Unit 3 Dynamics Things to Master

Question 1

What are the contact forces and non-contact forces?

Answer 1

There are 5 contact forces:
- friction
- air resistance
- normal force
- tension
- upthrust

There are three non-contact forces
- weight
- electrostatic force
- magnetic force

Question 2

What is Newton’s first law of motion?

Answer 2

Newton’s first law of motion states that an object at rest will remain at rest and an object in motion will continue in motion at constant velocity in the absence of a resultant force acting on it.

Question 3

What is Newton’s second law of motion?

Answer 3

Newton’s second law of motion states that the rate of change of momentum of a body is directly proportional to the resultant force acting on it and the direction of the resultant force is in the direction of the momentum

Question 4

What is Newton’s third law of motion?

Answer 4

If object A exerts a force on object B, then object B exerts an equal and opposite force on object A

Question 5

What conditions must an action-reaction pair of forces satisfy?

Answer 5

Same type
Equal in magnitude
Act in opposite directions
Act on different bodies

Question 6

What is the definition of mass?

Answer 6

Mass is a measure of the inertia of an object, which is the resistance of a body to change its state of rest or motion.

Question 7

What is the definition of weight?

Answer 7

A gravitational field is a region in which a mass experiences a force due to gravitational attraction.

Question 8

What is the definition of gravitational field strength?

Answer 8

Gravitational field strength, g, is gravitational force per unit mass placed at that point.

Question 9

What is the definition of terminal velocity?

Answer 9

The speed of a falling object increases due to a downward acceleration. When air resistance is not negligible, the falling object also experiences an increasing resistive force. Eventually, the amount of air resistance becomes the same as the weight of the object, and the velocity of the object ceases to increase. The final velocity attained is known as the terminal velocity.

Question 10

What are the positive effects of friction?

Answer 10

1. Walking
2. Holding an object
3. Braking

Of course, elaboration is required.

Question 11

What are the negative effects of friction?

Answer 11

1. Loss in useful energy
2. Generate heat (where heat is not desired)
3. Wear and tear

Question 12

What are the methods to reduce friction?

Answer 12

1. Ball bearings
2. Lubricants
3. Air layer

Question 13

Does friction occur when an object is not moving? What type of friction is this?

Answer 13

Yes. Friction can occur even when the two surfaces are not moving relative to each other. For example, imagine you are exerting a small force on a heavy stationary box. Initially, when the force applied on the box is small, the box remains stationary. Thus, the friction is equal to the applied force. This is called the static friction.

However, if you increase the applied force on a heavy box, the box will eventually start moving.

Question 14

What is kinetic friction?

Answer 14

Kinetic friction occurs when the two surfaces are moving relative to each other. This value is constant regardless of the velocity of the surface and it is smaller in magnitude than the static friction.

Question 15

What does friction depend on and what is it not affected by?

Answer 15

friction depends on
- the nature of the surfaces in contact
- how lightly the surfaces are pressed together

Friction is not affected by the area of the surfaces in contact