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Topic – Physics 2a Weight as a force Topic – Physics 2a Weight • Weight is the force due to the attraction of gravity. • The force of gravity, called weight, is pulling you downward, toward the center of the Earth. • weight (N) = Mass (kg) x g , • g depends on the planet and it also depends on the distance from the object to the center of the planet. Topic – Physics 2a G on Earth • At sea level, it is about 9.8 m/s2. • g is measured in m/s2 = m/s/s is the unit for acceleration • It means that an object falling on Earth, if we neglect air resistance, will speed up toward the ground at a rate of 9.8 m/s2. • Every second, the object will go 9.8m/s faster. Topic – Physics 2a Questions 1. John's mass is 65kg on earth. What his metric (a) mass and (b) weight on Mars, where g= 3.72m/s/s. m=____kg and W = _________ 1. On Mars, a falling object will speed up by ______ m/s every second. 2. Find out your mass in Kg. _________kg. What are your metric mass and weight on Jupiter where g = 25.9 m/s/s ? m = _________kg and W = ________N. Topic – Physics 2a Questions 4. Find the mass of a 11,500N automobile on mars. m = _____kg. Topic – Physics 2a 5. A student use a balance scaled in newton to find the weight of 1kg and then 2 kg. He finds that 1 kg = _______ N, 2kg = ________ N, 3 kg = __________ N ... 10kg = _________N The mass is proportional to the pull due to gravity (weight) and the constant of proportionality is_______ Topic – Physics 2a • We already know that mass is the amount of matter an object has, it can’t be changed unless matter is added or removed. What can be changed is the weight. Weight is the mass of a body acted on by acceleration due to gravity. • F = ma where Fw = weight (N) • g = a m = mass (kg) • then Fweight or Fw = mg g = acceleration due to gravity (g) • Example: • A 10.0 kg stone is first held on Earth (g = 9.8 ms-2) and on the moon (g = 1.6 ms-2). What is its weight in both cases? Fw Earth = mg = 10 x 9.8 Fw Moon = mg = 10 x 1.6 Fw Earth = 98 N Fw Moon = 16 N Topic – Physics 2a Lifts • Why do you feel heavier or lighter when a lift starts or stops? Your acceleration (from the lift) is added vectorially to the acceleration due to gravity. • When you accelerate up, gravity must be overcome so your apparent weight is • Fw = m (g + a) you feel heavier • When you accelerate down, it is helping gravity, so your apparent weight is Fw = m (g – a) you feel lighter • When you are moving at a constant velocity, there is no additional acceleration so your weight is simplyFw = mg Topic – Physics 2a Example 1 Topic – Physics 2a Example 2 Topic – Physics 2a Free body diagrams Topic – Physics 2a Free body diagrams Topic – Physics 2a Free body diagrams Topic – Physics 2a Activity Topic – Physics 2a