Capter 3: Dynamics

Question 1

What is mass?

Answer 1

Mass is a measure of the amount of matter in an object.
Consequently, this is the property of an object that resists change in motion.
The greater the mass of a body, the smaller the change produced by an applied force.
The SI unit for mass is the kilogram (kg).
1kg=1000g

…: Cambridge definition

Question 2

What is weight?

Answer 2

Weight is the effect of a gravitational field on a mass. Since it is a force, it is measured in newtons (N) and is a vector quantity.

Question 3

Weight formula

Answer 3

W=mg
g=acceleration of free fall (m/s^2)

The acceleration of freefall, g, on Earth, is 9.81 m/s^2

Question 4

What does it mean by an object falling under free fall?

Answer 4

An object in free fall is falling solely under the influence of gravitational attraction.
In the absence of air resistance, all bodies near the Earth fall with the same acceleration regardless of their mass.

Question 5

State the difference between gravitational field strength and acceleration due to gravity.

Answer 5

Gravitational field strength (in N/kg) and acceleration due to freefall (in m/s^2) are just two ways of describing the same thing.

They have the same value but have different units depending on the context.

Question 6

Explain the difference between mass and weight.

Answer 6

An object’s mass always remains the same, however, its weight will differ depending on the strength of the gravitational field at different locations within the Universe.
For example, the gravitational field strength on the Moon is 1.63 N/kg, meaning an object’s weight will be about 6 times less than on Earth.

Question 7

State the trend between size of plants and their gravitational field strength.

Answer 7

More massive planets and objects have a greater gravitational field strength.

Question 8

Force formula (Newtons 2nd Law)

Answer 8

F=ma

Note: acceleration is always in the same direction as the resultant force.

Question 9

State what is meant by a resultant force.

Answer 9

Since force is a vector quantity, every force on a body has a magnitude and a direction. The resultant force is therefore the vector sum of all the forces acting on the body. The direction of the force is indicated as either positive or negative.

Question 10

Linear momentum formula

Answer 10

p=mv

Question 11

Force definition in terms of momentum

Answer 11

Force is defined as the rate of change of momentum.

F=delta p/ delta t

delta p = p(f)-p(i)

Question 12

Explain the forces acting between a car and a wall upon impact. (Newtons 3rd Law)

Answer 12

The force exerted by the wall on the car will be equal in magnitude and opposite in direction to the force exerted by the car on the wall. F(car)=-F(wall)

Question 13

State Newtons First Law of Motion

Answer 13

A body will remain at rest or move with a constant velocity unless acted on by a resultant force.

Question 14

State what is meant by Newtons 2nd Law of Motion.

Answer 14

A resultant force acting on a body will cause a change in the objects motion in the direction of the force.

The rate of change of momentum is proportional to the magnitude of the force. F=ma

Question 15

State what is meant by Newton’s third law of motion.

Answer 15

When two bodies interact, the forces they exert on each other are equal and opposite.

Question 16

State what a Newton’s third law pair must be. (4 marks)

Answer 16

Equal in magnitude
Opposite in direction
Same type of force
Acting on different objects

Question 17

State what is meant by drag force.

Answer 17

Drag forces are forces axing in the opposite direction to an object moving through a fluid (liquid or gas).

Question 18

What is fluid?

Answer 18

Liquid or Gas

Question 19

State 2 examples of drag forces.

Answer 19

Fluid resistance (viscous drag) and air resistance

Question 21

Explain the driving force and frictional force acting on a car at different accelerations.

Answer 21

Accelerating: driving force > drag force

Constant velocity: driving force = drag force

Decelerating: driving force < drag force

Question 22

Explain a key component of drag forces

Answer 22

The magnitude of the drag force increases with the speed of the object.

Question 23

State 2 factors air resistance depends upon.

Answer 23

Shape of the object and the speed at which it is travelling.

Question 24

Explain how terminal velocity works.

Answer 24

For a body in free fall with no effects of air resistance (for example, on the moon), the only force acting on it is weight. The resultant force is equal to weight. Therefore, the body accelerates at g, acceleration of free fall.

For a body in free fall, when air resistance is a factor (for example on Earth):
Weight is greater than air resistance, so the resultant force is in the direction of motion
The body accelerates according to Newton’s second law, F = ma
As the velocity increases, the drag force increases
The resultant force decreases, therefore the acceleration decreases
When the drag force equals weight, the resultant force is zero
The body falls at a constant velocity called terminal velocity
Terminal velocity is the maximum velocity the body can reach

Question 25

How to draw terminal velocity graph.

Answer 25

Velocity time graph. Gradient decreasing and then constant.

Question 26

State the principle of conservation of linear momentum.

Answer 26

The total linear momentum before a collision is equal to the total linear momentum after a collision unless the system is acted on by a resultant external force. momentum before = momentum after

Question 27

What is an external force?

Answer 27

External forces are forces that act in a structure from outside e.g. friction and weight.

Question 28

What can a system with no external forces acting on it be described asv

Answer 28

A closed or isolated system.

Question 29

What are elastic collisions?

Answer 29

Elastic collisions are commonly those where objects colliding do not stick together and then move in opposite directions.

Question 30

What are inelastic collisions?

Answer 30

Inelastic collisions are where objects collide and stick together after the collision.

Question 31

What is an internal force?

Answer 31

Internal forces are forces exchanged by the particles in the system e.g tension in a string

Question 32

How to find out whether a collision is inelastic or elastic?

Answer 32

Compare the amount of KE before and after the collision. If the kinetic energy is conserved (stays the same), it is an elastic collision. If the kinetic energy is not conserved, it is an inelastic collisions.

Question 33

State kinetic energy formula

Answer 33

K.E. = 1/2mv^2

Question 34

What does KE depend on and explain perfectly elastic collision condition.

Answer 34

Kinetic energy depends on the speed of an object. In a perfectly elastic collision (such as a head-on collision): The relative speed of approach = the relative speed of separation.

Question 35

Define equilibrium

Answer 35

When a body or a system has constant velocity/ zero acceleration/ no resultant force/ no resultant torque.

Question 36

State the 3 forces acting on a hot air balloon

Answer 36

2 tension weight Buoyancy