Describe two processes in which an external force is exerted on a system and no work is done on the system. Explain why no work is done

Which phenomenon is caused by the magnetic field of the earth? a. magnets sticking to the steel door of a refrigerator b. one end of a magnetized needle pointing to the north c. iron becoming magnetized when rubbed with a strong magnet d. some magnets losing their magnetism when struck with a hammerlisa is standing outside her school. if an ambulance is approaching the school at fifty miles per hour with its siren on, how will the way lisa hears the sound compare to the way the driver of the ambulance hears the sound? a. lisa will hear the sound at a higher pitch than the driver. b. the driver will hear the sound after lisa hears the sound. c. lisa will hear the sound at the same wavelength as the driver. d. the driver will hear the sound at a higher frequency than lisa.what is the difference between an electromagnet and a bar magnet? a. a bar magnet's magnetic field is a perfect circle. b. a bar magnet weighs less than an electromagnet. c. an electromagnet's magnetic field can be switched on and off. d. an electromagnet has only one pole while a bar magnet has two.which statement accurately describes gravitational force? a. it is weak and long-ranged. b. it is strong and long-ranged. c. it is weak and short-ranged. d. it is strong and short-ranged.which of the following statements about magnets and magnetism is true? a. the north pole of a magnet is attracted to the south pole of another magnet. b. the north pole of a magnet can magnetize iron but the south pole cannot. c. the poles of a magnet result from the magnetic domains pointing in different directions. d. the poles of a magnet have weaker magnetic forces than the center of a magnet.the moving company loaded a crate that weighed 1800 n into a truck that was 1 meter off of the ground. they used a force of 600 n to push it up a ramp that had a slope of 4 meters. what was the efficiency of the moving company's ramp? a. 57% b. 68% c. 75% d. 86%which statement best explains why machines are inefficient? a. energy transfer in compound machines gives off electrical waste. b. during energy transfer in machines, some energy is always lost as heat. c. balanced forces in moving machines produce waste in the form of steam. d. when work is done by machines, gravity always causes a decrease in force.