Flashcards in Using physics to make things work Deck (30):

1

## What is a moment?

### A moment is the turning effect of a force.

2

## What equation gives the size of the moment?

###
M = F x d

M is the moment of the force in newton-metres, Nm

F is the force in newtons, N

d is the perpendicular distance from the line of action of the force to the pivot in metres, m.

3

## What two things can be done to increase the moment?

###
1) Either the force must increase.

2) Or the distance to the pivot must increase.

4

## Why is it easier to undo a wheel-nut by pushing on the end of a long spanner than a short one?

### It is easier because the long spanner increases the distance between the line of action of the force and the pivot.

5

##
Why is a lever used?

When using a lever what is the name of the force that we are trying to move?

What is the force applied to the lever called?

What does the lever do and how does this help the user?

###
We use a lever to make a job easier.

When using a lever the force we are trying to move is called the load and the force applied to the lever is the effort.

A lever acts a force multiplier, so the effort we apply can be much less than the load.

6

## What is the centre of mass?

### The single point where the mass of an object can be thought of as being concentrated.

7

##
What will happen to any object that is freely suspended?

What is the object in at this point?

### Any object that is freely suspended will come to rest with its centre of mass directly below the point of suspension, the object is then in equilibrium.

8

## How can the centre of mass of a thin irregular sheet of material be found? (5)

###
1) Suspend the thin sheet from a pin held in a clamp stand, because it is freely suspended it is able to turn.

2) When it comes to rest, hang a plumbline from the same pin.

3) Mark the position of the plumbline against the sheet.

4) Hang the sheet with the pin at another point and repeat the procedure.

5) The centre of mass is where the lines that marked the position of the plumbline cross.

9

##
On what does the centre of mass depend and where can it sometimes be?

For a symmetrical object where is the centre of mass?

What if object has more than one axis of symmetry?

###
The centre of mass depends on the shape of the object and can sometimes lie outside the object.

For a symmetrical object, its centre of mass is along the axis of symmetry.

If the object has more than one axis of symmetry, the centre of mass is where the axes of symmetry meet.

10

## What can be said about an object that is in equilibrium? (3)

###
1) It is balanced.

2) It is not turning.

3) Total clockwise moment and total anti-clockwise moment are equal.

11

## Give two everyday examples of the principle of moments.

###
1) Seesaws

2) Balance scales

12

##
Through what does the line of action of the weight of an object act?

What will happen if the line of action of the weight lies outside the base of an object?

###
The line of action of the weight of an object acts through its centre of mass.

If the line of action of the weight of an object lies outside the base of an object, there will be a resultant and the object will tend to topple over.

13

## What two factors will increase the distance an object has to tilt before the line of action of the weight moves outside the base?

###
1) Wide base

2) Low centre of mass

14

## By what equation is pressure given?

###
P = F/A

P is the pressure in pascals, Pa (or N/msquared)

F is the force in newtons, N

A is the cross-sectional area at right angles to the direction of the force in metres squared.

15

##
What can be said about liquids with regard to compressibility?

How is pressure transmitted in a liquid?

What does this mean about a force exerted at one point on a liquid and where is this made use of?

###
Liquids are virtually incompressible and the pressure in a liquid is transmitted in all directions.

This means that a force exerted at one point on a liquid will be transmitted to other points in the liquid. This is made use of in hydraulic pressure systems.

16

## On what three things does the force exerted by a hydraulic pressure system depend?

###
1) The force exerted on the system

2) The area of the cylinder on which the force acts

3) The are of the cylinder that exerts the force.

17

##
How can a hydraulic system be used a force multiplier?

What does this mean?

###
The use of different cross-sectional areas on the effort and load sides of a hydraulic system means that the system can be used as a force multiplier.

Therefore a small effort can be used to move a large load.

18

## What three things can be said about an object moving in a continuous circle?

###
1) It is continuously changing direction.

2) Therefore it is continuously changing velocity.

3) Therefore it is accelerating.

19

## What is the name given to the acceleration of an object moving in a continuous circle?

### The acceleration is called the centripetal acceleration.

20

##
When does an object accelerate?

What force acts on an object moving in a continuous circle and in which direction does the force act?

###
An object only accelerates when a resultant force acts on it.

The force is called the centripetal force and always acts towards the centre of the circle.

21

## What will happen if the centripetal force stops acting?

### If the centripetal force stops acting, the object will continue to move in a straight line at a tangent to the circle.

22

## What three factors will cause the centripetal force needed to make an object perform circular motion to increase?

###
1) Increase in the mass of the object.

2) Increase in the speed of the object.

3) Decrease in the radius of the circle.

23

## How does a pendulum move and what is this an example of?

### A pendulum moves to and fro along the same line, this is an example of oscillating motion.

24

##
What does a simple pendulum consist of?

What happens when the bob is dis placed to one side and let go?

What is the equilibrium position?

###
A simple pendulum consists of a mass called a bob, suspended on the end of a string.

When the bob is displaced to one side and let go, the pendulum oscillates back and forth, through the equilibrium position.

The equilibrium position is the position of pendulum when it stops moving.

25

## What is the amplitude of the oscillation of a pendulum?

### The amplitude of the oscillation is the distance from the equilibrium position to the highest position on either side.

26

##
What is the time period of the oscillation?

Give both explanations.

###
The time period of the oscillation is the time taken for one complete cycle, this is:

1) The time taken from the highest position on one side to the highest position on the other side and back to the start position.

OR

2) The time taken between successive passes in the same direction through the equilibrium position.

27

##
How can the time period be measured?

On what does the time period depend and how does this affect the time period?

###
To measure the time period of a pendulum we can measure the average time for 20 oscillations and divide the timing 20.

The time period depends only on the length of the pendulum and increases as its length increases.

28

## What is the frequency of oscillations for a pendulum?

### The frequency of oscillations is the number of complete cycles of oscillation per second.

29

## What equation relates time period and frequency?

###
T = 1/f

T is the time period in seconds, s

f is the frequency in hertz, Hz

30