Year 9 Science · Unit 4 · Lesson 16
Challenge Worksheet
Learning Goals
Read the graph
The bar chart below shows the braking force needed to stop four different vehicles from 100 km/h (27.8 m/s) in 4 seconds. Use it to answer the questions.
Data: calculated using F = ma, a = 6.95 m/s²; values rounded. Source: physics principles (Australian Design Rules, Austroads).
(a) Which vehicle requires the greatest braking force? How much greater is it than the small car's braking force? Calculate the ratio.
(b) All four vehicles stop from the same speed in the same time. Calculate the deceleration for any one vehicle and explain why the deceleration is the same for all four, even though the forces are very different.
(c) Explain why trucks need longer stopping distances in real life, even though the graph assumes they all stop in 4 seconds. Refer to Newton's Second Law and the limits of braking force.
Someone claims…
"Newton said that objects only move when a force acts on them. So if I'm rolling a ball across the floor and I take my hand away, the ball must slow down because of Newton's First Law — the law says objects stop when there's no force pushing them."
(a) Identify the error in this claim. State what Newton's First Law actually says, and explain why the ball slows down. Name the specific force responsible for the change in motion.
(b) Draw a free-body diagram in the box below for a ball rolling at constant speed on a flat surface. Label all forces acting on the ball, including their directions. Use arrow length to show relative magnitude. Explain what the net force is and why.
Draw your free-body diagram here. Use arrows from the centre of the object. Label each force with its name and direction.
(c) A 5 kg box is pushed across a floor by a 25 N applied force. Friction acts on the box with a magnitude of 10 N in the opposite direction. Using F = ma, calculate: (i) the net force on the box; (ii) the acceleration of the box. Show all working and include units.
Wrap Up
In one sentence, what was the main idea of this lesson?