, the larger the air resistance or drag.
• Density of the fluid
  In general, the higher the density of fluid, the larger the air resistance or drag.

Question 20

Define what is meant by ‘terminal velocity’

Answer 20

• A falling object eventually experiences zero acceleration when the air resistance is equal to the weight of the object
• When this happenes, the object travelxs with a constabnt veclocity called terminal velocity
• The object will continue falling at terminal velocity until it reaches the ground.