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Questions 3.6 What force is needed to give a car of mass 600 kg an acceleration of 2.5 m/s2? 3.7 A stone of mass 0.2 kg falls with an acceleration of 10.0 m/s2. How big is the force that causes this acceleration?
Activity 3.2 F, m and a
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Skills AO3.1 AO3.2 AO3.3 AO3.4
Using techniques, apparatus and materials Planning Observing, measuring and recording Interpreting and evaluating observations and data
If you change the force acting on an object, its acceleration changes. If you change the mass of the object, its acceleration changes. The picture shows one way to investigate this using a laboratory trolley, a light gate and a timer. The trolley is placed on a runway. A string passes over a pulley. Weights on the end of the string provide the force needed to make the trolley accelerate.
Two important points to note: ◆
3.8 What acceleration is produced by a force of 2000 N acting on a person of mass 80 kg? 3.9 One way to find the mass of an object is to measure its acceleration when a force acts on it. If a force of 80 N causes a box to accelerate at 0.1 m/s2, what is the mass of the box?
The force F pulling the trolley is the weight of the masses m hanging from the end of the string. Calculate the force using F = mg.
The mass m that is accelerating is the mass of the trolley plus the mass on the end of the string.
Investigate how the trolley’s acceleration a depends on the force F acting on it and on the mass m. 1 Set up the trolley on a runway, as shown. Decide how you will measure its acceleration. You can use a light gate and an interrupt card, or two light gates, or a motion sensor, and a data-logger and a computer. Alternatively, you could use a ticker-timer and ticker-tape. 2 Hang weights on the end of the string and release the trolley. Be ready to catch it when it reaches the end of the runway. Check that you can measure its acceleration. 3 To find out how the acceleration depends on the mass of the trolley, you must keep the force constant. Do not change the load on the end of the string. Increase the mass of the trolley by placing masses on top of it. 4 To find out how the acceleration depends on the force, you must change the number of masses on the end of the string. To keep the total mass constant, start with one mass on the string and nine masses on the trolley. Then, one by one, transfer masses from the trolley to the end of the string.
Original material © Cambridge University Press 2014
Chapter 3: Forces and motion
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