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.

Scientists use laser range-finding to measure the distance to the moon with great accuracy. a brief laser pulse is fired at the moon, then the time interval is measured until the "echo" is seen by a telescope. a laser beam spreads out as it travels because it diffracts through a circular exit as it leaves the laser. in order for the reflected light to be bright enough to detect, the laser spot on the moon must be no more than 1.0 km in diameter. staying within this diameter is accomplished by using a special large-diameter laser. part a if λ = 540nm , what is the minimum diameter of the circular opening from which the laser beam emerges? the earth-moon distance is 384,000 km. express your answer to two significant figures and include the appropriate units.

What type of electromagnetic wave is used in security pens? a. x-rays c. microwaves b. visible light d. ultraviolet radiation select the best answer from the choices provided

Juan created a chart to him study for a test. which headings best complete the chart? x: transparent objects y: opaque objects x: opaque objects y: transparent objects x: high frequency waves y: low frequency waves x: low frequency waves y: high frequency waves