Forces and Friction

• The weight of an object depends on the strength of gravity wherever the object is.

• The mass always stays the same.

Weight

Fw = mg

Gravity (9.8 m/sec2)

Mass (kg)

Weight force (N)

6.1 Calculate weight

• How much would a person who weighs 490 N (110 lbs) on Earth weigh on Jupiter?

• The value of g at the top of Jupiter’s atmosphere is 23 N/kg.

• (Since Jupiter may not actually have a surface, “on” means at the top of the atmosphere.)

Calculate force

• A 10-kilogram ball is supported at the end of a rope. How much force (tension) is in the rope?

Friction

• Friction results from relative motion between objects.

• Frictional forces are forces that resist or oppose motion.

Friction

Ff = Fn

Normal force (N)

Coefficient of friction

Friction force (N)

Calculate force of friction

• A 10 N force pushes down on a box that weighs 100 N. • As the box is pushed horizontally, the coefficient of sliding

friction is 0.25.• Determine the force of friction resisting the motion.

Sliding Friction

Ff = sFn

Normal force (N)

Coefficient of sliding friction

Friction force (N)

Table of friction coefficients

Calculate using friction

• The engine applies a forward force of 1,000 newtons to a 500-kg car.

• Find the acceleration of the car if the coefficient of rolling friction is 0.07.