Module 3 Dynamics

Question 1

What are the four fundamental forces?

Answer 1

Gravitational force
Electromagnetic force
Strong nuclear force
Weak nuclear force or weak interaction

Question 2

What is weight

Answer 2

the gravitational force acting through a centre of mass

Question 3

What is friction

Answer 3

force that arises when two surfaces rub against each other

Friction always acts in opposite direction to velocity

Question 4

What is drag?

Answer 4

resistive (frictional) force of an object travelling through an fluid

Drag in opposite direction to velocity

Question 5

What is tension

Answer 5

force within a stretched rope or cable

Question 6

What is normal contact force?

Answer 6

the force arising (perpendicular to the surface) when one object rests on another

Question 7

Which factors effects drag?

Answer 7

• Velocity - higher the v higher the drag
• Cross sectional area - more streamlined the object reduces drag (less contact surface area)
• Roughness of the surface area of the moving object
• Viscosity of the fluid

Question 8

What does F refer to in Newton’s 2nd law?

Answer 8

Resultant force (or net force)

Question 9

What are base the unit for Newtons?

Answer 9

kg*m s^-2

Question 10

How to calculate resultant force in newtons?

Answer 10

mass times acceleration

Question 11

What does Newton’s second law in the form F=ma apply to?

Answer 11

objects with constant mass

Question 12

Define a Newton

Answer 12

One Newton is the force that causes a mass of one kilogram to have an acceleration of one metre per second squared

Question 13

When is the equation F=ma no longer applicable?

Answer 13

when an object is travelling near the speed of light

Question 14

What is the effect on the mass of an object travelling near the speed of light?

Answer 14

mass increases

Question 15

Calculate the acceleration, and thus describe the motion of an object with the mass of 80kg in a body of water with the upthrust of 500N

Answer 15

• Weight 80x9.81 = 784.8N
• weight>upthrust therefore resultant force (direction of motion) is vertically down
• U-W = ma
• acceleration = (500-784.8)/80 = -3.56ms^-2
• Both acceleration and velocity are down therefore object is traveling and accelerating down so getting faster

Question 16

What is terminal velocity

Answer 16

when the resultant force of a moving object is 0

so constant velocity

Question 17

When do objects reach terminal velocity when falling?

Answer 17

when the drag force is equal but opposite to weight, there is no acceleration

(ignoring drag but may be included sometimes)

Question 18

What is a word used to describe an object moving at its maximum constant velocity?

Answer 18

terminal velocity

Question 19

How to calculate the resultant force of an accelerating object?

Answer 19

forces in the direction of acceleration - forces in opposite direction

Question 20

When an object is falling at terminal velocity, KE does not change but GPE does, explain what happens to this loss of GPE

Answer 20

GPE is converted to work done against drag as heat

KE constant (constant velocity)

GPE decreases if the object is falling

Question 21

When drag due to air resistance is taken into account, why can suvat equations not be used?

Answer 21

acceleration is not constant

Question 22

Describe the motion of a skydiver who jumps out of a plane and opens a parachute

(see velocity time graph in notes)

Answer 22

• Initially, only weight acts so a=g vertically downwards, no drag
  • Drag increases with increasing speed, W>D acceleration downwards but decreasing
  • Weight = Drag so resultant force = 0N, acceleration = 0. First terminal velocity
  • Parachute opens, large drag force caused by increase in surface area
  • Drag>weight so resultant force upwards, acceleration upwards and velocity downwards but decreasing
  • 2nd lower terminal velocity W=D, RF=0N, a=0

Question 23

Investigation on ball bearings through a viscous fluid

Answer 23

• a glass tube filled with a washing detergent
• insulating tape at equal distances across the tube

Question 24

Re investigating velocity of ball bearings through a viscous fluid

How will the arrangement of the markers on the tube change when placing them at equal time intervals?

Answer 24

will get wider and wider until distance is same as terminal velocity is reached

Question 25

Newton

Answer 25

force required to give a mass of 1kg an acceleration of 1ms-2 in the direction of the force.

Question 26

Upthrust

Answer 26

upward force that acts on a body fully or partially submerged in a liquid or gas.