Physics
Forces and motion
Key words
Learning objectives:
balanced forces equilibrium Newton’s first law resultant force
• understand what a force does • explain what happens to an object if all the forces acting on it cancel each other out • analyse how this applies to everyday situations.
An object may have several forces acting on it (Figure 5.18). A number of forces acting on an object may be replaced by a single force that has the same effect as all the original forces acting together. This single force is called the resultant force. If the forces are in balance they cancel each other out and the resultant force is zero. The object behaves as if there is no force on it at all. The object will be in equilibrium; it will not accelerate.
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Newton’s first law says that, if the resultant force acting on an object is zero, it will:
ABS (anti-lock braking system) brakes stop a car more quickly by rapidly pumping the brakes and preventing skidding.
tension force in the stalk
• if stationary, remain stationary • if moving, keep moving at a steady speed in a straight line. 4
If the resultant of two forces is zero, what must be true about:
Thinking about forces
a their size?
If we think about an object, we can usually identify the forces that are acting on it. For example, an apple growing on a tree (Figure 5.16) has the force of gravity acting downwards; this is its weight. There is another force acting, vertically upwards, through the stalk. These two forces are equal and opposite. They cancel out and the apple doesn’t move.
b their direction?
Let’s think of another situation: that of a parachutist. The parachutist jumped out of an aircraft and is falling. After a while, the parachutist reaches a steady speed called the terminal velocity. There is a downwards force weight, and an upwards force, which is called drag, or air resistance (Figure 5.17). When these two forces are equal and opposite, they cancel out. 1
The weight of the apple in Figure 5.16 is 1 N. What is the force upwards in the stalk?
2
When the parachutist in Figure 5.17 opens the parachute:
weight
Figure 5.16 Balanced forces on an apple hanging from a stalk
3
Is it always true that when all the forces on an object balance out it will be stationary?
Newton’s first law resistive forces
Look at the two forces shown in Figure 5.18. If the resultant of the two forces is zero, what must be true about the sizes of those forces?
6
In addition to the forces shown in Figure 5.18, there will also be weight, acting downwards, and a reaction force, acting upwards. How do these also obey Newton’s first law?
Deep in space, with no drag or gravitational forces to affect it, a spacecraft moving with its rockets off will keep moving forever without slowing down or speeding up.
REMEMBER!
drag
Applying Newton’s first law
weight
Figure 5.17 Balanced forces on a parachutist falling at a steady speed
a What will happen to the size of the drag force? b Will the two forces still be in balance immediately after the parachute opens?
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Did you know?
DID YOU KNOW? For a parachutist in free fall, before the parachute is released, the terminal velocity is around 50 m/s (180 km/h). However, by assuming a diving position this can increase to twice as much.
One of the most important questions to ask about a situation in which forces are acting on an object is whether the resultant force in a certain direction is zero. In this case Newton’s first law applies and a stationary object will remain at rest and if the object is moving it will continue to move in a straight line and at constant velocity. Examples of this are a boulder resting on the ground and a bicycle being pedalled along a level road at steady speed in a straight line. 7
Explain why there is a zero resultant force for a boulder on the ground.
8
Explain why there is a zero resultant force in the horizontal direction for a bicycle being pedalled along a level road at steady speed in a straight line.
9
Explain why, in the example of the bicycle, if the cyclist gets tired, the resultant force may no longer be zero.
10
Describe the motion of the Earth if the Sun suddenly vanished.
A normal contact force is exerted on an object by, for example, the surface of a floor or wall it is in contact with. The force is at right angles to the surface. In physics, ‘normal to’ means ‘at right angles to’.
forward force from engine
Figure 5.18 If the forces cancel out the resultant force is zero
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AQA GCSE Physics: Student Book
Google search: 'Newton’s first law'
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