9. Dynamics

Question 1

Mass

Definition

Answer 1

Mass is a measure of the amount of matter in a body.

Question 2

Weight

Definition

Answer 2

Weight is the gravitational force acting on an object that has mass.

Question 3

Gravitational Field

Definition

Answer 3

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

Question 4

Graviational Field Strength (g)

Definition

Answer 4

Gravitational field strength (g) is the gravitational force per unit mass placed at that point.

g = weight(N)/mass(kg)

Question 5

Differences between Mass and Weight

Answer 5

Mass is the measure of the amount of matter in a body.
Weight is the gravitational force acting on an object that has mass.

Mass is independent of the gravitational field strength.
Weight is dependent on the gravitational field strength.

Mass is measured with a beam balance or a calibrated electronic balance.
Weight is measured with a spring balance.

Mass is a scalar quantity.
Weight is a vector quantity.

Question 6

Newton’s First Law of Motion

Definition

Answer 6

Newton’s First Law of motion states that every object will continue in its state of rest or uniform motion in a straight line, unless a resultant force acts on it.

Question 7

Newton’s Second Law of Motion

Definition

Answer 7

Newton’s Second Law of motion states that the acceleration of an object is directly proportial to the net force acting on it and is inversely proportional to its mass. The object accelerates in the direction of the net force.

F_net= ma

Question 8

Newton’s Third Law of Motion

Definition

Answer 8

Newton’s Third Law of motion states that if body A exerts a force F_AB on body B, then body B will exert a equal and opposite force F_BA on body A.

Question 9

Inertia

Definition

Answer 9

Inertia of an object refers to the reluctance of the object to change its state of rest or motion, due to its mass.

Question 10

Friction

Definition + What it is affect by and not affected by

Answer 10

Friction is the contact force that opposes or tends to oppose motion between surfaces in contact
- depends on the material(s) in contact and the surface roughness
- proportional to the normal forces pressing the surface together
- independent of the area of contact

Question 11

Positive & Negative Effects of Friction & How to Reduce Negative Effects

Answer 11

Positive:
- To walk
- Use brakes in vehicles
- Prevents skidding on wet roads
- Hold a pen and pencil

Negative:
- Wear and tear
- Unwanted slowing down or stopping of motion
- Generates unwanted heat

How to reduce negative effects:
- Use smooth and polished surfaces
- Use wheels and rollers
- Use lubricants
- Adopt a streamline shape

Question 12

Describe the motion of a skydiver as he slides out from a plane, then opens his parachute some time later with reference to acceleration and terminal velocity

Answer 12

1. When the skydiver slides out from the plane, the only force acting on him is gravitational force (weight). Initial downward acceleration is uniform at 10 m/s².
2. As he falls, air resistance opposing his motion starts to increase. Net force downards is decreased. Hence, he falls downward at a decreasing acceleration.
3. Air resistance continues to increase until it is equal to his weight. At this point, there is no net force, and thus zero acceleration. He falls at terminal velocity subsequently.
4. The skydiver opens his parachute. The large surface area of the parachute gives rise to a larger air resistence. The net force is upwards, and the skydiver decelerates.
5. The skydiver continues to slow down. Air resistance decreases until it is equal to his weight. A lower terminal verlocity is reached, and the skydiver continues to the ground at this constant velocity.

Question 13

Factors Affecting Terminal Velocity

Answer 13

1. Weight
   - The object with a larger weight needs a larger air resistance to balance its weight.
   - Thus, it accelerates for a longer duration and reaches a larger terminal velocity.
   - Reaches the ground in a shorter time (for the same height) due to its larger average speed
2. Surface area
   - The object that has a larger surface area traps more air molecules underneath as it falls, hence air resistance increases at a faster rate
   - Final air resistance is equal for both cases as weight is equal
   - Thus, it accelerates for a shorter duration and reaches a smaller terminal velocity
   - Reaches the ground in a longer time (for the same height) due to its smaller average speed