Physics 1114 Summer 2020
Assignment 4
Start each problem on a new page. Refer to your Hand-in-Assignment Instructions.
1 On an ice rink, a puck slides past you at constant velocity.
For each of the following, explain if it is true or false.
a No forces act on the puck.
b A constant net force acts on the puck in the direction of the velocity.
c The net force acting on the puck is zero.
d The net force acting on the puck is its weight.
2 A 0.25kg ball is dropped from a height of 5m. It bounces back to a height of 4.05m. If the impact of the
ball with floor lasted 0.01s, find the average force that the floor exerted on the ball.
Steps:
• Find the velocity of the ball immediately before it hits the ground (use kinematics: therefore you need
a picture, define your coordinate system etc).
• Find the velocity of the ball immediately after it hits the ground (kinematics again) (use the same
coordinate system that you defined for the way down).
• BONUS draw position, velocity and acceleration versus times graphs for the bouncing ball.
• Then use the definition of aav=v
t to find the average acceleration.
• Draw a FBD of the ball hitting the floor including the direction of the acceleration. Apply Newton’s
second law in the vertical direction and solve for the normal force.
3 You are pushing a 12kg box across the floor with a constant force of magnitude
79.7N angled at 30∘
below the horizontal as shown. There is friction between the box
and the floor with coefficient of kinetic friction 0.4.
0 Assign appropriate symbols to numerical quantities.
a Given m , , k , and Fapp .
Find the acceleration of the box (symbols only) in terms of the givens.
b After you have found the completely symbolic expression for the acceleration, substitute in the numerical
values to calculate the numerical value of the acceleration.
c Are you working hard to push the box? Explain.
d Would it be easier to pull the box? Explain.
4 A mover applies a constant horizontal force F⃗ app of magnitude 500N to a 25kg box
accelerating it up a ramp angled at 40∘ at 0.75 m/s2
.
0 Assign appropriate symbols to numerical quantities.
a Given m , , Fapp and a . Find Fn , f k , then k .
b After you have found the completely symbolic expression for the coefficient of
kinetic friction, substitute in the numerical values to calculate its numerical value.
m
F
app
F
app
Checklist. For any force problem you must
• Draw a clear correct complete FBD of the object of interest. (No FBD, no marks!)
• Label each force with its correct unique name.
• Show the direction of the acceleration with an arrow like ⇒ .
• Define coordinate axes with the x-coordinate parallel to the direction of motion (or acc).
• Apply Newton’s second law in component form to the FBD: i.e., write ∑ Fx=m1 ax , ∑ F y=m1 ay .
Note that the only arrows that you write are on your FBDs.
• Use symbols first.
• Solve symbolically.
• Check the dimensions.
• If there are numerical values, substitute them in at the end with units (any dimensional numerical
quantities must have the proper units). Quote answers to 3 accurate digits.
• If appropriate, answer in a meaningful sentence.
• Is there enough information in your solution so the reader does not have to have this question sheet?
Ans 2 473N up. 3 a= F
app cosθ−μk ( m g+F
app sin θ)
m
, 0.5 m/s2. 4 μk= f k
Fn
= Fapp cosθ−m g sin θ−ma
F
app sinθ+m g cosθ ,
normal force has magnitude 509.267N, friction force, 206.63N, and coefficient of kinetic friction 0.406.
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