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Constant speed and acceleration for SQA Higher Physics

Constant speed and acceleration

This page covers the following topics:

1. Newton's first law

Newton's first law states that unless an object is acted upon by an external resultant force, it remains at rest or continue moving at a constant velocity. When there is no resultant force acting on an object, the object is said to be in equilibrium. Usually resultant force is found by combining forces on the same axis as vectors.

Newton's first law

1

State Newton's first law.

Newton's first law states than an object will remain at rest or continue to move at a constant velocity unless it is acted upon by an external resultant force.

An object will remain at rest or continue to move at a constant velocity unless it is acted upon by an external resultant force.

State Newton's first law.

2

A ladder of weight 200 N resting on rough ground is leaning against a smooth wall. The normal reaction exerted by the wall on the ladder is 200 N. Find the normal reaction and the frictional force exerted by the ground on the ladder.

Newton's first law states that an object will remain stationary unless it is acted upon by an external resultant force. Since the ladder is stationary, the forces acting on it must be balanced.

Resolving vertically: weight = N, therefore the normal reaction exerted by the ground on the ladder is 200 N.

Resolving horizontally: normal reaction at wall = F, therefore the frictional force exerted by the ground on the ladder is 200 N.

200 N, 200 N

A ladder of weight 200 N resting on rough ground is leaning against a smooth wall. The normal reaction exerted by the wall on the ladder is 200 N. Find the normal reaction and the frictional force exerted by the ground on the ladder.

3

An object is experiencing forces A, B, C and D as shown in the diagram. Explain the relationships which need to hold between these forces for the object to be in equilibrium.

By Newton's first law, the forces on an object must be balanced for it to be in equilibrium.
Therefore, the following equalities must hold: A = C and B = D.

balanced forces โ†’ A = C, B = D

An object is experiencing forces A, B, C and D as shown in the diagram. Explain the relationships which need to hold between these forces for the object to be in equilibrium.

4

A car is travelling at a constant velocity using a forwards thrust force of 1500 N. What is the size of the sum of frictional forces acting on the car?

Since the car is travelling at constant velocity, by Newton's second law, the forces acting on the car must be balanced. Therefore, the size of the sum of frictional forces acting on the car is 1500 N.

1500 N

A car is travelling at a constant velocity using a forwards thrust force of 1500 N. What is the size of the sum of frictional forces acting on the car?

5

A northward force of 50 N, a southwards force of 16 N and two eastward forces of 32 N and 45 N act on a particle. What are the missing forces for the particle to be in equilibirum?

By Newton's first law, for an object to be in equilibrium, it must not be experiencing any external resultant force.

Resolving vertically, resultant force = 50 N โˆ’ 16 N = 34 N northwards. Therefore, a force of 34 N southwards is needed to balance this out, so that there is no vertical resultant force.

Resolving horizontally, Resultant force = 45 N + 32 N = 77 N eastwards. Therefore, a force of 77 N westwards is needed to balance this out, so that there is no horizontal resultant force.

34 N southwards, 77 N westwards

A northward force of 50 N, a southwards force of 16 N and two eastward forces of 32 N and 45 N act on a particle. What are the missing forces for the particle to be in equilibirum?

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