Question

1) В каких случаях сила трения покоя совершает отрицательную работу: а) автомобиль трогается с места. б) автомобиль тормозит.2) Какую работу совершает человек: а) поднимая с постоянной скоростью на высоту 1 м ведро с водой общей массой 10 кг. б) удерживая это ведро на весу в течении 5 минут.

Answer 1

1(б)  Сила трения совершает отрицательную работу потому что сила трения направлена против движения автомобиля
2 Работа по подъему ведра A= m*g*h=10*10*1=100 Дж
Если человек держит ведро, то работа не совершается так как нет движения

Answer 2

Cases where static friction does negative work:

a) When a car starts moving from rest. b) When a car brakes.

Work done by a person:

a) When lifting a bucket of water with a total mass of 10 kg to a height of 1 m at a constant speed. b) When holding the bucket in place for 5 minutes against gravity.

Let's discuss each case in more detail.

Case a) When a car starts moving from rest: When a car is at rest and starts moving, the force of static friction acts in the opposite direction to the applied force. The work done by the force of static friction is negative because it acts in the opposite direction to the displacement of the car. This negative work done by static friction is necessary to overcome the initial inertia and bring the car into motion.

Case b) When a car brakes: When a car is in motion and brakes, the force of static friction acts in the direction opposite to the motion of the car. The work done by the force of static friction is negative because it acts in the opposite direction to the displacement of the car. This negative work done by static friction is necessary to slow down and eventually stop the car.

Case a) When lifting a bucket of water with a total mass of 10 kg to a height of 1 m at a constant speed: When a person lifts a bucket of water at a constant speed, the work done by the person is equal to the force applied multiplied by the displacement. In this case, the force applied is equal to the weight of the bucket, which is the mass of the bucket multiplied by the acceleration due to gravity. The displacement is the height to which the bucket is lifted. Since the bucket is lifted at a constant speed, the net force acting on the bucket is zero, and therefore, the work done by the person is zero.

Case b) When holding the bucket in place for 5 minutes against gravity: When a person holds a bucket in place against the force of gravity, no work is done. This is because the displacement is zero, as the bucket is not moving vertically. Even though the person is exerting a force to counteract the force of gravity, the work done is zero because there is no displacement in the direction of the force.