Forces – Chapter 4. What are forces? A force is a push or pull exerted by one object on another object Forces are vectors; they have magnitude and direction.

Forces – Chapter 4

Many orangutans spend their entire lives among the trees and are well adapted to move in this arboreal habitat. They have long arms (about two-thirds of their body height) and powerful chest muscles. Suppose an adult orangutan is hanging by its arms from a tree branch. The angle between each of the animal’s arms and the vertical is 15° with each arm exerting a force 430 N. The gravitational force acting on it is 830 N. Draw a free-body diagram of the animal. Farm = 430

NFarm = 430 N

Fgravity = 830 N

Θ = 30o

Newton’s 1st Law of Motion

An object at rest remains at rest, and an object in motion continues in motion with constant velocity unless the object experiences a net external force.

Force and Acceleration

Forces can cause accelerations (speeding up, slowing down, changing direction)

If the net force on an object is not zero, the object will accelerate

If the net force on an object is zero, the object will not accelerate

Inertia

Newton’s 1st Law is also called the Law of Inertia

Inertia is the tendency of an object to resist changes in its motion

Mass is a measure of inertia. The more mass an object has, the more inertia it has.

Newton’s Second Law of Motion

If a net force is applied to an object, this causes it to accelerate. The acceleration is directly proportional to the force, and inversely proportional to the mass.

Force = mass x acceleration

F = ma

Units

Force: Newtons Mass: kilograms Acceleration: m/s2

Because mass and acceleration are multiplied in F=ma, we would expect the units for force to be kg m/s2. A Newton is equivalent to a kg m/s2.

Newton’s Third Law

All forces occur in pairs, and these two forces are equal in magnitude and opposite in direction.

This is commonly paraphrased as “For every action force, there is an equal, but opposite, reaction force”.

Action-Reaction Forces Don’t Cancel

Force pairs do not cancel each other because they act on different objects

Acceleration Depends on Mass

Although the forces on each object are equal, the accelerations are not equal (unless the mass of both objects is the same).

The object that has less mass will experience a greater acceleration than the object with more mass.

Examples of Force Pairs

As Earth exerts a gravitational force on you, you exert a gravitational force on Earth.

Examples of Force Pairs

When walking, your foot pushes backward against the ground, and the ground pushes forward against your foot.

Examples of Force Pairs

A rifle pushes forward on a bullet, and the bullet pushes backward on the rifle (recoil).

Practice Questions

Determine the reaction force.

1. Athlete pushes bar upwards.

2. Bowling ball pushes pin rightwards.

3. Compressed air pushes balloon wall outwards.

Practice Questions

4. A 60 kg woman and 80 kg man push off each other while ice skating. Compare the force experienced by each one. Compare the acceleration experienced by each one.

Friction Opposes MotionFriction Opposes Motion

Friction acts in a direction opposite Friction acts in a direction opposite to an object’s motion.to an object’s motion.

If the object is not moving when If the object is not moving when there is a force attempting to make it there is a force attempting to make it move, friction acts in a direction move, friction acts in a direction opposite to that force.opposite to that force.

What causes friction?What causes friction?

Friction is caused by the roughness of Friction is caused by the roughness of the two surfaces. the two surfaces.

Even though a surface may feel Even though a surface may feel smooth, at the microscopic level it is smooth, at the microscopic level it is actually rough.actually rough.

The two surfaces areThe two surfaces are in contact in just a in contact in just a

few places.few places.

Types of FrictionTypes of Friction

Static friction: The force that resists Static friction: The force that resists the initiation of sliding motion the initiation of sliding motion between two surfaces that are in between two surfaces that are in contact and at restcontact and at rest

Kinetic friction: The force that Kinetic friction: The force that opposes the movement of two opposes the movement of two surfaces that are in contact and are surfaces that are in contact and are sliding over each othersliding over each other

Coefficients of Friction (Coefficients of Friction ())

A coefficient of friction is a ratio of A coefficient of friction is a ratio of the force of friction to the normal the force of friction to the normal force between two surfacesforce between two surfaces

Coefficients of friction depend on the Coefficients of friction depend on the material of the surfacesmaterial of the surfaces

Coefficients of friction do not have Coefficients of friction do not have unitsunits

Coefficients of FrictionCoefficients of Friction

**Notice that static friction is greater than kinetic friction

Calculating the Force of FrictionCalculating the Force of Friction

FFff = = NNFFf: f: Force of frictionForce of friction

FFn: n: Normal forceNormal force

: Coefficient of friction: Coefficient of friction

This means the frictional force is affected by the mass This means the frictional force is affected by the mass of the object and the roughness of the surfaces.of the object and the roughness of the surfaces.