Lesson 4 • Does the amount of force affect motion?

Recall the previous lesson, focusing on the effect of mass on the motion of an object.

Recall the model developed in the simulation: as the mass of an object increases, the speed (or motion) of the object decreases (with a constant force).

Examine the Forces and sport Poster and invite students to identify three examples of sports where mass can affect motion. Examples might include the sled pull (shown on the middle left), weightlifting (by the pool), or any ball sport.

Pose the question: How does increasing the applied force affect motion in sport and physical activity?

(Slide 37) Discuss how increased force applied on objects like the tennis ball, soccer ball and bow affect the motion of the balls or arrow.

Optional: (Slide 38) Play Gorri (“gor-ri”) - a moving target game.

Explain that many Aboriginal men and boys from all parts of Australia played games that used rolling balls or discs. For example, in the disc-rolling game common throughout Western Australia, a piece of rounded bark (disc) was rolled by one of the players for the other boys to aim at. The boy who set the disc rolling was about 15 metres away from the throwers and would call out gool-gool (going-going) as they started the disc rolling. The boy or young man who succeeded in piercing the disc took the place of the roller. Accuracy of eye and speed in casting the spear were easily learned from the disc game.

Invite students to reflect on their experience playing Gorri and consider whether throwing with more force made a difference.

(Slide 39) Pose the question: How does increasing force affect motion?

(Slide 40) Explain to students they will be conducting an investigation to determine the effect of force on the motion of an object. First, they need to understand the skill of using a Newton meter so they can measure forces.

Demonstrate the use of the Newton meter to the class.

Newton meter explanation

1. Show students the Newton meter (also called a spring balance or force meter). Explain that inside the device is a spring that will extend when something is pulled or lifted/suspended. Show different Newton meters (e.g. 5 N and 20 N) and explain that they all measure force but have different scales of measurement.

2. Explain that in this experiment, students will use one of the Newton meters to measure force. The amount of extension of the spring inside represents the amount of force being exerted. Point out that for this reason, students need to be careful not to affect the accuracy of the device by extending the spring too much during their experiment.

3. Draw attention to the scales on the side of the meter—one scale gives a measurement in grams and the other gives a measurement in Newtons. Explain that the measurements for this lesson will need to be read from the Newton scale.
4. Check that both Newton meters are working (the spring will extend if the hook is gently pulled) and that it reads 0 N before starting.
5. Discuss with students which Newton meter (5 N or 20 N) would be appropriate to use in this experiment, noting that the maximum force required is 5 N.

(Slide 41) Set up the force launcher for the investigation by making a Newton scale.

(Slide 42) Remind students of the question posed at the start of the lesson: How does increased force affect motion? Refine the question so it addresses the investigation: What happens to the distance an object travels if more force is applied?

✎STUDENT NOTES: Write a hypothesis for the investigation.

(Slide 43) Provide students with the Force and motion investigation Resource sheet. Allow time for students to form groups, allocate roles, collect equipment and conduct the investigation. 

Investigation steps

1. Stretch the elastic between the legs of a stool, table or chair.
2. Mark a chalk line from leg to leg of table/chair/stool and place the edge of the Newton scale A4 paper (from above) against this line.
3. Place the block into the force launcher elastic and pull the block and elastic backwards so the edge of the block is lined up with the 1 N line.
4. Release the block and measure the distance the edge of the block (that was in contact with the elastic) travels from the chalk line in centimetres. Record this value in the results table. Repeat this process another two times.
5. Repeat steps 3 and 4 for 2 N, 3 N, 4 N and 5 N of force.

✎STUDENT NOTES: Record the distance the block travels in each trial. Calculate the average distance the block travelled (rounded to the nearest 0.5 cm to make it easier to graph).

Discuss the main features of a graph and how to scale the vertical axis and plot points on the grid.

✎STUDENT NOTES: Graph the data collected in the investigation.

(Slide 44) Use the graph to discuss the patterns and trends shown in the investigation if student data is compromised due to the experimenter inexperience.

Discuss the pattern shown in the graph (or the graph’s appearance). Guide students to identify that the data increases at a steady rate, as it goes up by the same amount each time and the line is straight, not curved.

Invite students to describe the trend of the data (or what happens to the dependent variable as the independent variable is changed).

Use sentence starters such as: As the force used on the elastic ____, the distance travelled by the wooden block ____.

Discuss how applying extra force to the wooden block increased the distance it travelled. Relate this to the relationship between the variables. Invite students to describe this relationship using phrasing such as: As the amount of force was doubled on the block, the block moved twice the distance.

Example graph

Students should produce a graph showing a positive linear line-of-best-fit with a direct relationship between the force applied to the block and the distance it travels. A sample graph is shown below.

✎STUDENT NOTES: Complete the conclusion and discussion questions on the Force and motion investigation resource sheet.

If students need support, then the following prompts could be used as temporary scaffolds and could be reduced in future investigations as students become more comfortable with the scientific terminology.

(Slide 45) Pose the questions: Which sports rely on increased force for extra distance or extra speed? Which sports rely on increased force to stop motion or to suddenly change direction?

Discuss how the forces applied to objects in the pictures relate to the motion of people and equipment. If required verbally model the thinking process for an image. You could, for example, highlight:

• what happens to the snowboarder’s direction when they apply more force through the board into the snow.
• when the wheelchair athlete pushes the ball harder, the ball’s speed as it leaves their hand is much faster as compared to a softer push.
• kicking the football with more force makes it leave the boot with more speed, and it goes farther than a soft kick.
• increased pulling force by the footballer would stop the kicker from being able to complete the kick.
• the bike image suggests that a large force has been applied, and as a result, the rider has suddenly stopped moving forward.

You could also model inferential thinking by asking questions like:

• If the person pushed or kicked with twice as much force, what do you predict would happen to the distance or speed of the ball or movement?
• Which sports in the images require the greatest force to change motion, and what evidence supports your decision?
• Looking at the wheelchair, what design features can you notice that might help the athlete move quickly or turn sharply so they can apply force in the right direction?
  • The wheelchair’s large, outward‑angled wheels increase stability and help the athlete turn quickly without tipping. Their position closer to the body places the hand rims at a more efficient angle, allowing stronger, more effective pushes. The lightweight design also helps the athlete build up speed more quickly.

(Slide 46) Discuss examples of force changing motion in students’ chosen sport or physical activity, and how they might represent this on their infographic in the Act phase.

✎STUDENT NOTES: Write a list of what to include on your infographic about forces, and a list of important movements in your chosen sport or physical activity.

Reflect on the lesson

You might ask students to:

• review the learning goals for the lesson and check for understanding.
• go back to the PhET Forces and motion: basics ‬computer simulation model and change the amount of force applied to a mass.
  • What effect does this have on the speed of the skateboard?
  • Determine if the data from this simulation shows the same trend as the class investigation.
• review force diagrams and draw three force diagrams for different motion scenarios.