250 likes | 478 Views
NAME: _______________________________________ Forces Do Now. Review-Calculate net force. Four people are pulling on the same 200 kg box with the forces shown. Review-Calculate force. A woman is holding two dogs on a leash.
E N D
Review-Calculate net force • Four people are pulling on the same 200 kg box with the forces shown.
Review-Calculate force • A woman is holding two dogs on a leash. • If each dog pulls with a force of 80 newtons, how much force does the woman have to exert to keep the dogs from moving?
Friction • Frictionis a force that resists the motion of objects or surfaces. • It is the force that two surfaces exert on each other when they rub against each other.
Friction Depends on the: • Types of surfaces • Force between the surfaces • When the hockey puck slides on ice, a thin layer of water between the rubber and the ice allows the puck to slide easily.
Identifying friction forces • Friction is a force, measured innewtonsjust like any other force. • Static frictionkeeps an object atrest from moving.
Identifying friction forces • Sliding frictionis a force that resists the motion of an object moving across a surface. • The object is already moving in sliding friction.
Friction Friction is greater... • between rough surfaces • when there’s greater force between surfaces (such as more weight)
Reducing the force of friction • Unless a force is constantly applied, friction will slow all motion to a stop eventually. • It is impossible to completely get rid of friction, but it can be reduced.
Useful friction • Friction is also important to anyone driving a car. • Grooved tire treads allow space for water to be channeled away from the road-tire contac point, allowing for more friction in wet conditions.
Useful friction • Shoes are designed to increase the friction between their soles and the ground. Why do you think these shoes increase friction? • Players wearing cleats can apply much greater forces against the ground to help them move and to keep from slipping.
Friction and energy • Friction changes energy of motion into heat energy.
Friction and energy • Each time two moving surfaces touch each other, tiny bits of material are broken off by friction. • Breaking off bits of material uses energy. Friction has affected which pile of stones more?
Video and Practice • http://science.howstuffworks.com/transport/engines-equipment/maglev-train.htm • Answer questions 1(a, b, c, d) and 2 (a and b) with your partner.
Friction Equation Normal force (N) Ff = m Fn Friction force (N) Coefficient of friction
What is the normal force? • A normal force is created whenever an object is in contact with a surface. • The normal force has equal strengthto the force pressing the object into the surface, which is often the object’s weight. The normal force is sometimes called the support force.
G: Given U: Undefined E: Equation S: Substitute S: Solve
A box weighs 100 N, which is the NORMAL FORCE. As the box is pushed horizontally, the coefficient of sliding friction is 0.25. Determine the force of friction resisting the motion. • G: Fn= 100N us=0.25 • U: Ff • E: Ff = usFn • (Frictional Force = Coefficient of Friction x Normal Force) • S: Ff= 0.25 x 100N • S: Ff= 25 N
A steel pot with a weight of 50 N sits on a steel countertop. The coefficient of friction is 0.74. How much force does it take to start the pot sliding? Calculate using friction
A steel pot with a weight of 50 N sits on a steel countertop. The coefficient of friction is 0.74. • How much force does it take to start the pot sliding? G: U: E: S: S: