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.