Which of the following is a wrong example of friction?
A person lifting an heavy object
Understanding Friction and Its Examples
Friction is a force that opposes motion or attempted motion between surfaces in contact. It always acts parallel to the surfaces that are in contact.
Analyzing the Given Scenarios
Let's examine each option to determine whether it involves friction opposing motion between surfaces in contact.
- Washing machine pushed along a floor: When a washing machine is pushed along a floor, there is relative motion between the bottom surface of the washing machine and the surface of the floor. Friction acts between these two surfaces, opposing the sliding motion. This is an example of kinetic friction.
- Sled sliding across snow: As a sled slides across snow, there is relative motion between the runners of the sled and the surface of the snow. Friction acts between these surfaces, opposing the sliding motion. This is also an example of kinetic friction, although the friction on snow is often lower than on other surfaces.
- A person sliding down a slide: When a person slides down a slide, there is relative motion between the person's body (or clothing) and the surface of the slide. Friction acts between these surfaces, opposing the sliding motion. This is another example of kinetic friction.
- A person lifting an heavy object: When a person lifts a heavy object, they are applying an upward force to overcome the downward force of gravity acting on the object. The primary force being overcome during the lifting action is gravity, not friction between surfaces attempting to slide past each other relative to the direction of the lift. While there might be static friction involved in the person's grip to prevent the object from slipping down their hands, or internal friction within the body, the lifting motion itself (vertical movement) is fundamentally about countering gravity, not opposing friction between the object and another surface it is sliding *across* or *along* during the lift.
Identifying the Wrong Example of Friction
Based on the analysis, the scenarios involving pushing or sliding across a surface (washing machine, sled, person on a slide) are clear examples where friction acts to oppose the motion between the contacting surfaces. Lifting an object, however, primarily involves overcoming gravity. While friction might be present in other ways (like grip), the lifting motion itself isn't directly opposed by surface-to-surface friction in the same way that sliding or pushing is.
Therefore, lifting a heavy object is the scenario that is not a typical or direct example of friction opposing the primary intended motion between contacting surfaces, unlike the other options.
| Scenario | Involves Friction Opposing Motion Between Surfaces? | Type of Friction (if applicable) |
|---|---|---|
| Washing machine pushed | Yes | Kinetic Friction |
| Sled sliding | Yes | Kinetic Friction |
| Person sliding | Yes | Kinetic Friction |
| Person lifting object | No (Primarily overcoming gravity) | Grip friction (Static) might be present, but not opposing the lift itself. |
Conclusion on Friction Examples
The action of lifting a heavy object is primarily about applying force to counteract gravity. The other actions, pushing a washing machine or sliding a sled or person, directly involve friction between contacting surfaces opposing the motion. Thus, lifting an object is the scenario that is not a typical or direct example of friction in the context of motion opposing surface contact as presented in the other options.
The scenario that is a wrong example of friction opposing motion between surfaces in the context of the other options is a person lifting an heavy object.
Revision Table: Key Concepts of Friction
| Concept | Description | Formula (Simplified) |
|---|---|---|
| Friction Force ($\vec{f}$) | Force opposing relative motion or attempted motion between surfaces in contact. Acts parallel to surfaces. | N/A (vector) |
| Static Friction ($f_s$) | Friction that prevents surfaces from sliding relative to each other when a force is applied. Maximum value before motion starts. | $f_s \le \mu_s N$ |
| Kinetic Friction ($f_k$) | Friction that opposes the relative sliding motion between surfaces that are already in motion. Typically less than maximum static friction. | $f_k = \mu_k N$ |
| Coefficient of Static Friction ($\mu_s$) | A dimensionless value representing the ratio of the maximum static friction force to the normal force. Depends on the materials in contact. | N/A |
| Coefficient of Kinetic Friction ($\mu_k$) | A dimensionless value representing the ratio of the kinetic friction force to the normal force. Depends on the materials in contact. | N/A |
| Normal Force ($N$) | The component of the contact force exerted by a surface on an object which is perpendicular to the surface. | Depends on situation (e.g., $N=mg$ on a flat horizontal surface with no other vertical forces). |
Additional Information: Forces in Lifting
When a person lifts a heavy object, the main forces involved are:
- Gravity: The force pulling the object downwards towards the Earth. This is the object's weight, calculated as $W = mg$, where $m$ is mass and $g$ is the acceleration due to gravity.
- Applied Force: The upward force exerted by the person on the object to lift it. To lift the object at a constant velocity, this applied force must be equal in magnitude to the weight, $F_{applied} = W$. To accelerate it upwards, the applied force must be greater than the weight.
- Normal Force: If the object was initially resting on a surface, there was a normal force from the surface supporting it. This normal force reduces to zero as the object is lifted clear of the surface.
While friction might be involved in maintaining a grip on the object (static friction preventing it from sliding out of the hands) or internal to the person's body (muscles, joints), it is not the primary force opposing the vertical lifting motion itself in the same way that kinetic friction opposes horizontal sliding motion across a surface.
Important Questions from Common forces in mechanics
Which of the following is NOT true about Frictional force?
A. Friction is the force which opposes the relative motion of two surfaces in contact.
B. The force of friction that acts when a body is moving (sliding) on a surface is called sliding friction.
C. Friction in machines wastes energy and also causes wear and tear.
D. Rolling friction is much more than sliding friction, the use of ball bearings in a machine considerably reduces friction.
Which one of the following statements about friction is incorrect?
- An electric lift with a maximum load of $2000\text{ kg}$ (lift + passengers) starts from rest and accelerates upwards at $0.2\text{ ms}^{-2}$. The frictional force opposing the motion is $3000\text{ N}$. The instantaneous power delivered by the motor when the lift reaches a speed of $1.5\text{ ms}^{-1}$ is:
(Assume $g = 10\text{ ms}^{-2}$) When a wheel is rolling on a level road, the direction of frictional force between the wheel and road is in:
The resistive force between a vehicle’s tyres and the road is known as ______.
