Unit 4 Activity 1 Extension A Quiz

Two skateboarders start themselves moving at the same speed side-by-side. After their initial pushes, they slow down gradually due to the frictional force acting on each of them. The masses of the skateboarders and the strengths of the frictional forces are shown in the force diagrams below. Assuming they are initially moving at the same speed, which of the skateboarders will slow to a stop first, and why? (Hint: Think carefully about whether the first one to stop will have a higher or lower rate of change in his speed.)

A. Skateboarder 2 will stop first because the quantity force strength/mass is higher for him.

Frank and Hector are moving two boxes across a rough floor. Frank pushes a 20 kg box with a force of 85 N while a frictional force of 75 N acts on it in the opposite direction. Meanwhile, Hector pushes on the 30 kg box with a force of 105 N while a frictional force of unknown strength acts on it. Frank and Hector find that as their boxes are moving, they remain side-by-side, so they realize they must both be changing speed at the same rate. What is the strength of the frictional force acting on the 30 kg box?

B. 90 N

When shopping for a new car, a professor was surprised to learn that a certain small compact car (mass=800 kg) will speed up (accelerate) at a faster rate than a large SUV (mass=2200 kg), although the SUV has the more powerful engine. What is the reason that the compact car can speed up at a faster rate than the SUV, despite having a less powerful engine?

B. The quantity pushing force / mass is greater for the compact car.

Shown below are force diagrams for three carts that have different masses and are being pushed forward by different strength fans. If these three carts were in a race, which one would speed up at the slowest rate and hence lose the race? Select the cart that you think will finish LAST in a race between them.

Cart C

At the beginning of this extension activity, we asked how we could tell which of the two boxes below would speed up at the higher rate. The ideas you have developed in this assignment should help you decide the answer to this question.

D. The 35 kg box will speed up at a higher rate because the quantity net force strength/mass is greater for this box.