Chapter 3.2 - Forces in Action

Question 1

Equation for weight

Answer 1

W=mg

Question 2

Tension

Answer 2

A force experienced by a string or wire that is being pulled

Question 3

Normal contact force

Answer 3

A force that acts perpendicular to the point of contact between two surfaces

Question 4

Upthrust

Answer 4

The upward force that a liquid or gas exerts on a body due to the liquid or gas being displaced

Question 5

Friction

Answer 5

A force that occurs between two surfaces that opposes motion. It is a result of intermolecular forces.

Question 6

What you show on a free body diagram (3)

Answer 6

• The bodies
• The directions of each force
• The value of each force

Question 7

Fluid

Answer 7

A liquid or a gas

Question 8

Drag

Answer 8

The frictional force experienced by an object travelling through a fluid

Question 9

Does friction depend on velocity

Answer 9

No they’re independant.

Question 10

Does drag depend on velocity

Answer 10

o yes

Question 11

How is drag related to velocity

Answer 11

D = kv^2

Question 12

Factors affecting drag (4)

Answer 12

• Density of fluid
• Coefficient of drag
• Cross sectional area of object
• Velocity

Question 13

The motion of an object falling in the presence of drag

Answer 13

• Initially accelerates because force of gravity acts downwards and there is minimal drag
• As speed increases the upwards force of drag increases
• Eventually the force of weight is equal to the force of drag and the object no longer accelerates and is moving at its terminal velocity

Question 14

What happens when a parachute is opened while falling at terminal velocity

Answer 14

The drag force increases due to the increased cross sectional area. This results in a net upwards force and therefore an upwards acceleration, which causes the skydiver to slow down. As they slow down the drag force decreases until a new terminal velocity is reached

Question 15

Terminal velocity

Answer 15

The speed of an object at which the weight is equal to the drag

Question 16

Experiment to determine terminal velocity of a body in a fluid

Answer 16

Set up a cylinder filled with fluid and wrap elastic bands around it at regular intervals. Drop a ball of known mass into the tube and time how long it takes to reach each elastic band. Towards the bottom the times should remain constant and this value is used to calculate terminal velocity

Question 17

Equilibrium

Answer 17

The resultant force acting on an object is zero

Question 18

How to find the resultant of two forces

Answer 18

Add them

Question 19

Moment

Answer 19

The turning effect of a force

Question 20

Equation for a moment

Answer 20

moment = force * perpendicular distance

Question 21

Principle of moments

Answer 21

For a body to be in equilibrium, the sum of the clockwise components must equal the sum of the anticlockwise components

Question 22

Couple

Answer 22

Pair of forces of equal size that act in parallel but opposite directions, resulting in a turning force (torque)

Question 23

Equation for torque of a couple

Answer 23

T = (magnitude of one force) * (perpendicular distance between the forces)

Question 24

Weight

Answer 24

Force experienced by a mass due to a gravitational field

Question 25

Centre of mass

Answer 25

A single point that you can consider the entire mass to exist at, regardless of orientation

Question 26

Centre of gravity

Answer 26

A single point through which the entire weight of an object can be thought to act

Question 27

When are the centre of mass and the centre of gravity the same

Answer 27

For a small body in a uniform gravitational field

Question 28

Experiment to find centre of gravity

Answer 28

Hang the object freely from a point and draw a vertical line down it. Repeat from another point. Where these two lines cross will be the centre of gravity

Question 29

Three forces act on an object in equilibrium. What must these forces be?

Answer 29

coplanar (same plane)

Question 30

Density

Answer 30

Mass per unit volume

Question 31

Pressure

Answer 31

Force per unit area

Question 32

Unit of pressure

Answer 32

Pascal (Newtons per metre squared)

Question 33

Pressure in a fluid

Answer 33

p=hρg
p - pressure
h - depth
ρ - density
g - acceleration due to gravity

Question 34

Upthrust

Answer 34

An upward force exerted by a fluid on an object that is fully or partially submerged

Question 35

Archimedes principle

Answer 35

The upthrust is equal to the weight of fluid displaced

Question 36

Equation for upthrust

Answer 36

F = (h2 - h1)ρgA
or
F = sρgA where s is the height of the object

Question 37

Remember

Answer 37

Pressure increases as you go deeper into a fluid but upthrust is the same wherever you are (as long as you are fully submerged)