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
Weight
force (N)
Fw = mg
Gravity (9.8 m/sec2)
Mass (kg)
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
Friction force (N)
Ff = m Fn
Normal force (N)
Coefficient of friction
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
Friction force (N)
Ff = msFn
Normal force (N)
Coefficient of
sliding friction
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.