If only an external force can change the velocity of a body, how can the internal force of the brakes bring a car to rest? 1. it is the driver, not the car itself who causes the breaking, so it is an external force. 2. the engine is stopped, so the car has no force to run further. 3. some internal forces can change the velocity of a body. 4. it is the force of the road on the tires (an external force) that stops the car.

4. It is the force of the road on the tires (an external force) that stops the car.

Explanation:

If there is no friction between the road and the tires, the car won't stop.

You can see this, for example, when there is ice on the road. You can still apply the brakes (internal force), but since there is no friction (external force) the car won't stop.

The force of the brakes on the wheels is not what makes the car stop, it is the friction of the road against still tires that makes it stop.

answer: good job it's all correct !

explanation:

it is given that trillions of microbes make up to 1% to 3% of the body mass.

let's take average of this percentage: 2%

consider the average mass of the human body as 70 kg.

then, trillions of microbes would have mass  

therefore, 100 trillion micro-organisms would weigh:

this is an approximate value and could change depending on the mass of the human body and percentage of the microbes present in it.

average mass per microorganism =