P3- Using physics to make things work

Question 1

What is a moment?

Answer 1

Turning effect of a force.

Question 2

How can the size of a moment be calculated?

Answer 2

M= F x d

M= Moment of force in newton-metres (Nm).

F= Force in N.

d= Perpendicular distance from the line of action of the force to the pivot (m).

Question 3

How can you increase a moment?

Answer 3

• Increase the force.
• Increases the distance to the pivot.

Question 4

What is the centre of mass of an object?

Answer 4

The point where the mass of an object can be thought to be concentrated.

Question 5

What is meant by an object being in ‘equilibrium’?

Answer 5

When an object is freely suspended and comes to rest with its centre of mass directly below the point of suspension.

Question 6

How can you find the centre of mass of a thin irregular sheet of material?

Answer 6

1) Suspend sheet from a pin held in a clamp stand.
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 procedure at least twice more.
5) The centre of mass is where the lines that marked the position of the plumbline cross.

Question 7

Where is the centre of mass of a symmetrical object?

Answer 7

Along the axis of symmetry.

Question 8

What is the principle of moments?

Answer 8

Sum of anticlockwise moment= Sum of clockwise moment.

Question 9

Aimie sits 2m from the centre of a seesaw. Leo weighs twice as much as Aimie.

How far from the centre must he sit to balance the seesaw?

Answer 9

1m.

Question 10

If someone sits in the centre of a seesaw, the moment about the pivot is zero.

Why?

Answer 10

Because the perpendicular distance to the pivot is zero.

Question 11

How can the stability of an object be increased?

Answer 11

• Making the base as wide as possible.
• Making the centre of mass as low as possible.

Question 12

Why does an object topple over?

Answer 12

If the line of action of its weight is outside its base.

If the resultant moment about its point of turning is not zero.

Question 13

Why does hanging heavy bags from the handle of a pushchair make it more likely to topple over?

Answer 13

The bags raise the centre of mass so the pushchair will not have to tilt so far before the line of action of the weight moves outside the base.

Question 14

How can pressure be calculated?

Answer 14

P= F/A

P= Pressure in pascals (Pa or N/m²).

F= Force (N).

A= Area (m²).

Question 15

What does the force exerted by a hydraulic pressure system depend on?

Answer 15

• Force exerted on system.
• Area of cylinder on which this force acts on.
• Area of cylinder that exerts force.

Question 16

What properties of a liquid make it useful in a hydraulic system?

Answer 16

Liquids are virtually incompressible, and the pressure in the liquid is transmitted equally in all directions.

Question 17

What is the velocity of an object moving in a circle at a constant speed?

Answer 17

Velocity is continually changing as the object’s direction is continually changing.

Question 18

What is centripetal acceleration?

Answer 18

The acceleration towards the centre of the circle of an object moving round the circle.

Question 19

What is centripetal force?

Answer 19

The resultant force acting towards the centre of the circle when an object is moving in a circle.

Question 20

How can centripetal force increase?

Answer 20

• If the mass of object increases.
• If the speed of object increases.
• If the radius of the circle decreases.

Question 21

A student is whirling a conker around on a piece of string, in a horizontal circle.

What force provides the centripetal force?

Answer 21

Tension in the string.

Question 22

What does the time period of a simple pendulum depend on?

Answer 22

The length of the pendulum.

Longer pendulums have greater time periods than shorter pendulums.

Question 23

How does a simple pendulum work?

Answer 23

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.

Question 24

What is the amplitude of the oscillation in a pendulum?

Answer 24

The distance from the equilibrium position to the highest position on either side.

Question 25

What is the time period for a pendulum?

Answer 25

The time taken for a pendulum to swing from one side to the other and then back again to its original position.

Question 26

What is the frequency of a pendulum?

Answer 26

Number of complete swings (cycles of oscillation) per second.

Question 27

How can the time period of a pendulum be calculated?

Answer 27

T= 1/f

T= Time period (s).

f= Frequency (Hz).

Question 28

How can the time period of a pendulum be measured?

Answer 28

Measure the average time for 20 oscillations and divide the timing by 20.

Question 29

A playground swing is an example of an oscillating motion. If not pushed repeatedly, why will the swing come to rest?

Answer 29

Because energy is transferred due to friction at the top of the swing and air resistance.

Question 30

What force provides the centripetal force as a car travels around a bend in the road?

Answer 30

Friction.

Question 31

A racing car should not roll over when racing.

Suggest two reasons why.

Answer 31

• Low centre of mass.
• Wide base.

Question 32

The bus will topple over if the ramp is tilted at too great an angle.

Explain why (2 marks).

Answer 32

The line of action of the weight lies outside the base (1) so there is a resultant moment acting on the bus (1).

Question 33

What is the unit for a moment?

Answer 33

Nm.