P3.2 Using Physics to Make Things Work

Question 1

What is the centre of mass of an object?

Answer 1

The point at which all the mass of the object is concentrated

Question 2

What acts through the centre of mass of an object?

Answer 2

Its weight

Question 3

What is a freely suspended object?

Answer 3

An object that rotate freely around one point of suspension

Question 4

When a freely suspended object is at rest, where will the centre of mass be in comparison to the point of suspension?

Answer 4

When at rest, the centre of mass of the object will be vertically below the point of suspension

Question 5

How do you find the centre of mass of an object?

Answer 5

Drawing lines according to the plumb line - the point where the two lines meet is the centre of mass

Question 6

When does an object which is freely suspended swing?

Answer 6

When the object’s centre of mass isn’t vertically in line with its suspension point

Question 7

What is the time period of a pendulum?

Answer 7

The time taken for the pendulum to swing from one side to the other and back

Question 8

Why are pendulums used in clocks?

Answer 8

Because their time periods are always the same

Question 9

What is the formula for working out a time period?

Answer 9

T = 1 / f ==> Time period in s = 1 / Frequency in Hz

Question 10

How does the length of a pendulum affect its time period?

Answer 10

The longer the pendulum, the greater the time period

Question 11

What is the length of a pendulum?

Answer 11

The distance from its suspension point to its centre of mass

Question 12

What are three uses of pendulums?

Answer 12

Clocks, playground swings and fairground rides

Question 13

What is a moment?

Answer 13

The turning effect of a force

Question 14

What is the line of action of a force?

Answer 14

The straight line passing through the point at which F acts and in the same direction as F

Question 15

What is the equation for finding a moment?

Answer 15

M = F x d ==> Moment of force in Nm = Force in N x Perpendicular distance in m

Question 16

What is the perpendicular distance?

Answer 16

A line from the line of action of the force to the pivot in m

Question 17

How do levers ‘multiply the force’ put into them?

Answer 17

By increasing the distance from the pivot at which the force is applied

Question 18

What are two examples of levers?

Answer 18

Wheelbarrows and scissors

Question 19

When doesn’t an object turn/when is an object balanced?

Answer 19

When the total anticlockwise moments on an object about any pivot are equal to the total clockwise moments, the object won’t turn

Question 20

For a balanced object, how do the anticlockwise/clockwise moments relate?

Answer 20

Total anticlockwise moment(s) = Total clockwise moment(s)

Question 21

What do the most stable objects have? (2)

Answer 21

A wide base and a low centre of mass

Question 22

When will an object tip over?

Answer 22

When the line of action of its centre of mass moves beyond the edge of its base

Question 23

How can you use moments to describe how an object topples over?

Answer 23

Once the line of action is beyond the edge of an objects base, the base acts as a pivot and a resultant moment causes the object to rotate around it (topple over)

Question 24

How can you prevent objects from toppling over? (2)

Answer 24

By giving them a heavy round base

Question 25

How does a heavy base help to stop an object from toppling over?

Answer 25

The centre of mass is closer to the ground

Question 26

How does a round base help to stop an object from toppling over?

Answer 26

The outer edges of the base (the pivot) moves away from the centre of mass when you push it (as the object rolls) basically giving it a wider base

Question 27

What does a liquid being incompressible mean?

Answer 27

You can’t squash it, the volume and density stay the same

Question 28

What happens when a force is applied to one point in a liquid? Why?

Answer 28

Because they are incompressible, a force applied to one point in the liquid will be transmitted to other points in the liquid

Question 29

How is pressure transmitted throughout a liquid?

Answer 29

Pressure in a liquid is transmitted equally in all directions

Question 30

How do you calculate the pressure caused by a force?

Answer 30

P = F/A ==> Pressure in Pa = Force in N / Area in m²

Question 31

Why is the force in a hydraulic system multiplied?

Answer 31

The pressure is equal at both pistons (the slave and master piston) as it contains a liquid, and due to the second piston having a larger cross-sectional area, it produces a greater force

Question 32

What are hydraulic systems used in? (3)

Answer 32

Car breaking systems, hydraulic car jacks, and the manufacturing/deployment of landing gear on some aircraft

Question 33

What is velocity?

Answer 33

Both the speed and direction of an object

Question 34

What happens when an object is travelling in a circle?

Answer 34

It is constantly changing direction

Question 35

Why does an object when travelling in a circle?

Answer 35

Because, even if its speed isn’t changing, its velocity is constantly changing

Question 36

In what direction does an object accelerate towards when going round a circle?

Answer 36

It accelerates towards the centre of the circle

Question 37

What is the centripetal force?

Answer 37

When an object travels around a circle, the centripetal force is a resultant force that is acting on an object which causes it to accelerate towards the centre of the circle

Question 38

Which 3 forces provides the centripetal force in a given situation?

Answer 38

It can be tension, friction or gravity

Question 39

When will friction be the force causing the centripetal force acting on an object?

Answer 39

E.g. a car around a bend - the centripetal force comes from the friction between the car’s tyres and the road

Question 40

When will tension be the force causing the centripetal force acting on an object?

Answer 40

E.g. a bucket whirling around on a rope - the centripetal force comes from the tension in the rope

Question 41

How does speed affect centripetal force?

Answer 41

The faster an object’s moving, the bigger the centripetal force has to be to keep it moving in a circle

Question 42

How does mass affect centripetal force

Answer 42

The larger the mass, the bigger the centripetal force has to be to keep it moving in a circle

Question 43

How does the radius of the circle affect centripetal force

Answer 43

The smaller the circle it’s moving in, the bigger the centripetal force has to be to keep it moving in a circle (as it has more ‘turning’ to do)