Everyday applications: weight, normal force, friction and road safety

A lamp is sitting still on a bedside table and does not fall to the floor, as shown in the diagram.

The Earth pulls down on it with its weight: what other force acts on the lamp, who exerts it, and in what direction and sense?

You hold a bag with $2\ \text{kg}$ of oranges in your open hand, keeping it still, as shown in the diagram. Take $g \approx 10\ \text{m/s}^2$.

1. a) How much does the bag weigh?
2. b) With what force does your hand push upwards? Justify your answer considering that the bag is not accelerating.

A $60\ \text{kg}$ washing machine is sitting still on the kitchen floor. Take $g \approx 10\ \text{m/s}^2$.

1. a) Calculate its weight.
2. b) What is the magnitude of the normal force exerted by the floor on it? Justify your answer.
3. c) Weight and the normal force are equal and opposite forces, but they do NOT form an action-reaction pair: explain why (hint: notice which body each acts upon).

Name two everyday situations where friction helps us (without it we could not do something) and explain each one briefly.

Explain why it is so easy to slip when walking on an icy pavement. What does friction have to do with it? Why is salt or sand spread over the ice?

To drag the sofa across the living room, you have to push hard at first, but once it moves, it slides with less effort.

1. a) Explain the difference between static friction and kinetic friction.
2. b) What would happen to the required force if you removed the cushions and emptied the drawers beforehand? Justify your answer.

Car tyres have a tread pattern and must be changed when they are 'bald'. Explain the purpose of this tread pattern and the danger of driving with bald tyres, especially on wet roads.

Order these situations from shortest to longest stopping distance and justify the order considering the speed and the available friction:

1. a) A car travelling at $50\ \text{km/h}$ on a dry road surface.
2. b) The same car travelling at $50\ \text{km/h}$ on a wet road surface.
3. c) The same car travelling at $100\ \text{km/h}$ on a dry road surface.

Two identical cars (same mass) travel, one at $50\ \text{km/h}$ and the other at $100\ \text{km/h}$, and both brake at the same time with the same braking force.

1. a) Using Newton's second law, explain why both decelerate equally.
2. b) Reason which of the two will need more time to stop and, therefore, more metres.
3. c) Using this idea, explain why speed limits in towns are $30\ \text{km/h}$ or $50\ \text{km/h}$ rather than $120\ \text{km/h}$.

Explain the purpose of a bicycle helmet by considering what would happen to your head in a fall, with and without a helmet. What is the advantage of making the impact last a little longer?

In a crash, the driver's head tends to keep moving forward (first law). The airbag inflates in a fraction of a second to slow it down.

1. a) Why is stopping the head with the airbag less harmful than against the steering wheel?
2. b) Relate your answer to Newton's second law: what happens to the force if the braking action is spread over a longer time?

Analyse a head-on collision at $50\ \text{km/h}$ using Newton's laws:

1. a) Explain, using Newton's first law, what happens to the body of an unbelted passenger at the moment of impact.
2. b) Explain two things the seatbelt does to protect them: holding them in place and distributing the force across strong parts of the body.
3. c) The airbag extends the duration of the head's deceleration: using Newton's second law, explain why this reduces the force experienced by the head.