Um tanque cilíndrico, de diâmetro D = 2 m, contém água até um nível h = 2 m acima de um pequeno orifício de diâmetro d = 1 cm, conforme ilustra a figura a seguir. O tanque está sobre um carro que pode se movimentar livremente sobre um trilho horizontal, com atrito desprezível. A água começa a sair pelo orifício em um instante inicial t = 0. Na figura, V denota a velocidade de descida da superfície livre do tanque e v, a velocidade de saída da água no pequeno orifício. A massa total do tanque com a água e o do carrinho é M, a densidade da água é de 1.000 kg/m³ e a aceleração da gravidade é !$ g = 10 m/s^2 !$.

In a world where many of us are glued to our smartphones, Dulcie Cowling is something of an anomaly — she has ditched hers. The 36-year-old decided at the end of last year that getting rid of her handset would improve her mental health. So, over Christmas she told her family and friends that she was switching to an old Nokia phone that could only make and receive calls and text messages.

She recalls that one of the pivotal moments that led to her decision was a day at the park with her two boys, aged six and three: “I was on my mobile at a playground with the kids and I looked up and every single parent — there was up to 20 — were looking at their phones, just scrolling away,” she says.

“I thought ‘when did this happen?’. Everyone is missing out on real life. I don’t think you get to your death bed and think you should have spent more time on Twitter, or reading articles online.”

Ms Cowling, who is a creative director at London-based advertising agency Hell Yeah!, adds that the idea to abandon her smartphone had built up during the covid-19 lockdowns.

“I thought about how much of my life is spent looking at the phone and what else could I do. Being constantly connected to lots of services creates a lot of distractions, and is a lot for the brain to process.”

She plans to use the time gained from quitting her smartphone to read and sleep more.

About nine out of 10 people in the UK now own a smartphone, a figure broadly replicated across the developed world. And we are glued to them — one recent study found that the average person spends 4.8 hours a day on their handset.

Yet for a small, but growing number of people, enough is enough.

Alex Dunedin binned his smartphone two years ago. “Culturally we have become addicted to these tools,” says the educational researcher and technology expert. “They are blunting cognition and impeding productivity.”

He has become happier and more productive since he stopped using a smartphone, he says.

Mr Dunedin doesn’t even have an old-fashioned mobile phone or even a landline anymore. He is instead only electronically contactable via emails to his home computer.

“It has improved my life,” he says. “My thoughts are freed up from constantly being cognitively connected to a machine that I need to feed with energy and money. I think that the danger of technologies is that they are emptying our lives.”

Yet, while some worry about how much time they spend on their handset, for millions of others they are a godsend.

“More than ever, access to healthcare, education, social services and often to our friends and family is digital, and the smartphone is an essential lifeline for people,” says a spokesperson for UK mobile network Vodafone.

“We also create resources to help people get the most from their tech, as well as to stay safe when they’re online — that’s hugely important.”

Suzanne Bearne.

The people deciding to ditch their smartphones. Internet: <www.bbc.com> (adapted).