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Transcript

Ch. 10 Forces Forces What is a force • When you ride a bike, your foot pushes against the pedal. The push makes the wheels of the bike move. • When you drop something, it is pulled to the ground by gravity. What is a force • Is a push or a pull. • a force is described by a push or a pull in particular direction. • Forces are measured in Newton. • Named after Isaac Newton. • You exert one Newton of force when lifting a lemon. • What SI unit measure forces? Forces • • • • • • Can make objects move Can make objects move faster Can make objects move slower Can make objects stop moving Can make objects change direction Can make objects change shape Combining forces • Often, more than a single force acts on an object at one time. • A combination of forces on an object is called the net force. • When forces that act in the same direction, the net force can be found by adding the strengths of the individual forces. • When forces act in opposite directions, they also combine to produce a net force. (subtract) Combining Forces Unbalanced Forces • Whenever there is a net • Top is balanced force acting on an • Bottom is unbalanced object, the forces are unbalanced. • These forces can cause an object to start moving, stop moving, or change direction. Balanced Forces • Equal forces are acting on one object in opposite directions are called balanced forces. Answer the following Questions Find the Net Force? 1. 2. 3. 5N 10 N 5N 3N 10 N 32 N 5N 4. 5N 10N Section 2 Friction and Gravity • Friction is the force that two surfaces exert on each other when they rub together. • The strength of the force of friction depends on two factors. How hard they push together and they type of surfaces involved. A smooth surface is much easier to slide on. Friction Gravity • A force that pulls two objects towards each other. • Two factors that affect gravity 1. Mass- the more mass the greater the gravitational pull. 2. Distance- the farther apart two objects are the less gravitational pull. Gravity Section 3 Newton’s 1st law of motion or law of inertia • An object at rest will remain at rest, and an object at a constant velocity will stay at a constant velocity unless acted on by an unbalanced force. • Inertia is the tendency of an object to resist motion. Newton’s 2nd law • Acceleration depends on the object’s mass on the net force acting on the object. • Acceleration=net force/ mass • Net force = mass X acceleration Calculate • A speedboat pulls a 55 kg water skier. The force causes the skier to accelerate at 2.0 meters per second per second. Calculate the net force that causes this acceleration. Newton’s 3rd law • States that if one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object. • For every action there is an equal and opposite reaction. Momentum • Characteristic of a moving object that is related to the mass and the velocity of the object. • Momentum of a moving object can be determined by multiplying the object’s mass and velocity. • Momentum = mass X velocity Momentum • Which has more momentum: a 3.0 kg sledgehammer swung at 1.5 m/s, or a 4.0 kg sledgehammer swung at 0.9 m/s? Conservation of Momentum • Conservation refers to the conditions before and after some event. • This law states that, in the absence of outside forces, the total momentum of objects that interact does not change. • It is the same before and after they interact. Collisions • Look at page 358 in your book.