4.9 Force and acceleration A force is a push, or pull, or twist that changes motion. The size of a force is measured in newtons (symbol N). Some common measures of force are: ■ switching a light switch, 5 N ■ removing a ring tab from a can, 20 N ■ hitting a tennis ball with a racquet, 2000 N ■ the force of a jet engine, 200 000 N force to switch on a light force to open a drink can 20 N 5N 2000 N force from a jet engine force on a tennis ball 200 000 N Measures of force EXPERIMENT When you are riding in a car, you are being accelerated all the time. Moving away from traffic AIM: To investigate the relationship between force, acceleration and mass For this experiment, you can use either a collision trolley or a toy car, string and masses. If you are using a collision trolley, the force is applied by a spring under the trolley. Hold the trolley against a wall or solid barrier, and release the spring. To estimate the acceleration, measure the distance that the trolley moves. Do this three times and take an average. lights, going around a corner and stopping all involve acceleration. Remember that acceleration is a change in speed or direction. Deceleration is slowing down, a negative acceleration. A force is a push or a pull that causes a change in motion. A change in motion is acceleration. To change the acceleration a force is needed. In a car, force comes from the engine, the brakes and the tyres. The force exerted by the engine will accelerate the car from rest. The force exerted by the brakes will cause the car to stop. The force exerted by the tyres will cause the car to change direction. The force exerted by a cyclist to move a bike is not large enough to move a car. The force exerted by a car engine is hardly large enough to move a truck. The experiment below is about investigating the relationship (connection) between force, acceleration and mass. car. (Since toy cars have such a small mass, it is best to use small forces to move them. Twenty-cent coins taped onto string work well, as do metal washers tied onto string.) Estimate the acceleration of the car from its speed across the table. Repeat for different forces and different masses on the toy car. How can you make the acceleration of the toy car greater? What reduces the acceleration? Consider the force and mass of the car. place books on trolley to increase the mass different tensions in spring = different forces Mass on string. Start with about 20 g mass. Different masses give different forces Estimating acceleration Repeat for different forces and different masses on the trolley. How can you make the acceleration of the trolley greater? What reduces the acceleration? If you are using a toy car, string and masses, the gravitational force of the masses will move the toy 120 Add other cars or plasticine to change the mass Investigating how mass and force affects acceleration Write your results in a table. Suitable columns would be Size of force, Mass of car or trolley and Size of acceleration. Combine your results and make some conclusions about the experiment. Write your conclusion in your note book. MOTION In the experiment you would have noticed that a big force produces a big acceleration. But the size of the acceleration also depends on the mass. A big mass reduces the acceleration. We can understand this with some common examples. A car with a powerful engine will accelerate faster than a car with a less powerful engine. Powerful engines give more force and more acceleration. The acceleration of the car also depends on the mass of the car. A car loaded with heavy luggage for holidays has more mass, and will not have the same acceleration as when it is unloaded. The loaded car will not stop as well as an unloaded car. The braking force in cars is provided by the brakes. The car with the greater mass will CHECKPOINT: Ocean going ships have a large mass. In spite of huge engines generating a large force, the acceleration is small 121 take longer to stop. Again the mass reduces the deceleration. The mass is like inertia. It resists the change in motion caused by the force. The acceleration caused by a force is increased if the force is increased. The acceleration caused by the same force is reduced if the mass is increased. Mathematically, F a= m This is usually written as the mathematical equation F = ma. Force is measured in the unit called Newton, mass is measured in kilograms, and acceleration is measured in metres per second squared. The abbreviations are N, kg and m s–2. Isaac Newton summarised this information in his second law of motion. It says that: ‘A large force produces a large acceleration, provided that the mass remains the same. A large mass reduces the acceleration of the same force.’ In a car crash the occupants stop moving quickly. There is a large deceleration. A large force is needed for this to happen. A large force can be enough to break bones and cause internal injuries. Air bags, padded dashboards and crumple zones in cars absorb a lot of the forces in collisions, and reduce the forces applied to occupants as they suddenly stop. COPY AND COMPLETE When you are riding in a car you are being ___________ all the time. Remember that acceleration is a ______ in _____ or _________. Deceleration is _______ down, a ________ acceleration. A force is a ____ or a ____ that causes a ______ in motion. A change in motion is ____________. To ______ the acceleration a _____ is needed. In a car, force comes from the ______, the ______ and the _____. The ____________ caused by a _____ is increased if the _____ is increased. The ____________ caused by the same _____ is reduced if the ____ is increased. This is usually written as the ____________ equation _ = __. QUESTIONS 1 What, in your own words, is Newton’s second law of motion? d Which car takes the longer distance to stop? Explain clearly. 2 What two factors determine the acceleration of a car from rest? 4 The modern unit of force is called the Newton. Who does this name honour? What did he do? 3 Imagine two similar cars, one of which is towing a caravan. The mass of the caravan is equal to the mass of the car. Compare the car that is on its own to the car towing the caravan in these situations. a What is the difference in acceleration? b What is the difference in stopping force when the brakes are applied? c What is the difference in decelerating force when the brakes are applied? 5 How would the acceleration of a car change if a truck motor replaced the usual motor? Consider the change in force and mass. 6 Which accelerates faster: a 400 000 kg Boeing 747 jet whose engines exert a total force of 800 000 N, a sports car that exerts a force of 7800 N with a mass of 1300 kg, or a motorbike of mass 300 kg with an engine force of 3000 N?
