Isaac Newton and Me
Problem:
What relationship exists between mass, force and acceleration?
Materials:
Scooter Boards from PE, skateboards, or rollerblades
Spring scales, stopwatches, meter sticks.
Procedure:
1. Select a starting point and measure distances of 5 m, 10 m, and 15 m from that point.
2. Place a student on the cart or skateboard. That student must hold onto one end of the spring scale.
3. The student pulling the cart/skateboard must pull with the spring scale at a constant value and with the scale level.
4. Place students with stopwatches at each of the meter marks. When signaled, each timer starts the stopwatch and the person pulling the cart/skateboard starts to run. Timers stop their stopwatches when the cart/skateboard passes them.
5. Repeat 4 times, varying the force with rider #1 twice and using one of the forces applied to rider #1 with two other riders of different masses (riders #2 and #3).
Data Table
Rider #
/Force applied N
/Time for 5 m
/Time for 10 m
/Time for 15 m
11
1
2
3
Summing Up:
1. Compare the amount of time it takes to travel 15 m for each of the forces applied to rider # 1. How does varying the applied force relate to the change in velocity?
2. Compare the time it takes to travel each 5 m segment. What does this difference in time infer? (SUBTRACT THE TIME FOR 5 m FROM THE TIME FOR 10 m AND THE TIME FOR 10 M FROM THE TIME FOR 15 m)
3. How does acceleration (a = Δv/Δt) relate to the net force?
4. Sketch a graph of applied force versus acceleration. What type of relationship does this graph represent?
5. Compare the amount of time it takes to travel 15 m for each of the 3 riders pulled with force #2. How does the acceleration of the object relate to mass of the object?
6. Sketch a graph of mass versus acceleration. What type of relationship does this graph represent?
7. If a force of 20 N is applied to the cart/skateboard and the rider and no movement results, what do you believe is the problem?
8. Ask the person pulling the cart/skateboard and the person riding the cart/skateboard to describe the physical sensations they had as the distance and applied forces increased.
Isaac Newton and Me
Teacher’s Notes
This activity is of high interest and requires approximately one class period for data collection. To insure safety have students sit on the skateboards or scooter cars or if they are using roller skates or blades have another student jog along beside them as a catcher to reduce the possibility of falls. If you do not have large spring scales you may use a parallel combination of smaller scales or calibrate a bungy cord on a meter stick by applying a force and measuring the stretch in cm of the cord.
An understanding that a nonzero net force produces an acceleration that is related to the mass is the goal of this activity. Prior to the lab readdress the differences between velocity and acceleration. A constant net force does not produce a constant velocity. Introduce that the unit of the force is a Newton (N). 1 N = 1kgm/s2. An unbalanced force (N) causes a mass (kg) to accelerate (m/s2).
Students should recognize that there is a force that opposes the motion of the cart and students, friction. In this instance it is considered negative if the direction of the velocity is positive. If directed to pull the cart and rider at a constant velocity, the force of friction can be measured numerically. Have students pull students having different masses at a constant velocity to develop the concept that the force of friction has a dependence on the weight of the object being moved. Review the concept that weight is a downward force resulting from the acceleration of gravity on a mass as in the Inertia Balls demonstration.
Answers to Summing Up:
1. How does varying the applied force relate to the change in velocity?
Ans: The final velocity is greater with a greater applied force.
2. What does this difference in time infer?
Ans: If the force is constant, the time it takes to travel successive 5 m distances will decrease. Hence the students are accelerating.
3. A greater net force produces a greater acceleration.
Ans: How does acceleration (a = Δv/Δt) relate to the net force?
4. Sketch a graph of applied force versus acceleration. What type of relationship does this graph represent?
Ans: Students should sketch a linear or direct relationship for their graph.
5. How does the acceleration of the object relate to mass of the object?
Ans: The greater the mass the less the acceleration.
6. Sketch a graph of mass versus acceleration. What type of relationship does this graph represent?
Ans: Students should sketch an inverse or indirect graph.
7. If a force of 20 N is applied to the cart/skateboard and the rider and no movement results, what do you believe is the problem?
Ans: There is not enough force to overcome inertia or friction is a likely response.
8. Ask the person pulling the cart/skateboard and the person riding the cart/skateboard to describe the physical sensations they had as the distance and applied forces increased.
Ans:The student pulling the cart/skateboard often states that he/she felt like they were going to be run over. The person riding often expresses concerns about going too fast with greater force and a fear of crashing.
Jan Mader and Mary Winn
Diagram by Russel Davison, King High School, Tampa, FL