SP9: forces and their effects

Question 1

what are contact forces?

Answer 1

forces that act between two objects that are physically touching each other

Question 2

normal contact force

Answer 2

an object at rest on a surface experiences a reaction force perpendicular (normal) to the surface

Question 3

tension

Answer 3

-an object that is being stretched experiences a tension force
-pulling force exerted by each end of an object

Question 4

friction

Answer 4

a force that opposes or prevents movement and converts kinetic energy into heat

Question 5

air resistance

Answer 5

a force of friction produced when an object moves through the air

Question 6

what happens when a contact force acts between two objects?

Answer 6

both objects experience the same size force, but in opposite directions

Question 7

what is a non-contact force?

Answer 7

a force that acts between two objects that are not physically touching each other

Question 8

magnetic force

Answer 8

a force experienced by any magnetic material in a magnetic field

Question 9

electrostatic force

Answer 9

experienced by any charged particle in an electric field

Question 10

gravitational force

Answer 10

-experienced by any mass in a gravitational field
-masses are attracted towards each other by gravitational force

Question 11

what is a free body diagram & how is it drawn?

Answer 11

-a diagram that models the forces acting on an object
-the object is usually shown as a box or a dot
-the forces are shown as thin arrows pointing away from the centre of the box/dot

Question 12

how do you draw a vector diagram?

Answer 12

[CHECK PHYSICS BOOK ITS MUCH CLEARER]

Question 13

what is resultant force?

Answer 13

the combination of all forces acting on an object to make one force

Question 14

if the weight of a box (acting downwards) is 50 N and the normal reaction force (acting upwards) is 50 N what is the resultant force?

Answer 14

0

Question 15

if the weight of the box (acting downwards) is 50 N and the air resistance (acting upwards) is 20 N, what is the resultant force?

Answer 15

30N downwards

Question 16

what is a moment?

Answer 16

-the turning effect of a force
-moments act about a point in a clockwise or anticlockwise direction

Question 17

what point does a moment act about?

Answer 17

any point on the object, but the pivot (fulcrum) is usually chosen

Question 18

equation for a moment

Answer 18

force x distance

Question 19

units for the equation of a moment

Answer 19

-moment in newton-metres (Nm)
-force in newtons (N)
-distance in metres (m)

Question 20

which distance is used while calculating moments?

Answer 20

-the perpendicular distance from the pivot to the line of action of the force (shortest distance between the pivot and the line of action of the force)

Question 21

a force of 15N is applied to a door handle, 12 cm from the pivot. calculate the moment of the force

Answer 21

15 x 0.12

1.8 Nm

Question 22

the principle of moments

Answer 22

if the object is balanced: total clockwise moment = total anticlockwise moment

Question 23

a parent and child are at opposite ends of a playground see-saw.
the parent weighs 750 N and the child weighs 250 N.
the child sits 2.4 m from the pivot.

calculate the distance the parent must sit from the pivot for the see-saw to be balanced

Answer 23

child’s moment = force × distance

250 N × 2.4 m = 600 Nm

parent’s moment = child’s moment

distance = moment/force

distance = 600/750

distance = 0.8m

Question 24

what does a lever consist of?

Answer 24

a pivot
an effort
a load

Question 25

what does a lever do?

Answer 25

-make use of moments to act as a force multiplier
-they allow a larger force to act upon the load than is supplied by the effort

Question 26

when will a lever be most effective?

Answer 26

the longer the lever, and the further the effort acts from the pivot, the greater the force on the load will be.

Question 27

a solid beam 0.5 m long is laid across a pivot to form a simple lever.
the pivot is 0.1 m from the end of the beam
calculate the heaviest load that could be lifted using a force of 500 N

Answer 27

1. calculate the greatest distance from the pivot: 0.5 – 0.1 = 0.4 m
2. calculate the moment:
   m = fd | m = 500 x 0.4 | m = 200nm
3. use the answer above to calculate the maximum force 0.1 m from the pivot: f = m/d | f = 200/0.1 | f = 2,000

heaviest load = 2,000N

Question 28

gears

Answer 28

-wheels with toothed edges that rotate around an axle
-the teeth of one gear fit into the teeth of another gear
-this lets one gear turn the other, meaning one can be used to turn another axle

Question 29

how to calculate the speed of gears:

Answer 29

-speed is the reversed ratio of teeth

eg: if the ratio of teeth is 10:30 (1:3) the speed is 3:1, so every time the smaller gear moves three times, the bigger one moves once

Question 30

a gear with a radius of 0.1 m is turned by a gear with a radius of 0.05 m. the moment of the smaller gear is 20 Nm. calculate the moment of the larger gear.

Answer 30

1. calculate the force on the teeth of the smaller gear: f = m/d | f = 20/0.05 | f = 400N
2. use the answer above to calculate the moment of the larger gear:
   m = fd | m = 400 x 0.1 | m = 40Nm

(gears have same force for some reason)

Question 31

reducing unwanted energy transfer

Answer 31

-any surfaces in contact in a machine will dissipate energy through friction
-this can be reduced with lubrication so that the machine transfers energy more efficiently and there is less wear on the moving parts
-oily or greasy liquid that holds the surfaces apart a little, and allows them to slide past each other can be used