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Forces in all directions A force is needed to start anything moving. An interaction force arises between two objects. The green arrow shows the force of the weightlifter pushing upwards. The red arrow shows the downwards force of the weights. The two forces in a pair are called interaction forces. Interaction forces are: 1.Equal in size 2.Opposite in direction 3.Act on different objects Interaction forces are: 1.Equal in size 2.Opposite in direction 3.Act on different objects

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How things start moving When objects explode, the pieces usually travel outwards in all directions, however when an object is designed so that everything that comes out of it goes in one direction………. Then you have a rocket!! Fi rstly, the engine has to make the wheels turn, this causes a forward force on the car. Force exerted on car Force exerted by tyre on road.

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Friction Friction adjusts its size depending on the situation until it reaches a limit. When two surfaces are put together, the bumps on one fit into another and make it hard to move over each other. (think of sliding two brushes over each other) 25N 50N 70N Friction at its maximum Box moves

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Reaction of surfaces When you drop an object the force pulling it downwards is called Gravity However when you put an object on the table, it does not fall. Therefore something must be stopping gravity. The table must exert an upward force on the ball that balances gravity. Adding forces If forces acting on an object balance each other, then they add to zero. The resultant force is the direction that the larger force is facing. 5N 3N Resultant force = 2N

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More adding forces 5N 3N6N 5N 3N Resultant force = 0 Resultant force = 3N Resultant force = 8N

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Speed Average speed = distance travelled Time taken Instantaneous speed = the speed at a particular moment How to measure vehicle’s speed: Gatso speed cameras- uses radar. The camera takes two photographs of the car half a second apart. Truvelo speed cameras- detector cables. Pressure sensors detect when the car passes over. It measures the time taken to travel in-between these cables. Police radar guns- bounce microwaves off approaching cars.

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Picturing motion Distance time graph- shows how far a moving object is from its starting point. Speed time graph- shows the speed of the moving object. Distance time graphs Speed- time graph

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Velocity= Speed in a certain direction Forces and motion If an object is heavy, it will move slower, whereas the lighter ones will move quicker. Momentum= mass x velocity (Kg m/s) (kg) (m/s) Change of momentum = force x time for which it acts (kg m/s) (N) (s) Imagine you are pushing an object hard enough for it to move. If you keep pushing it, it will get faster. Its momentum is increasing.

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Example: A football has a mass of around 1 kg. A free kick gives it a speed of 20 m/s. What is its momentum? Momentum = mass x velocity – 1 kg x 20 m/s – – = 20 kg m/s Question 1: What is the momentum of a hockey ball of mass 0.4 kg moving at 5 m/s? Question 2: What is the momentum of a jogger of mass 55kg, running at 4 m/s?

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Car safety Crumple zones- the front part of the car is designed to absorb the energy of the impact. It is designed to crumple slowly, therefore making the force on the car smaller. In a collision, the momentum becomes zero as the car stops. The size of the force exerted on the car depends on the time the collision lasts. The BIGGER the time, the smaller the force. Seat Belts and air bags- are designed to make the change of momentum take longer. So the force that causes the change is less.

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Laws of motion Law 1: If the resultant force acting on an object is zero, the momentum of the object does not change. Law 2: If there is a resultant force acting on an object, the momentum of the object will change. Driving forceCounter- force As you go faster, air resistance force gets bigger, so does the counter force. Eventually you reach a speed where the counter force balances with the driving force.

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Work and Energy Work done by a force = force x distance moved in direction of force (J) (N)(m) If you are pushing your car: 50 m 600 N Work done = force x distance moved = 600N x 50m = 30,000 J

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Gravitational potential energy When you lift an object up, its gravitational potential energy increases. Gravitational potential energy = weight x vertical height difference (J) (N)(m) If you push a trolley you are transferring your energy to the trolley which is stored as kinetic energy: Kinetic energy = ½ x mass x (velocity)² (J) (kg) (m/s) For example when a roller coaster runs down a slope it loses gravitational potential energy and gains kinetic energy.

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Chapter 3 Forces.

Chapter 3 Forces.

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