Force and Friction: Part 1 Review

Force and Friction: Part 1 Review

Reviewing concepts and problems involving free body diagrams, Newton's 2nd Law (F = ma), and friction.

Types of Forces

• Applied

• Gravity

• Normal

• Friction

• Drag

• Tension

Multiple Choice

A man pushes a box across the floor at a constant velocity. Which formula describes the vertical forces on the box?

•  $F_P\ -\ F_g\ =\ 0$ 

• $F_p+F_g\ -\ F_f\ =\ 0$

•  $F_n\ -10F_g\ =\ 0$ 

•  $F_n-mg\ =\ 0$ 

Multiple Choice

A man pushes a box across the floor at a constant velocity. There is a force from the man pushing on the box. This is the ...

• force of gravity

• normal force

• force of friction

• applied force

Multiple Choice

A man pushes a box across the floor at a constant velocity. There is a force on the box resisting its motion. This is the ...

• force of gravity

• normal force

• force of friction

• applied force

Multiple Choice

A man pushes a box across the floor at a constant velocity. There is a force on the box from the floor pushing the box up. This is the ...

• force of gravity

• normal force

• force of friction

• applied force

Multiple Choice

A man pushes a box across the floor at a constant velocity. There is a force on the box pulling it toward the center of the earth. This is called the

• force of gravity

• normal force

• friction force

• applied force

FBD: Free Body Diagrams

• Analyze problem for ALL present forces

• Balanced forces cancel out such as the case here with gravity and the normal force.

• This results in NO vertical acceleration due to a net force of 0 N.

• Applied and friction forces act horizontally opposed...this allows capacity for acceleration...IF there is a net force above 0 Newtons.

• 
  

• 
  

Friction Formula

• Friction is based on a formula using the Normal Force (N or $F_n$  )

• The "number" associated with the strength of the friction force based on the surface is called the friction coefficient 

• Static friction coefficient =  $\mu_s$  

• Kinetic (moving) friction coefficient =  $\mu_k$  

Multiple Choice

A man pushes a box across the floor at a constant velocity. If another box were put on top of this box, what would happen to the force of friction?

• It would increase

• It would decrease

• It would remain unchanged

Multiple Choice

What is the unit called that measures force?

• weight

• meter

• Newton

• kilogram

Multiple Choice

What direction does friction act relative to motion?

• in the same direction

• in the opposite direction

• need more information

• depends on the force

Calculate Force due to Friction

• You need the Normal Force

• You need the friction coefficient

Multiple Choice

 What is the force of friction between a block of ice that weighs 950 N and the ground if μ = .12? 

• 65 N
• 226 N
• 175 N
• 114 N

Multiple Choice

What is the coefficient of static friction if it takes 44 N of force to move a box that weighs 86 N? 

• 0.78
• 0.51
• 0.78 N
• 0.58 N

Applied Forces vs. Friction

• Using the given coefficient for STATIC friction, we first consider: "Is the push or pull strong enough to overcome friction?"

• If not, the object doesn't move. We are done!

• If it does, then we move on to using the KINETIC coefficient to determine net acceleration.

Multiple Choice

What is the NET FORCE of this diagram?

• 17 N, right

• 17 N, left

• 23 N, right

• 12 N, up

Multiple Choice

Name the missing force...

• applied force
• weight force
• tension force
• drag

Multiple Choice

Name the missing force...

• frictional force
• weight force
• thrust
• buoyant force

Multiple Choice

A box slides across a floor at a constant velocity. What forces act on the box?

• friction, gravity
• friction, tension, gravity
• friction, normal force, gravity
• friction, normal force, net force

Sample Problem: Does it Move?

A 5 kg box sits on a floor that has a static friction coefficient of 0.45 and a kinetic coefficient of 0.32. If I push the box horizontally with a force of 20N, will it move?

Sample Problem 2: Does it Move? How fast?

The same 5kg box in the same room with the static coefficient = 0.45 and the kinetic = 0.32 is pushed. This time, I use a 30N force. Will it move? If so, what is the net acceleration?

Fill in the Blanks

A 10kg desk sitting a carpeted floor. Jovani wants to push it but can only apply a maximum of 80N horizontally. Based on the static friction coefficient of 0.75, determine the maximum force friction can apply.

(Round to the nearest tenth.)

Multiple Choice

A 10kg desk sitting a carpeted floor. Jovani wants to push it but can only apply a maximum of 80N horizontally. Based on the static friction coefficient of 0.75, will Jovani be able to move the desk?

• yes

• no

• can't conclude yet

Multiple Choice

A 10kg desk sitting a carpeted floor. Jovani wants to push it but can only apply a maximum of 80N horizontally. Based on the static friction coefficient of 0.75 and a kinetic friction coefficient of 0.60, what net force is Jovani actually going to be applying to the desk as it moves?

• 80 N

• 21 N

• 6.5 N

• 25 N

• 32N

Multiple Choice

A 10kg desk sitting a carpeted floor. Jovani wants to push it but can only apply a maximum of 80N horizontally. Based on the static friction coefficient of 0.75 and a kinetic friction coefficient of 0.60, with what acceleration will the box move?

• 0 m/s/s

• 2.1 m/s/s

• 0.210 m/s/s

• 0.65 m/s/s