# Set 5 PHYS

## Set 5 PHYS

5
Set
5

Problem Solution
1
m N – is a product of two units, metres and force. mN – is milliNewtons
2
Torque = force ! perpendicular distance
Torque = 160 N ! 0.75 m
Torque = 120 N m
88
3
= perpendicular
=
= 220 N
0 .4

4

5
6a
6b

7
8a

In a bus or truck more turning force (torque) is needed to turn the wheels. The same force
on a larger steering wheel will provide a greater torque than the same force on a smaller
steering wheel. Racing cars have a smaller steering wheel, a smaller movement of the
steering wheel causes a greater amount of turning in the car, but a larger force is needed.
Leaning forward ensures that the centre of mass of hiker + backpack is over the feet.
Otherwise the pack may cause the hiker to fall backwards.
Amanda’s torque about pivot = 24 kg ! 9.8 N kg-1 ! 2.5 m = 590 N m
Father’s torque about pivot point = 60 kg ! 9.8 N kg-1 ! 1.6 m = 940 N m
Combined torque of Ben & Amanda = 940 N m
Torque of B & A = 590 N m + (WB! 2.0 m)
Torque B = 940 N m – 590 N m = 350 N m
350 N m /2.0 m = 175 N (Ben’s weight)
175 N / 9.8 N kg-1 = 18 kg
With a smaller radius the distance from the axle to the road is reduced and a greater force
is transferred to the road. Greater acceleration is achieved but a lower top speed.
Use shoulder as pivot point
450 ! 10-3 m ! F =120! 10-3 m ! 36 kg ! 9.8 N kg-1
F = 94 N
120mm
Hand

Shoulder
36kg ! 9.8 N kg-1

8b

9

Use centre of mass of pipe as pivot point
C.W.M. = A.C.W.M.
120 ! 10-3 m ! F = 94 N ! 570 ! 10-3 m
F = 447 N
Use chair leg nearest the person as pivot point, chair will tip when turning force of person
is greater than turning force due to weight of chair.
Fperson ! 0.3 m = 25 kg ! 9.8 N kg-1 ! 0.55 N
Fperson = 450 N
Therefore maximum mass of person is 46 kg
1.9m

400mm
100mm

400mm
Leg

Weight
Bench

Leg
Weight
Person

10a

If platform is not moving then net force is 0 N.
Upwards force = Downwards force
320N + 590 N = 280 N + John...