4 - Forces in action

Question 1

What is mass?

Answer 1

A physical property of an object which describes how much matter it contains.

Question 2

Define net force.

Answer 2

The sum of all of the forces acting on an object.

Question 3

What does a net force cause?

Answer 3

Acceleration in the direction of the force.

Question 4

What is weight?

Answer 4

The gravitational force acting on an object.

Question 5

Explain ‘centre of mass’.

Answer 5

A point in/on/around an object through which its entire weight can be thought to act.

Question 6

How would you carry out an experiment to determine an object’s centre of mass?

Answer 6

• Hang object - free to spin
• Hang plumb line and mark line
• Repeat in two other positions
• Point where lines cross is centre of mass.

Question 7

What is each force represented by in a free body diagram?

Answer 7

A labelled arrow, proportional to the size of the force.

Question 8

What is friction?

Answer 8

The force that arises when two surfaces rub against each other.

Question 9

What is drag?

Answer 9

The resistive force acting on a travelling object, such as air resistance on a car.

Question 10

What is tension?

Answer 10

The force within a stretched cable or rope.

Question 11

What is upthrust?

Answer 11

The upwards force exerted on an object floating in liquid or gas.

Question 12

What is normal contact force?

Answer 12

A force arising when one object rests against another object.

Question 13

What is drag proportional to?

Answer 13

velocity^2 x surface area

Question 14

What is the drag force experienced by objects travelling through air commonly called?

Answer 14

Air resistance

Question 15

Define terminal velocity.

Answer 15

When an object stops accelerating and reaches a constant velocity (because its drag is equal to its pushing force; newtons first law of motion).

Question 16

Describe how a falling object reaches terminal velocity.

Answer 16

NOTE: CLARIFY ANSWER
- At release, acceleration is equal to g, downward force = mg, net force = mg

• As object accelerates, velocity increases, downward force = mg, upwards force = D, net force = mg - D
• Eventually, drag becomes equal to weight and so net force = 0, drag cannot increase because speed is no longer increasing

Question 17

What is the only factor that can affect an objects terminal velocity when it is already travelling at terminal velocity?

Answer 17

Surface area

e.g. parachute

Question 18

What is a moment?

Answer 18

The turning effect of a force around a point.

Question 19

How do you calculate a moment?

Answer 19

Moment= Force x distance from pivot

Question 20

What must you always ensure (and sometimes have to calculate) before calculating a moment?

Answer 20

That the force and the distance are perpendicular.

Question 21

What is the principle of moments?

Answer 21

When a body is at rotational equilibrium, the sum of the clockwise moments must be equal to the sum of the anticlockwise moments.

Question 22

What is a couple?

Answer 22

A pair of equal, parallel forces acting in opposite directions.

Question 23

What is torque?

Answer 23

The moment of a couple.

Question 24

How do you calculate torque?

Answer 24

One of the forces x distance between forces.

Question 25

What does coplanar of forces mean?

Answer 25

All the forces lie in the same plane e.g. 2d

Question 26

Define density.

Answer 26

density=mass/volume; mass per unit volume

Question 27

Define pressure.

Answer 27

Normal force exerted per unit area.

Question 28

How could you determine the volume of a irregularly shaped object?

Answer 28

• Fill bucket of water with water up to the brim
• Place the object in the water
• Collect all water displaced by object
• Volume of water collected will equal volume of object; calculate volume of water.

Question 29

What is a fluid?

Answer 29

A substance that flows. Gases or liquids

Question 30

What is the equation for pressure under a column of liquid?

Answer 30

P = hpg
Pressure= height x density x gravitational field strength

Question 31

Does the cross-sectional area of the column of water affect the pressure?

Answer 31

NO

Question 32

What is pressure under water proportional to?

Answer 32

The depth the object is at, h.`

Question 33

In what direction is pressure the same at a certain depth under water?

Answer 33

All directions

Question 34

What is pressure caused by?

[VERIFY]

Answer 34

Particles moving and colliding with objects.

Question 35

What must you always remember to include when calculating pressure underwater?

Answer 35

The pressure exerted by the atmosphere above the water.

Question 36

What is upthrust caused by?

Answer 36

A difference in pressure above and below an object.

Question 37

When will an object float?

Answer 37

If the upthrust force is greater than the weight of the object.

Question 38

How do you calculate upthrust?

Answer 38

F = pxgA

pressure below - pressure above

Pb = pgh + pgx Pa = pgh

P = pgh + pgx - pgh = pgx

p = F/A F = pA

F = pgxA

Question 39

What does Archimedes’ Principle state?

[VERIFY]

Answer 39

The upthrust (bouyant) force exerted on a body is equal to the weight of fluid that the object displaces.

Question 40

What conditions (density) must be met in order for an object to float?

Answer 40

The average density of the object must be less than the density of the fluid.