# PHY102 Motion Exercises week 1

PHY-102: Motion Exercises

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**1. Jane is collecting data for a ball rolling down a hill. She measures out a set of different distances and then proceeds to use a stopwatch to find the time it takes the ball to roll each distance.**

**a. What is the independent variable in her experiment?**

**answer:***The time of the ball rolling down the hill*

**b. What is the dependent variable in her experiment?**

**answer:** *The hill the ball rolls down*

**c. Give one control variable for her experiment.**

**answer:** *The distance the ball rolls depends on the time taken to roll down the hill*

**2. Consider an experiment where you drop an object.**

**a. Briefly describe your proposed experiment. (Make sure it is controlled).**

**answer:** *Collecting data from dropping a water balloon from stairs. Measure out the distances from different levels of floors, use a stopwatch and find out how long it will take for the water balloon to drop from each floor.*

**b. What would be the independent variable for your experiment?**

**answer:** *The independent variable would be the time it took the balloon to fall*

**c. What would be the dependent variable for your experiment?**

**answer:** *The dependent is the speed in which the balloon will go*

**d. Give one control variable for your experiment.**

**answer:** *Control variable is the distance it took for the balloon to fall down to the ground*

**3. Consider a freely falling object.**

**a. What is the acceleration (in m/s2) after 5 seconds of fall?**

**answer:** *9.8 m/s^2*

**b. What is the acceleration (in m/s2) after 10 seconds of fall?**

**answer:** *9.8 m/s^2*

**c. What is the velocity (in m/s) after 5 seconds of fall?**

**answer:** *49 m/s*

**d. What is the velocity (in m/s) of 10 seconds of fall?**

**answer:** *98 m/s*

**4. A sign is hung between two cables as illustrated below. If the sign weighs 350 N, what is the tension (in N) in each cable?**

M1F1

*Tension on each cable is 125N*

**5. A construction worker on a high-rise building is on a platform suspended between two cables as illustrated below. The construction worker weighs 850 N, the plank weighs 450 N, and the tension in the left cable is 550 N.**

**a. What is the tension (in N) in the right cable?**

**answer:** N = 750

**b. Explain your answer.**

*Applying static means that the sum of the forces in the equilibrium without movement must be equal to zero*

– 450n + 850n – 550n

– 1300n – 550n =750n

**6. Two forces of 50 N and 30 N, respectively, are acting on an object. Find the net force (in N) on the object if
**

**a. the forces are acting in the same direction**

*So, if the forces are in the same direction, it’ll be 50N+30N= 80N!*

**b. the forces are acting in opposite directions.**

*50N-30N= 20N!*

**7. A box is pulled straight across the floor at a constant speed. It is pulled with a horizontal force of 48 N.**

**a. Find the net force (in N) on the box.**

**answer:**

*F_net=ma*

*The box is pulled at constant speed, so it acceleration is zero. Therefore F_net=0*

**b. Find the force of friction (in N) from the floor on the box.**

**answer:**

*F_net=F-F_frict*

*So F_frict=F=48 N*

**c. The person pulling on the box stops pulling and the box comes to a rest. Find the force of friction (in N) on the box when at rest.**

**Answer:*** When the box on the horizontal floor is at rest, the friction force is zero.*

**8. A bowling ball rolls 32 meters in 0.8 seconds. Find the average speed (in m/s) of the bowling ball in m/s.**

**Solution:**

Average speed :v=st (1)

,where s– all the way, t– all the time

Of (1) v= 40 m/s

**Answer:** 40 m/s

**9. A car accelerates from 3.5 m/s to 17 m/s in 4.5 seconds. Find the acceleration of the car in m/s2.**

**Answer and Explanation**

The acceleration of the car in m/s2 is given by the formula;

The acceleration=(V-U)/t, where V is the final velocity of 17 m/s,U is the initial velocity of 3.5 m/s and t is the time taken to accelerate which is 4.5 seconds.

The acceleration=(V-U)/t

The acceleration=(17 m/s- 3.5 m/s)/4.5 m/s

The acceleration=*3.0 m/s^2*

**10. Rank the following from lowest to highest:**

a. The support force on you standing in an elevator at rest.

b. The support force on you standing in an elevator accelerating upward.

c. The support force on you standing in an elevator accelerating downward.

