4.1 A force is a push, a pull or a twist

Learning intentions

By the end of this topic, you will be able to: • explain why a measuring device must be calibrated • provide examples of forces in reallife situations.

gravity the force of attraction between objects due to their masses

Figure 2 Spring balances are used to measure force.

spring balance a device consisting of a spring and a scale, used to measure forces

calibrate check the accuracy of a meter or measuring device against known measurements

newton the unit used to measure force; symbol N

Key ideas

• A force is a push or pull that happens when two objects interact. • The effect of a force can be measured. • Gravity affects the movement of objects on Earth.

Forces in action

Forces act on everything around us all the time. Usually, more than one force is acting on an object at one time, but often we do not notice them. You have many forces acting on you at the moment. Gravity is pulling you towards the centre of the Earth. The chair you are sitting on is pushing back against you, changing the shape of your leg muscles. Because the forces acting on you are in balance (the same strength), you do not move. You sit still on the chair. When you kick or throw a ball, you use energy to generate a push force. This force causes the ball to move (Figure 1). When you catch a ball, you still give it a push. This time, the push force causes the ball to stop moving. Forces act on everything around us all the time. Forces cause objects to: > begin to move > speed up

> slow down or stop moving > change direction > change shape > spin > remain still. Examples of these forces are shown in Figures 3–9. Measuring forces One way to ‘see’ a force at work is to measure it. In the kitchen, cooks use scales to measure how much the Earth’s gravity pulls on the ingredients. Twenty grams of fl our is pulled to the centre of the Earth, causing the fl our to push down on the scales. In the laboratory, force is measured using a spring balance (Figure 2). A stiff spring in the balance stretches when an object pulls on it. This moves the marker so that the amount of force can be measured. A rubber band can measure the size of forces in a similar way to a spring balance. DRAFT

Figure 1 The force of Ronaldo kicking the ball is easy to identify and describe, but what is pulling him towards the centre of the Earth?

Before we can use a rubber band to measure a force, it must be calibrated .

This means matching the stretch of the rubber band to the force pulling on it. The unit used to measure forces is called the newton after

English physicist Sir Isaac Newton (1642–1727), who fi rst described Figure 5 Slows the force used to pull an apple down. The brakes from a tree. Spring balances are also on this bicycle wheel sometimes known as newton meters. push down on the rotor

Scientists around the world have agreed to this standard measurement so that they can communicate with one another. In every of the wheel, causing the wheel to slow down. This in turn brings the bicycle to a stop. country, the force of 100 g being pulled to the centre of the Earth is about 1 newton (N). This is about the same as one large chocolate bar sitting on your hand.

Figure 8 Spins.

Figure 6 Changes direction. The tennis racquet pushes the ball The hand turns the knob to open the door. in a different direction.

Figure 3 Begins to

move. The golf club pushes the ball. The club exerts a force on the ball, causing it to begin to move. If the club misses the ball, there is no new force on the ball from the club and the ball stays still.

Figure 4

Speeds up. When skateboarders want to move faster, they use their feet to exert a force on the ground. Figure 9 Remains still. The gravity pulling down on the pot plant and the weight of air above it are in balance with the force of the ground pushing up on the pot. in turn brings the bicycle to a stop. The gravity pulling down on the pot plant and the weight of air above it are in balance with the force of the ground pushing up on the pot.

Figure 7 Changes shape.

The hands push the plasticine into a different shape. When the hands stop pushing, the plasticine no longer changes. Retrieve 1 Defi ne the term ‘force’. 2 Recall seven things that forces can do. 3 State the unit used when measuring a force. 4 Name the person whom the unit of force is named after. Comprehend 5 Describe how force can be measured. 6 Use an example to describe how you can see the effects of a force, but not see the force. 7 Many measuring instruments have to be calibrated. Use an example to explain why calibration of equipment is important. (HINT: Describe the consequences of not calibrating the equipment and how this would affect the results of an experiment.) Analyse 8 Rank these forces from biggest to smallest. a a truck hitting a pole b a rocket being launched c typing one letter on a computer keyboard

d kicking a soccer ball 4.1 Check your learning DRAFT e pushing a car along the street Apply 9 A student was using the force measurer in Experiment 4.1 when the rubber band broke. Predict how using a different rubber band would affect the results. Quiz me Complete the Quiz me to check how well you’ve mastered the learning intentions and to be assigned a worksheet at your level.