Types of forces

Forces acting on an object

All forces are measured Newtons. Not all forces are present in every

situation. Identify the forces that apply to each

particular situation.

Weight

Weight = mass x gravity The force of weight acts on every

mass that is attracted due to gravity. What is the weight of a 5 kg object

here on Earth? (5kg)(9.8 m/s2) = 49 N

Normal force

The normal force is the opposite but equal force of Newton’s 3rd Law.

An object resting on a desk exerts a force downward on the desk due to gravity. The desk exerts a force upward on the object.

How do we know these forces are equal in size?

Normal force

The upward force in this example is the normal force.

The normal force exists between any 2 objects in contact, and is always perpendicular to the surfaces in contact.

For a rigid horizontal surface, the normal force equals the weight (equal in size but opposite in direction). N = W = mg

Normal force on inclined surface•For an object resting on an inclined surface, the normal force does NOT equal the weight. •By law of similar triangles, the angles are equal (show why).•Then mg can be resolved into parallel and perpendicular components.•The perpendicular component of the weight is the same size as the normal.•You can see that N=mgcosθ

Normal force on inclined surface

For an object resting on an inclined surface, the normal force does NOT equal the weight.

N = mgcosθ (equation used to find size only)

θ is the angle between the surface and the horizontal.

example

Normal force activity

Materials: wood mass, spring scale, protractorTask: 1. Gather and record enough data to calculate the

normal force felt by a wood mass resting on an inclined plane.

2. Then measure and record the actual normal force (it will be the force to just perpendicularly pull the mass off the inclined plane.

3. Calculate percent difference between two values.4. Repeat for a plane inclined at a different angle.5. Use independent thought to write a paragraph

analyzing what was learned, what went wrong, how to make it better, etc

Tension

Tension is the force in a rope, cable, or wire that is attached to an object.

Tension always points away from the object along the rope, cable, or wire.

Tension drawings.

Upthrust

Upthrust is the force experienced by a body immersed in a fluid (a gas or a liquid). Also known as the buoyant force (FB).

Upthrust pushes up on the body. If upthrust ≥ weight, the body will float.

The amount of upthrust is equal to the weight of the fluid displaced by the body.

Note the relative size of each arrow

Air Resistance

Air resistance is the force that opposes motion of bodies through the air.

The amount of air resistance depends upon the speed, shape, and size of the body.

When air resistance = weight, the body attains terminal velocity and no longer accelerates while falling.

A more spread-out shape means more air resistance and a smaller terminal

velocity.

Free Body Diagrams

In a free body diagram (FBD), the object is drawn as a point.

All the forces acting on the object are drawn as arrows radiating from the point.

Using FBDs

Steps to using FBD to solve a problem: Step 1: Resolve each force into its x and y

components. Step 2: Add all the x components together -

watch signs! If the object is at equilibrium (not accelerating) – the sum of the forces is equal to 0.

Step 3: Repeat step 2 for the y components. Step 4: Solve simultaneous equations.

Equations used with FBDs

Fx = Fcosθ

Fy = Fsinθ

∑ Fx = 0 if the object is not accelerating in the x direction (ax = 0 m/s2)

∑ Fy = 0 if the object is not accelerating in the y direction (ay = 0 m/s2)

Examples

Draw an FBD for each problem, and an equation for ΣFx and ΣFy

1. A 3.0-kg textbook rests on a horizontal table.

2. A 3.0-kg textbook rests on a plane that is inclined at 35° from the horizontal. There must be a force holding the book in place – draw that and label it Fapplied. Include it in your equations.

3. A 3.0-kg textbook is suspended from the ceiling by a string.

FBD activity

Normal force as a result of other forces

Using FBD’s, we can see the result that an applied force will have on the magnitude of the normal force.

If an object has an upward force acting on it, the normal force will be reduced.

If an object has a downward force acting on it, the normal force will be increased.

examples

Friction

Friction is a force that opposes motion (points in the opposite direction as motion).

Friction exists between 2 masses in contact.

Friction tends to slow motion down.

2 kinds of friction

• Two kinds of friction: Static friction – the frictional force

between 2 objects that aren’t moving with respect to each other.

Kinetic friction – the frictional force between 2 objects that are moving with respect to each other.

2 kinds of friction

It is harder to get an object moving than to keep it moving.

The force due to static friction is always larger than the force due to kinetic friction.

Friction equation

Friction is fun! f = μN f is the force due to friction (in

Newtons). μ is the coefficient of friction (unitless) N is the normal force (in Newtons).

Coefficient of friction

μ is a characteristic of the 2 surfaces in contact.

A rougher, bumpier surface would have more friction and a bigger μ. Ex

A smoother surface would have less friction and a smaller μ. Ex

μ is unitless and always < 1.0.

2 coefficients of friction

μs is the coefficient of static friction.

μk is the coefficient of kinetic friction. Which one will be greater? Coefficient of static friction is always

greater than kinetic friction.

Friction examples

What is the frictional force acting on a box if I am pushing on it with a force on 4 N but the box isn’t moving?

4N

f smax

f smax is the maximum static friction force that must be overcome before the object can begin to move.

f smax = μs N

Once the friction force is larger than f smax, and the object begins to move, then the object experiences kinetic friction.

Examples

Friction activity

Find the frictional forces of stationary and moving objects on horizontal and inclined surfaces.