Chapter 4-Forces in motion Definitions

Question 1

Force

Answer 1

A push/pull on an object
units: N

F=ma

Question 2

Weight

Answer 2

The gravitational force on an object
units: N

W=mg

Question 3

Centre of mass

Answer 3

A point through which any externally applied force produces straight line motion but no rotation

Question 4

Centre of gravity

Answer 4

An imaginary point at which the entire weight of an object appears to act

(can find COM using plumline experiment)

Question 5

Plum-line

Answer 5

A string with a weight used to provide a vertical reference line

Question 6

Free-body diagram

Answer 6

A diagram that represents the forces acting on a single object

Question 7

Normal contact force (R)

Answer 7

The force exerted by a surface on an object, which acts perpendicularly to the surface.

units: N

Question 8

Friction

Answer 8

The force that arises when two surfaces rub against each other

acting in opposite direction to motion
( if object on rough surface)
f= mui* R

Question 9

Drag

Answer 9

the resistive force on an object travelling though a fluid (same as friction)

depends on: -speed, shape and texture of object
-density of fluid travelling through
larger cross-sectional area–> greater drag

(drag) directly proportional to (speed^2)

in air–> drag= air resistance

Question 10

Tension

Answer 10

The force within a stretched cable or rope

Question 11

Upthrust

Answer 11

an upwards buoyancy force act8ng on an object when it is in a fluid

Question 12

Terminal velocity

Answer 12

The constant speed reached by an object when drag force( and upthrust) is equal and opposite to weight of object.

acceleration =0
resultant force=0
drag=weight

Question 13

Moment

Answer 13

= force* perpendicular distance of the line of action. of force from axis or point of rotation
=Fd
units:Nm

Question 14

PRINCIPLE OF MOMENTS

Answer 14

For a body in rotational equilibrium, the sum of anticlockwise moments about a point= sum of clockwise moments about same point.

Question 15

Couple

Answer 15

A pair of equal and opposite forces acting on a body but not in same straight line

Question 16

Torque (of a couple)

Answer 16

The product of one of the forces of a couple and the perpendicular distance between the forces

Torque= Fd

Question 17

Triangle of forces

Answer 17

Three forces acting on a point in equilibrium, represented by sides of triangle

resolve horizontally and vertically

Question 18

Density

Answer 18

The mass per unit volume of a substance
units: kgm-3

density= m/V

(experiment to determine density)

Question 19

Fluid

Answer 19

A substance that can flow, including liquids and gases

Question 20

Pressure

Answer 20

The force exerted per unit cross-sectional area
units: Pascals ( Pa) or Nm-2

P=F/A

in fluids–> P=hdensity9.81

Question 21

ARCHIMEDES’ PRINCIPLE

Answer 21

The upthrust on an object in a fluid is equal to the weight of fluid it displaces.

upthrustobject will SINK
upthrust>weight –>object will FLOAT