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Transcript
PHYSICS 231 Lecture 8: Forces, forces & examples Remco Zegers Walk-in hour: Monday 10:00-11:00 am Room W109 Cyclotron Lab PHY 231 1 Forces seen in the previous lecture • Gravity: Force between massive object • Normal force: Elasticity force from supporting surface n=-FgL FgL=mgcos Fg//=mgsin Fg=mg PHY 231 2 Jumping! The pelvis has a mass of 30.0 kg. What is its acceleration? PHY 231 3 Tension T The magnitude of the force T acting on the crate, is the same as the tension in the rope. Spring-scale You could measure the tension by inserting a spring-scale... PHY 231 4 Problem What is the tension in the string and what will be the acceleration of the two masses? PHY 231 5 Friction Friction are the forces acting on an object due to interaction with the surroundings (air-friction, ground-friction etc). Two variants: • Static Friction: as long as an external force (F) trying to make an object move is smaller than fs,max, the static friction fs equals F but is pointing in the opposite direction: no movement! fs,max=sn s=coefficient of static friction • Kinetic Friction: After F has surpassed fs,max, the object starts moving but there is still friction. However, the friction will be less than fs,max! fk=kn k=coefficient of kinetic friction PHY 231 6 PHY 231 7 Problem A)If s=1.0, what is the angle for which the block just starts to slide? B)The block starts moving. Given that k=0.5, what is the acceleration of the block? PHY 231 8 All the forces come together... If a=3.30 m/s2 (the 12kg block is moving downward), what is the value of k? PHY 231 9 General strategy • If not given, make a drawing of the problem. • Put all the relevant forces in the drawing, object by object. • Think about the axis • Think about the signs • Decompose the forces in direction parallel to the motion and perpendicular to it. • Write down Newton’s first law for forces in the parallel direction and perpendicular direction. • Solve for the unknowns. • Check whether your answer makes sense. PHY 231 10