TOPIC 4- Forces And Energy

Question 1

Efficiency of an energy transfer

Answer 1

The proportion of total energy supplied that ends up in useful energy stores.
(Useful energy/total energy)

Question 2

A closed system

Answer 2

A system with a net change in energy of zero.

Question 3

How is energy transferred by heating?

Answer 3

Energy is transferred to the thermal energy stores of the object, increasing its temperature.

Question 4

What happens when a force moves an object through a distance?

Answer 4

Work is done on the object and so energy is transferred mechanically from one store to another.

Question 5

Work done (J) formula

Answer 5

Force (N) * distance moved in direction of force (M)

Question 6

One joule of work

Answer 6

Force of one newton causes an object to move a distance of one meter.
1J =1Nm

Question 7

Why does work done often lead to a rise in temperature?

Answer 7

Energy is dissipated to the thermal energy stores of the moving object and its surroundings.

Question 8

Efficiency formula

Answer 8

Useful energy transferred by device / total energy supplied to device.

Question 9

Power definition
(Unit?j

Answer 9

Rate of energy transfer (how much work is done per second).

Measured in watts

Question 10

Power formula

Answer 10

Work done (J) / time taken (s)

P=E/T

Question 11

Force

Answer 11

A vector (push/pull) that is caused by it interacting with something.

Question 12

Contact forces

Answer 12

Objects that need to be touching for a force to act.

Question 13

Non-contact forces

Answer 13

Forces that can act between objects that aren’t touching.

Usually caused by interacting fields.

Question 14

What happens to interacting magnetic fields?

Answer 14

Cause attraction or repulsion between magnetic objects and electrostatic force between charges is caused by the fields interacting.

Question 15

Newton’s third law

Answer 15

Whenever 2 objects interact, they both exert an equal but opposite force called an interaction pair.

Question 16

Free body force diagram

Answer 16

An isolated body and all forces acting on it.
Includes every force acting on the body but none it exerts. Sizes of arrows show relative magnitudes of forces and their directions.

Question 17

Resultant force

Answer 17

The overall force on a point/object.

Question 18

Resultant force of objects in equilibrium

Answer 18

Zero

Question 19

How to use scale drawings to find resultant forces

Answer 19

1. ) draw a scale drawing of the forces acting (w a sensible scale) w arrows.
2. )draw net force from point of each end of forces and measure the length to scale to find the force in N.
3. ) use a protractor to measure the direction as a bearing.

Question 20

How do you split a force into components?

Answer 20

Split the angle into two components perpendicular to each other, acting together will have the same effect as the single force.

Question 21

Moment

Answer 21

The turning effect of a force.

Question 22

Moment of a force formula

Answer 22

Force (N) * distance (m)

Nm = N*m

Question 23

How do you achieve the maximum moment?

Answer 23

Push at right angles to the spanner, meaning a larger distance. (Normal to direction of the force)

Question 24

Sum of clockwise moments =

Answer 24

The sum of anti-clockwise moments

Question 25

Levers

Answer 25

Transfer the turning effect of a force so that it is easier to do work (increases distance from pivot at which force is applied)

Question 26

How does lever length affect force needed to give a moment?

Answer 26

Longer the lever, smaller the force needed to give the same moment.

Question 27

Gears

Answer 27

Circular cogs w ‘teeth’ around edge which interlock to cause others to turn upon turning (in opposite direction).

Question 28

What are gears used for?

Answer 28

To transmit the rotational effect of a force from one place to another.

Question 29

How does gear size affect the moment?

Speed of spin?

Answer 29

Larger gears give larger moments as the distance from the pivot is further.

Larger gear = slower spin