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Force Pairs Date ________ Forces always exist in pairs. Force of the stake on the __________ A hammer exerts a force on the stake. There must be a force to stop the hammer. The stake exerts a force on the hammer. When one object exerts a force on a second object, the second object exerts a force equal in magnitude and opposite in direction on the first object. Force of the hammer on the __________ If the forces are equal in magnitude and opposite in direction, why don't the forces cancel each other out? 1 The two forces are acting on __________ ___________________ _____________. Forces cancel each other when they are equal in magnitude and opposite in direction acting on the SAME object. When two forces are equal in magnitude and opposite in direction and acting on DIFFERENT objects they do not cancel out. They make objects move!! 2 When two forces interact with one another the forces that the objects exert on each other are called an actionreaction pair. The force object 1 exerts on object 2 is the action force. The force object 2 exerts on object 1 is the reaction force. It does matter which is which since the action force and reaction forces occur AT THE SAME TIME. The action reaction forces are equal in magnitude and opposite in direction, but either object may still have a ________ _______________ acting on it 3 Force pairs also apply to field forces (gravitational force, electrical force, magnetic force) The gravitational force of the Earth pulls the rock downward and the gravitational force of the rock pulls the Earth upward with a force of the same magnitude. The mass of the Earth is much greater than the mass of the rock. The acceleration of the Earth is negligible but the rock does accelerate. 4 When a rifle is fired, the force exerted on the bullet has the same magnitude as the force exerted on the rifle. The same force on different objects produce different accelerations. Bullet: force = acceleration small mass Rifle: force = acceleration large mass 5 A rocket accelerates because as the rocket throws out burned rocket fuel at high velocity, the fuel exerts a force against the rocket. Why is a rocket easier to accelerate as it travels in space away from the Earth? 6 Arnold Strongman and Suzie Small pull on opposite ends of a rope in a tug of war. The greatest force exerted on the rope is by a) Arnold b) Suzie c)... both the same 7 Two 100 N weights are attached to a spring scale as shown. What does the spring scale read? Would the spring scale reading change if the left pulley was removed and the left side of the string was attached to a stationary vertical pole? 8 Suppose two carts, one twice as massive as the other fly apart when the compressed spring that joins them is released. 1) How does the force exerted by the spring on the 1m cart compare to the force exerted by the spring on the 2m cart? 2) How fast does the 2m cart roll compared to the smaller 1m cart? 9 Sprinting near the end of a race, a runner with a mass of 60 kg accelerates from 6.0 m/s to 7.0 m/s in 2 seconds. 1) What is the runner's average acceleration? 2) To gain speed the runner produces a backward force on the ground. What is this force? 3) What is the force that the ground exerts on the runner? 4) Why don't these two forces cancel out? 10