Topic 4 - Forces and Energy

Question 1

How can energy of a system be changed

Answer 1

Mechanically (forces doing work), Electrically (by moving charge doing work), by heating or by radiation

Question 2

How do you measure work done by a force

Answer 2

force x distance

Question 3

What is work done

Answer 3

a force which does work over a distance

Question 4

What does power mean

Answer 4

How much work is done per second

-It is the rate of energy transfer and is measured in Watts

Question 5

How do you work out power

Answer 5

work done/ time taken

Question 6

describe an example when work is done mechanically on an object and why is leads to a rise in temperature

Answer 6

A person pushing a box along a carpet
-work is done against frictional forces which transfers energy to kinetic store but also to the thermal store due to friction so overall temperature increases

Question 7

Whats is the effect of a gravitational field on objects

Answer 7

non contact force- the gravittional attraction between objects is caused by their gravitational fields interacting

Question 8

What is the effect of magnetic fields on objects

and electrostatic force

Answer 8

interacting magnetic fields cause attraction and repulsion between magnetic objects
-electrostatic force causes attraction and repulsion between electrical charges and their electric field

Question 9

What are the forces that occur when objects are in contact with each other

Answer 9

both objects feel an equal and opposite force called an interaction pair (3rd law)
-these pairs can be shows with vectors (arrows diagram)

Question 10

What is the resultant force

Answer 10

Overall force acting on an object

-at least 2 forces are acting on an object

Question 11

How do you work out resultant forces on a straight line

Answer 11

adding those in same direction and subtracting any in opposite

Question 12

Objects in equilibrium have a resultant force of…..

Answer 12

zero (this can be stationary or moving at constant speed)

Question 13

Describe a situation where energy is transferred Mechanically (by doing work)

Answer 13

if a box is lifted off the floor work is done against gravity which causes energy to be transferred mechanically to objects kinetic and GPE stores

Question 14

Describe a situation where energy is transferred electrially

Answer 14

electric ions carry charge electrically from mains power supply to thermal energy store of metal plates

Question 15

Describe a situation where energy is transferred by heating

Answer 15

A pan of water on a stove-

energy is transferred by heating to thermal energy stores of pan and water

Question 16

Describe a situation where energy is transferred by radiation

Answer 16

Toaster- coil of wire with high resistance heats and glows giving off infrared radiation and cooks the bread

Question 17

What is a moment

Answer 17

The turning effect of a force

Question 18

How do you calculate a moment

Answer 18

force x distance

Question 19

How would someone get the maximum moment/turning effect

Answer 19

you could push at right angles as any other angle means smaller distance so smaller moment

Question 20

What is the principle of moments for a an object in equilibrium

Answer 20

Sum of clockwise moments is = sum of the anticlockwise moments

Question 21

What do leavers do

Answer 21

They make it easier to do work by transferring the turning effect by increasing the distance from the pivot where a force is applied

Question 22

Explain how gears transmit rotational effects of forces

Answer 22

They fit together to transmit turning effects by interlocking,
a larger gear is slower but creates a large moment
a small gear is faster but has smaller moment

Question 23

How can we work out speeds and moments of gears

Answer 23

gear ratios