**Answer:** **(c) < (a) < (b)**

**Step-by-step explanation**

~ In (a) Elevator is at rest, so acceleration of elevator = 0 m/s^{2}

so support force = N_{a} = mg ( where m = mass, g = gravitation acceleration )

~ in (b) elevator is accelerating upward,

so support force = N_{b} = mg + ma ( where a is acceleration of elevator )

~ in (c) elevator is accelerating downward,

so support force = N_{c} = mg – ma

we see that,

**N**_{c}** < N**_{a}** < N**_{b}

hence correct order from lowest support force to highest force is,

**(c) < (a) < (b)**

**11. Describe the speed and acceleration of the ball released from the top of the track shown in the figure below.**

*Slope downward, speed will increase as it falls down the track once it hits the ground it will slow down since the track is straight down. Increase speed and decrease acceleration.*

**12. Describe the speed and acceleration of the ball released from the top of the track shown in the figure below.**

*The balls speed will increase until it runs out of motion, increasing speed and increased acceleration due to the center being curved*

**13. Describe the speed and acceleration of the ball released from the top of the track shown in the figure below.**

*Increase speed and decrease acceleration with the curving of the line, it will have a slower speed going straight down, decrease acceleration*

**14. You throw a ball upward with a speed of 14 m/s. What is the acceleration of the ball after it leaves your hand? Ignore air resistance and provide an explanation for your answer.**

*– 9.8 m/s^2 as it goes downward due to gravity pulling it down*

**15. How would your answer to the previous question change if you take into account the effects of air resistance?**

*The objects speed would be higher after the start with 9/8 m/s^2 as it got closer down speed would decrease.*

**16. Describe the speed and acceleration of a person sky diving. Include in your explanation a description of the motion before the parachute is opened as well as a description of the motion after the parachute is opened.**

*Newton’s second Law explains how the ski divers speed would accelerate. Due to falling downwards the force of gravity is pushing him towards the ground. But once the parachute is opened. Then switches to velocity which will push him upwards and allow him to control the parachute*

**17. A net force of 24 N is acting on a 4.0-kg object. Find the acceleration in m/s**^2**.**

*F=ma*

*– mass= 4Kg*

*– net force= 24N = Kgm/ S^2*

*– 24kgms/s= 4kga*

*– Divide both by 4*

*– 24 kgm/s^2 /4kgs/4kg*

– *A=6m/s^2*

**18. A person pulls horizontally with a force of 64 N on a 14-kg box. There is a force of friction between the box and the floor of 36 N. Find the acceleration of the box in m/s2. Show your work.**

*F=ma*

*– 64N-36N=14a*

*– 28=14a A=28/14*

*– =2m/s^2*

The remaining questions are multiple-choice questions:

**Answer:**

**19. One difference between a hypothesis and a theory is that a hypothesis**

A. is a guess that has not been well tested, whereas a theory is a synthesis of well-tested guesses.

B. is testable, whereas a theory is not testable.

C. can be revised, whereas a theory cannot be revised.

D. is not testable, whereas a theory is testable.

**Answer:** *is a guess that has not been well tested, whereas a theory is a synthesis of well-tested guesses.*

**20. A car starts from rest and reached a speed of 24 m/s in 6 seconds. What is the acceleration of the car?**

A. 144 m

B. 6 m/s2

C. 4 m/s2

D. 10 m/s2

E. 0 m/s2

**Answer:** 6 m/s2

**21. Which of the following forces is NOT a contact force?**

A. *Friction force*

B. Support force

C. Force of gravity

D. Tension force

**Answer:** *Friction force*

**22. If you pull horizontally on a desk with a force of 150 N and the desk doesn’t move, the friction force must be 150 N. Now if you pull with 250 N so the desk slides at constant velocity, the friction force is**

A. more than 150 N, but less than 250 N.

B. 250 N.

C. more than 250.

**Answer:** *250 N.*

**23. Suppose a particle is accelerated through space by a constant 10 N force. Suddenly the particle encounters a second force of 10 N in a direction opposite to that of the first force. The particle**

A. is brought to a rapid halt.

B. theoretically accelerates to speeds approaching the speed of light.

C. continues at the speed it had when it encountered the second force.

D. gradually slows down to a halt.

**Answer:*** continues at the speed it had when it encountered the second force.*

**24. Newton’s First Law of Motion applies to**

A. objects at rest only.

B. moving objects only.

C. both moving and non moving objects.

**Answer:** *both moving and non moving objects.*

**25. A freely falling object starts from rest. After falling for 2 seconds, it will have a speed of about**

A. 5 m/s

B. 10 m/s

C. 20 m/s

D. 40 m/s

**Answer:** *20 m/s*

**26. Suppose an object is in free fall. Each second the object falls**

A. the same distance as in the second before.

B. a larger distance than in the second before.

C. with the same instantaneous speed.

D. with the same average speed.

**Answer:** *a larger distance than in the second before.*

Answered Exam Questions Exam: 02.10 Module Two Exam

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