Everyday forces

Calculate the weight of a rucksack of $5\ \text{kg}$ (take $g = 9,8\ \text{m/s}^2$) and indicate the direction in which this force acts.

A pair of bathroom scales reads $60\ \text{kg}$.

1. a) What do the scales actually measure: mass or weight?
2. b) What is this person's weight in newtons (take $g = 9,8\ \text{m/s}^2$)?
3. c) What would the scales read on the Moon, where gravity is weaker? Reason your answer.

A book of $2\ \text{kg}$ is at rest on a table (take $g = 9,8\ \text{m/s}^2$).

1. a) Draw the forces acting on the book in your exercise book.
2. b) Calculate the weight of the book and the normal reaction force exerted by the table.

You can do it on paper and send a photo with your answer.

A cup is resting on the kitchen worktop. Which object exerts the normal force on the cup? In which direction does it point? Why do you think it is called a "normal" force?

A crate of $20\ \text{kg}$ is pulled along a floor for which the friction force is given by $F_r = \mu \cdot N$, with $\mu = 0,3$ (take $g = 10\ \text{m/s}^2$).

1. a) Calculate the normal force on the crate.
2. b) Calculate the friction force.
3. c) What is the minimum horizontal force that must be applied for the crate to move forward at a constant speed? Justify your answer.

Two teams pull on a rope in opposite directions, each with a force of $350\ \text{N}$, and the rope does not move, as shown in the diagram.

1. a) What is the tension at any point in the rope? (Be careful: it is not the sum of the two forces; explain why.)
2. b) If one of the teams lets go of the rope, what happens to the tension and to the movement of the rope? Explain it.

A lamp of $3\ \text{kg}$ hangs from the ceiling suspended by a cable (take $g = 9,8\ \text{m/s}^2$).

1. a) What forces act on the lamp and what exerts them?
2. b) Calculate the tension in the cable.

A goods lift raises a load of $60\ \text{kg}$ with an acceleration of $2\ \text{m/s}^2$ (take $g = 10\ \text{m/s}^2$).

1. a) Draw the forces acting on the load in your exercise book.
2. b) Calculate its weight.
3. c) Apply Newton's second law to calculate the tension in the cable.

You can do it on paper and send a photo with your answer.

When you submerge a beach ball in a swimming pool and let go of it, it shoots upwards. What force, besides weight, acts on the ball while it is submerged? What exerts it and in which direction does it point?

A person of $60\ \text{kg}$ is standing on bathroom scales inside a lift (take $g = 10\ \text{m/s}^2$), as shown in the diagram.

1. a) What do the scales read with the lift at rest?
2. b) When the lift accelerates upwards at $2\ \text{m/s}^2$, the normal force is $N = m \cdot (g + a)$: calculate what the scales read in this case.
3. c) Has the person's real weight changed? Explain what has changed.

A worker pulls a crate of $10\ \text{kg}$ along a horizontal floor with a force of $60\ \text{N}$, and the friction is $30\ \text{N}$ (take $g = 10\ \text{m/s}^2$). The diagram shows the forces acting on the crate.

1. a) Calculate the weight of the crate and the normal force from the floor.
2. b) Calculate the net horizontal force and the acceleration of the crate.

A crate of $5\ \text{kg}$ is on a horizontal floor and is pushed with a horizontal force of $40\ \text{N}$; the friction is $20\ \text{N}$ (take $g = 10\ \text{m/s}^2$).

1. a) Draw the four forces acting on the crate in your exercise book, naming each one and the object that exerts it.
2. b) Calculate the magnitude of all four forces.
3. c) Calculate the net force on the crate and its acceleration.

You can do it on paper and send a photo with your answer.