4 : Forces in action

Question 1

net force

Answer 1

• a resultant force acting on a object will make an object accelerate in the direction of the force
• f = ma

Question 2

weight of an object

Answer 2

• force experienced by a mass due to a gravitational field
• W = mg

Question 3

normal contact force

Answer 3

• an object exerts a force on a surface
• the surface exerts an equal and opposite force on the object
• acts perpendicular to the surface

Question 4

equilibrium

Answer 4

-forces on body are balanced

Question 5

tension

Answer 5

• if string is pulled tight
• force pulling equally on the objects at either end of the string

Question 6

Friction

Answer 6

• acting opposite direction of motion

Question 7

mass

Answer 7

• an amount of matter
• the greater mass the greater the resistance to a change in velocity

Question 8

centre of mass

Answer 8

• the single point the whole weight of object acts through
• object will balance around this point

Question 9

centre of mass and balance

Answer 9

• stable if it has low centre of mass and wide base area and lest sable vice versa
• an object will topple over f a vertical line drawn downwards from centre of mass fals outside its base area

Question 10

drag

Answer 10

• the resistive force on an object travelling through a fluid (air or water)
• the same as friction

Question 11

upthrust

Answer 11

• an upwards buoyancy force acting on an object when it is in a fluid

Question 12

frictional forces

Answer 12

• act in opposite directiont o the motion of the object
• convert kinetic energy into heat

Question 13

contact friction

Answer 13

• between solid surfaces

Question 14

fluid friction

Answer 14

• drag
• depends on viscosity of fluid
  increases as speed increases (directly proportional)
• depends on shape and size of pbject as well (larger area greater resistive force)

Question 15

terminal velocity

Answer 15

• driving force that stays the same all the time
• frictional force tht increases with speed
• resistive force is equal to driving force
• no resultant force and no acceleration
• so constant velocity

Question 16

Skydiver question

Answer 16

• skydiver leaves a plane and will accelerate until the air resistance equals weight
• traveling at terminal velocity
• they will open parachute which increases the air resistance (as surface area increases) so it is now bigger than weight
• this slows him down until speed drops enough for air resistance is equal to his weight
• new terminal velocity is smaller

Question 17

affects terminal velocity

Answer 17

• liquid - tv will be lower in more viscous liquids as drag is greater
• size - large surface area tv will be lower
• shape - drag force is smaller on moe streamlined shapes
• mass - heavier objects reach a faster tv as greater drag force is needed to balance extra weight

Question 18

density

Answer 18

• d = m/v
• Kg m^ -3
• mass per unit volume
  density of water is 1g cm^ -3

Question 19

floating

Answer 19

• density of an object determines whether it floats
• float if the fluid has a lower density than the object

Question 20

Pressure

Answer 20

• amount of force applied per unit area
• Pa (N m^ -2)
• the extra pressure acting n the object due to a fluid depends on the depth of the object the density and the acceleration due to free fall

Question 21

bodies experiencing upthrust

Answer 21

• upwards force that fluid exert on objects that are completely or partially submerge in a fluid
• top and bottom of object are at different depths
• create a pressure difference which causes upthrust
• Axpg
• x = distance between top and bottom surface

Question 22

Archimedes principle

Answer 22

• upthrust is equal to the weight of the fluid it has displaced
• (h2 - h1)pgA

Question 23

Moment of a force

Answer 23

• Nm
• force x perpendicular distance from the pivot to the line of action of the force
• f * x

Question 24

principle of moments

Answer 24

• in equilibrium
• sum of the clockwise moments equals the sum of anticlockwise moments

Question 25

lever

Answer 25

an effort force acts against a load force rotating around a pivot

Question 26

a couple

Answer 26

• a pair of forces of equal size which act parallel to each other but in opposite directions
• produces a turning force

Question 27

torque of a couple

Answer 27

• Nm
• size of one of the forces x perpendicular distance between the forces